National Aeronautics and 

Space Administration 

HUMAN ADAPTATION TO SPACE FLIGHT:
 
 

THE ROLE OF NUTRITION

 

Scott M. Smith 

Nutritionist; 


Manager for Nutritional Biochemistry 


Nutritional Biochemistry Laboratory 


Biomedical Research and 
 


Environmental Sciences Division
 
 

NASA Johnson Space Center

 

Houston, Texas USA
 
 

Sara R. Zwart 

Senior Scientist; Deputy Manager 


for Nutritional Biochemistry

 

Nutritional Biochemistry Laboratory 


Biomedical Research and 
 


Environmental Sciences Division
 
 

NASA Johnson Space Center

 

Houston, Texas USA
 
 

Martina Heer 

Nutritionist

 

Adjunct Professor of Nutrition Physiology
 
 

University of Bonn

 

Institute of Nutritional and Food Sciences

 

Bonn, Germany

 

Table of Contents 

Preface .......................................................................................................................... iv

 

1. Introduction..............................................................................................................1

 

Conducting Nutrition Research on ISS ..........................................................................2

 

Blood Collection..........................................................................................................2

 

Urine Collection...........................................................................................................3

 

Biological Sample Stowage and Return .....................................................................4

 

Food Intake Monitoring...............................................................................................5

 

Body Mass ..................................................................................................................6

 

2. Energy and Fuel Metabolism..................................................................................7

 

Energy Expenditure and Requirements..........................................................................7

 

Energy Intake..................................................................................................................8

 

Implications for Inadequate Energy Intake.....................................................................9

 

Carbohydrate ............................................................................................................... 13

 

Fat (and Fatty Acids) .................................................................................................... 14

 

Cofactors in Energy Metabolism .................................................................................. 15

 

3. Muscle and Protein ............................................................................................... 17

 

Protein Intake ............................................................................................................... 17

 

Vitamin B6 .................................................................................................................... 18

 

Muscle Loss and Protein Turnover in Microgravity ...................................................... 18

 

Ground Analog Studies ................................................................................................ 19

 

Muscle Loss Countermeasures.................................................................................... 21

 

Mechanical................................................................................................................ 21

 

Pharmacological ....................................................................................................... 22

 

Nutritional.................................................................................................................. 25

 

Protein and Bone ...................................................................................................... 26

 

4. Bone........................................................................................................................ 29

 

Bone Loss .................................................................................................................... 29

 

Bone Metabolism ......................................................................................................... 30

 

Bone Loss Countermeasures.......................................................................................31

 

Exercise..................................................................................................................... 31

 

Gravity....................................................................................................................... 35

 

Vibration .................................................................................................................... 35

 

Pharmacological Agents ........................................................................................... 35

 

Nutritional Countermeasures .................................................................................... 36

 

i
Nutrients and Bone Health...........................................................................................37

 

Calcium ..................................................................................................................... 37

 

Vitamin D...................................................................................................................37

 

Vitamin K...................................................................................................................40

 

Phosphorus............................................................................................................... 40

 

Magnesium................................................................................................................ 41

 

Zinc (and Lead) ......................................................................................................... 42

 

Unique Aspects of Calcium and Space Flight.............................................................. 42

 

Urine Processing and Water Reclamation ................................................................ 42

 

Natural Calcium Isotope Composition of Bone ........................................................ 43

 

5. Iron and Hematology............................................................................................. 45

 

Iron ............................................................................................................................... 45

 

Copper ......................................................................................................................... 48

 

6. Cardiovascular Health........................................................................................... 51

 

Energy .......................................................................................................................... 51

 

Magnesium................................................................................................................... 53

 

Ongoing and Future Research ..................................................................................... 53

 

Oxidative Stress ........................................................................................................ 53

 

Omega-3 Fatty Acids ................................................................................................ 54

 

Healthier Diets........................................................................................................... 54

 

7. Sodium, Potassium, Fluid, and Renal Stone Risk .............................................. 55

 

Sodium and Chloride.................................................................................................... 55

 

Potassium..................................................................................................................... 58

 

Fluid.............................................................................................................................. 59

 

Renal Stone Risk .......................................................................................................... 61

 

8. Space Flight Ophthalmic Changes and Nutrition...............................................63

 

Ophthalmic Changes.................................................................................................... 63

 

One-Carbon Metabolism.............................................................................................. 63

 

Folate............................................................................................................................ 65

 

Vitamin B12................................................................................................................... 66

 

Biotin ............................................................................................................................ 67

 

Vitamin A ...................................................................................................................... 67

 

9. Immune Function, Inflammation, and Nutrition.................................................. 71

 

Energy Intake................................................................................................................ 71

 

Protein and Amino Acids..............................................................................................71

 

Vitamin D ...................................................................................................................... 72

 

ii
Vitamin B12................................................................................................................... 73

 

Sodium ......................................................................................................................... 73

 

Vitamin A ...................................................................................................................... 74

 

Vitamin C......................................................................................................................74

 

Vitamin E ...................................................................................................................... 74

 

Copper ......................................................................................................................... 74

 

Zinc............................................................................................................................... 75

 

Polyphenols.................................................................................................................. 75

 

Iron ............................................................................................................................... 75

 

Polyunsaturated Fatty Acids ........................................................................................ 76

 

10. Antioxidants and Oxidative Stress.....................................................................77

 

Hypoxic Conditions......................................................................................................77

 

Extravehicular Activity .................................................................................................. 77

 

Reactive Oxygen Species and Exercise....................................................................... 78

 

Radiation Exposure ...................................................................................................... 78

 

Oxidative Damage Markers During Space Flight and in Ground Analogs ................... 79

 

Antioxidants and Related Nutrients: Selenium, Vitamin E, Vitamin C .......................... 79

 

11. Pharmacology and Drug-Nutrient Interactions ................................................ 83

 

Supplements versus Whole Foods............................................................................... 83

 

Dietary Factors ............................................................................................................. 84

 

Metabolism of Nutrients ...............................................................................................85

 

Monoamine Oxidase Inhibitors .................................................................................... 86

 

Antacids and Proton Pump Inhibitors .......................................................................... 86

 

Summary of Pharmacology and Drug-Nutrient Interactions........................................86

 

12. Summary .............................................................................................................. 87

 

13. Appendices ......................................................................................................... 89

 

References ................................................................................................................... 89

 

Authors and Editor .....................................................................................................131

 

Acknowledgments .....................................................................................................132

 

Nutrient Intake Data ...................................................................................................134

 

List of Figures.............................................................................................................135

 

Abbreviations..............................................................................................................140

 

iii
Preface 

Our first effort at compiling a book took place in 2009. The product of that effort is 

available for (free) download through the link http://go.nasa.gov/QS1KW1. That initial 

effort came from a push within the NASA Human Research Program to have each of 

the disciplines or "Elements" comprising the Program prepare an “evidence report” 

describing, as thoroughly as possible, “everything” known about the discipline (in our 

case, Space Nutrition), including gaps in our knowledge that needed to be filled before 

we could take the next steps of exploration. Details were provided to allow definition of 

space flight requirements, and to that end, the text of the Space Nutrition book included 

basics of nutrient metabolism and requirements on Earth, and much of the available data 

related to space flight. Because, to a degree, the format was thrust upon us, and the 

timeline was short, the 2009 book reads a little like an encyclopedia. 

The current volume is not a second edition of that first book, but rather an evolution. 

It is our hope that this book reflects a (brief) review of the history of and current state 

of knowledge about the role of nutrition in human space flight. We have attempted to 

organize this from a more physiological point of view, and to highlight systems, and 

the nutrients that support them, rather than the other way around. 

The other main difference between the 2 books is that a tremendous amount of work 

has been completed and published in the past 6 years, including many reports of 

ground-based studies and International Space Station (ISS) flight research findings. 

New risks to human health have been identified, including one related to vision changes 

in astronauts on ISS. We detail herein data suggesting a tie-in of the folate- and vitamin 

B12-dependent 1-carbon metabolism pathway with these changes. Recent publications 

have documented the effects of good nutrition and heavy resistance exercise on bone 

metabolism during space flight. After more than a half century of human space flight, 

this is the first evidence of the ability to mitigate the loss of bone mineral density in 

astronauts on long-duration missions. Although more work remains to be done, any 

progress is incredibly exciting. 

We hope we have captured in this book the state of the field of study of the role 

of human nutrition in space flight, along with the work leading up to this state, and 

some guideposts for work remaining to be done and gaps that need to be filled. 

iv
1

The importance of nutrition in exploration has been documented repeatedly throughout 

history, on voyages across oceans, on expeditions across polar ice, and on treks across 

unexplored continents. Scurvy is a prime example of exploration-induced nutritional 

discovery. While most people understand that scurvy is associated with vitamin C 

deficiency, and that this was an issue for sailors centuries ago, few realize the magnitude 

of this problem. If one examines the roughly 400-year period between Christopher 

Columbus’ voyage in 1492 and the invention of the steam engine, scurvy resulted in 

the deaths of more sailors than all other causes of death combined (1). Ships that sailed 

with a crew of hundreds often returned with tens. It is estimated that more than 2 million 

sailors perished from scurvy during this time (1). A key difference between these difficult 

journeys and future space exploration is that astronauts are not likely to find food along 

the way. Thus, understanding the nutritional requirements of space travelers and the role 

of nutrition in human adaptation to microgravity are as critical to crew safety and mission 

success as any of the mechanical systems of the spacecraft itself. 

There are many facets to nutrition and health on Earth. Space flight introduces further 

complications, and many gaps remain in our knowledge of the relationships between 

nutrition and health in space that need to be filled before we can safely embark on 

“exploration” missions, that is, missions beyond low Earth orbit. At the surface of these 

unknowns is the need to understand and define basic nutrient requirements during 

extended stays in microgravity. Beyond this lies the need to characterize the role of 

nutrition in the adaptation of physiological systems to microgravity, and/or the impact 

of these changes on nutrition. Additionally, environmental impacts (including radiation, 

and spacecraft and spacesuit atmospheres) can alter nutritional status and nutritional 

requirements of space flight. For surface missions (on, for example, the moon and Mars), 

partial gravity may complicate the situation further. Finally, many potential targets for 

nutritional countermeasures exist, where modified dietary intake may help to counteract 

or mitigate some of the negative effects of space flight on the human body. 

In 2009, at the urging of NASA’s Human Research Program, the authors published a 

book (2) with the aim of reviewing the existing knowledge and history of human nutrition 

for space flight, with a key goal of identifying gaps in the knowledge base required to 

provide confidence that the risk of an inadequate food system or inadequate nutrition 

to support humans on expeditions to the moon and Mars is as low as possible. A brief 

history of space programs and space food systems was also included. This volume is 

available for free download through the link http://go.nasa.gov/QS1KW1. 

What we have attempted to provide here is not a second edition, but rather an updated 

overview of space nutrition from a more physiological perspective. Its division into 

chapters is based on physiological systems (eg, metabolism, muscle, bone), and 

highlighted in each chapter are the nutrients that are particularly associated with that 

system. We present data from ground-based analog studies, conducted in laboratories 

on Earth by exposing human subjects to one or more environmental conditions similar 

to those produced by space flight. While bed rest may be the most common analog of 

space flight, recent nutrition research from Antarctica and undersea habitats has helped 

Introduction 1
2

expand our understanding of nutritional changes during space flight simulations. We will 

also review space flight research, including data from now-historical flights on the Space 

Shuttle, as well as the Russian space station Mir and earlier space programs such as 

Apollo and Skylab missions. In recent years, the International Space Station (ISS) has 

provided (and continues to provide) a wealth of nutrition findings from flights of 4 to 

6 months, and these will be reviewed in detail. We also present a brief overview of 

the techniques used for conducting this research on orbit. 

Key areas of nutrition concerns are primarily nutrition- or nutrient-related changes, 

including loss of body mass and depletion of body nutrient stores because of 

inadequate food supply, inadequate food intake, increased metabolism, and/or 

irreversible loss or degradation of nutrients. Physiological changes that involve nutrition 

(to some degree) include bone and muscle loss, cardiovascular degradation, impairment 

of immune function, and neurovestibular changes. Vision changes have recently been 

identified in ISS crewmembers, and are a major (if not the major) health concern for 

long-term space missions (3). Environmental issues, including radiation exposure and 

cabin environment (O2/CO2, temperature, and humidity) can have profound effects on 

nutrition, and/or may provide areas where nutrients may serve as countermeasures. 

When crewmembers are outside the spacecraft (that is, on spacewalks, or extravehicular 

activity), the spacesuit becomes a spacecraft as well, with associated concerns (high 

oxygen exposure, limited water availability, inability to eat for up to 8 to 10 hours at a 

time while in the suit). Drug-nutrient interactions are of concern on Earth, and take on 

added importance during space flight. We will review all of these areas in general, and 

in detail with respect to the role of nutrition and specific nutrients, highlighting existing 

knowledge as well as gaps where additional research is needed. 

Conducting Nutrition Research on ISS 

Blood Collection 

The first crew of ISS took up residence there in November 2000, but the turning point for 

nutrition research on ISS came in 2006, when the capability for collection, processing, 

and frozen storage of blood and urine samples was brought to orbit. Crewmembers are 

trained in procedures and use of the equipment required, and depending on the planned 

crew complement and schedules, many are trained for autophlebotomy, that is, drawing 

their own blood. In October 2006, early in the Expedition 14 mission, Mike Lopez-Alegria 

collected the first blood samples to be drawn on ISS and captured the image (Figure 1). 

After they are collected, samples are allowed to clot, and are then centrifuged (Figure 2). 

Figure 1. Mike Lopez-Alegria collects the 

first blood sample on the International Space 

Station on October 5, 2006, having inserted 

the needle himself. The collection tubes can 

be seen (one in hand, the others in elastic 

bands on his belt assembly), along with a 

sharps container and detailed procedures 

(both Velcroed to the wall). He had set 

the camera to automatically take pictures 

periodically, snapping this one in the process. 
3

Figure 2. The refrigerated centrifuge on ISS. 

Urine Collection 

Urine is collected by using urine collection 

devices, UCDs (Figure 3; Figure 4). A UCD 

is essentially a bag with a one-way valve on 

one end, allowing urine entry while voiding, 

and a port at the other end to withdraw 

syringe samples. Each void, typically over a 

24-hour period, is collected in this manner, 

and 2 or 3 syringes are used per void to 

withdraw about 6 to 7 mL of urine. Before 

flight, a small amount of lithium chloride 

solution is added to the bags, and after the 

void is complete, the UCD is kneaded to 

facilitate mixing of the LiCl with the void. 

After the samples are returned to Earth, 

the lithium concentration is determined, 

to allow back-calculation of the urine void 

volume so that 24-hour pools may be 

created from the individual voids. 

Figure 3. Left: Urine Collection Device (UCD), shown here with a syringe on the collection port, 

and a female adapter. Right: a UCD with a male adapter shown floating on ISS. 

After the syringes are collected, the UCD 

is placed in a ziplock bag to provide another 

layer of containment, and the ziplock is placed 

in a urine containment bag (UCB, Figure 

5). Typically, after the day’s collections are 

complete, the UCBs are stowed for eventual 

disposal with other ISS trash. Most trash is 

discarded in departing cargo vehicles that are 

not designed for reentry (that is, they burn up 

coming back through the atmosphere). Before 

the Space Shuttle was retired in 2011, some 

trash was brought home on Shuttle vehicles. 

Figure 4. Sunita Williams shown here 

with a UCD, contained in a ziplock bag 

to provide another layer of containment. 
4

Biological Sample Stowage and Return 

The other key piece of hardware launched 

to ISS in 2006 was the freezer, the minuseighty (degrees) laboratory freezer for ISS, 

or MELFI. There are currently 3 MELFIs 

on ISS, to allow increased stowage volume 

for intervals between returns to Earth. 

Each MELFI has 4 dewars (double-walled 

containers). On the outside of the MELFI, 

the dewars are covered (Figure 6 shows 

a crewmember opening the lid of one of 

the dewars). Each dewar has 4 trays, which 

are pulled out to store samples (Figures 

7-9). The MELFIs are primarily designed 

for ultracold storage (temperatures 

lower than -80°C), and typically maintain 

temperatures close to -96°C. They are 

capable of refrigerated storage as well; 

typically one of the 4 dewars will be at 

refrigerator temperature, while the others 

will be at -96°. 

Figure 7. External view of a MELFI, showing the 

interior of one of the dewars, with 1 tray partially 

removed. The 2 urine syringes to be placed in 

the freezer can be seen Velcroed to the wall. 

Figure 5. Urine Containment Bag, used for 

holding discarded UCDs until they can be 

disposed of along with other trash from ISS. 

Figure 6. External view of a MELFI, showing 
 

the covers for the 4 dewars. 
 

Whenever possible, samples are returned 

to Earth. The Space Shuttle was the only 

ride home for frozen blood and urine 

samples before its retirement in 2011. 

On the first few return flights, samples 

were transferred from the MELFI to 

Double Cold Bags (Figure 9), which 

Figure 8. Closer view of the MELFI tray, with 

compartment open for sample insertion. 

The mesh bags are designed to contain the 

samples from a given blood collection session. 

Blood samples are shown in this image. 

Figure 9. Crewmember Greg Chamitoff 

(left) can be seen with the MELFI tray under 

his right hand, performing sample transfers 

from the MELFI into the Double Cold Bag 

(Sandy Magnus has her hand inside the 

DCB in this image). 
5

included specially designed ice packs to maintain samples in a frozen state (specifically, 

at a lower temperature than -30°C) from the time when samples were taken out of the 

MELFI through the time when they were received at the landing site, usually 2 to 3 hours 

after landing. The ice packs would maintain required temperatures for more than 125 

hours. This duration was required because the Space Shuttle usually stayed on orbit 

for 1 to 2 days after undocking from ISS, and could potentially be held up from landing 

because of weather issues at landing sites for another 1 to 2 days. On many sample 

return flights, in addition to the Double Cold Bags, a powered -96°C freezer is also 

flown to increase the volume of returning samples. 

Since the retirement of the Space Shuttle, the only space cargo vehicles that have been 

able to return payloads are the SpaceX “Dragon” capsules. Other cargo vehicles exist, 

but they are only designed to bring supplies and equipment to ISS, not to return them 

to Earth. Thus, these other vehicles will burn up in the atmosphere on return, by design. 

The SpaceX capsules splash down in the ocean off the coast of California. 

Food Intake Monitoring 

Methods of recording food intake have evolved over the course of human space flight. 

On Skylab missions, which included metabolic balance studies, crewmembers called 

down daily to mission control and reported any off-nominal intake for the day. When it is 

required for specific research protocols, detailed dietary intake data has been obtained 

through written food intake logs or even with barcode scanning. Barcode scanning often 

seems an attractive alternative, but it comes with its own set of issues when used during 

space flight. Not all space foods have barcode labels on them, or for many commercially 

packaged items, the label is not in the database on the reader, and thus item information 

won’t display on the screen. Additionally, many of the space foods do not have flat 

surfaces on their packaging, and wrinkles in the label can be difficult for the barcode 

reader to detect. 

It was determined during the Shuttle-Mir program that for long-duration missions it 

would be valuable for flight surgeons to have a means of having the crews easily, and 

quickly, report typical dietary intake. To this end, a Food Frequency Questionnaire (FFQ) 

(Figure 10; Figure 11) was 

designed with the goal of 

having crews report typical 

intake once per week, with 

a computerized questionnaire 

that would take 5 to 15 

minutes to complete. The FFQ 

was designed to report 6 key 

nutrients: energy, fluid, protein, 

calcium, sodium, and iron. 

Later a seventh nutrient, 

potassium, was included as 

well. The development, testing, 

and data from the FFQ have 

been reported (4-6). Although 

some crewmembers 

expressed concern over the 

accuracy of a 7-day recall 

questionnaire, the FFQ had 

Figure 10. Peggy Whitson with the opening screen of 

the Food Frequency Questionnaire. After a crewmember 

enters name or initials, the second screen opens with a list 

of food categories, under each of which is a list of food 

items grouped together according to nutrient content. 
6

Figure 11. A screen shot from the Food Frequency 

Questionnaire. Available foods are grouped according 

to key nutrient content. Shown here are 2 groups: “Fruit” 

and “Beans, Soups.” The foods provided by the European 

Space Agency, the Canadian Space Agency, the Japanese 

Aerospace Exploration Agency, and NASA are shown in black 

font, and foods provided by Russia are shown in blue. 

several things in its favor: 

the ISS food system is 

rather limited, the portion 

sizes are known, and 

the available foods 

repeat with a high rate 

of frequency (with a 

menu cycle of 8 to 11 d). 

While this text was being 

written (2013-2014) an 

iPad App was being 

developed for potential 

flight to ISS. Dubbed the 

ISS Food Intake Tracker, 

it has the purpose of 

enabling crews to more 

easily and accurately 

collect complete dietary 

intake data, in support 

of both medical 

operations and research 

experiment purposes. 

Body Mass 

Determining body mass is one of the most basic overall measures of health, as 

noted every time one visits a physician’s office on Earth. Determination of “weight” in 

weightlessness, however, presents some unique challenges, as described in a recent 

report (7). There are two devices on ISS: a Body Mass Measuring Device, which uses 

spring oscillation, and a Space Linear Acceleration Mass Measuring Device, which uses 

the physics of the equation: force = mass x acceleration. Images of the two devices 

available on ISS are shown in Figure 12. 

Figure 12. Karen Nyberg 

(left), Expedition 36 

Flight Engineer, using 

the Space Linear 

Acceleration Mass 

Measurement Device. 

Tom Marshburn 

(right), Expedition 

34 Flight Engineer, 

using the Body Mass 

Measurement Device. 

Photo credits: NASA. 
7

Adequate energy intake is perhaps the single most important aspect of astronaut 

nutrition. This is not only because energy in and of itself is more important than other 

nutritional factors, but also because if enough food is consumed to meet energy needs, 

then generally other nutrients will also be consumed in reasonable amounts. There 

are many facets to maintaining eucaloric intake during space flight, including energy 

requirements; physiological changes in taste and satiety; scheduling issues of allotting 

time for meal preparation, consumption, and cleanup; food quality; and even food 

availability. 

Little research has been done on differences in fuel components (protein, carbohydrate, 

fat) during space flight, or on cofactors (eg, vitamins) of energy utilization. We review 

these here, highlighting what has been done and potential areas of future research. 

Energy Expenditure and Requirements 

Energy expenditure is often hypothesized to be lower during flight than on the ground, 

because of the presumed relative hypokinesia in space (8). An early example of this is 

that lower energy expenditure was observed during extravehicular activity (EVA) on the 

lunar surface than during similar activities at 1g (9). This was determined through indirect 

calorimetry in the space suit. However, Space Shuttle crewmembers during EVA did not 

have any change in energy expenditure relative to before flight (10). 

Studies of total energy expenditure of Shuttle astronauts documented that in-flight 

energy expenditure was unchanged from preflight levels (11), or in cases where these 

crews performed intensive exercise during the mission, energy expenditure during 

flight was higher than before flight (12). For these studies, the doubly-labeled water 

(water enriched with deuterium and 18O) technique was used to determine oxygen 

consumption (13). The benefits of this technique are that it is noninvasive and it takes 

into account the energy cost of all activities over a period of several days. The drawback 

of the method is that information about the individual components of total energy 

expenditure (TEE), such as resting, sleep, and exercise, is not available. The range of 

differences between preflight and in-flight TEE makes it important to have information 

about the components of TEE. Although it is assumed that moving the body mass 

around the cabin requires less expenditure of energy during weightlessness than at 

1g, other metabolic activities, such as maintaining resting metabolic rate and responding 

to stress, may require increased energy expenditure during weightlessness. 

In ground-based bed rest studies, an analog of space flight, resting energy expenditure 

did not change, but TEE was less during bed rest than before bed rest (14). Because 

TEE during flight is unchanged (11) or increased (12) from preflight levels, the lower 

TEE during bed rest may indicate that bed rest is not an appropriate model for studies 

of energy metabolism during flight. One possible explanation for this difference between 

bed rest and space flight is the lack of a metabolic response to stress during bed 

rest (15). Attempts have been made to improve the utility of bed rest studies by 

administering a metabolic stressor (such as triiodothyronine or cortisol) to provide 

2 Energy and Fuel Metabolism 
8

a better ground-based model than bed rest alone for the metabolic effects of space 

flight on energy and fuel metabolism (16). 

Energy requirements are typically estimated using standard equations, including the 

World Health Organization (WHO) (17) and Dietary Reference Intake (DRI) (18), using 

a “moderately active” or “active” adjustment for activity level for these 2 equations, 

respectively. The DRI equation includes the effects of age, sex, weight, and height in 

estimating energy requirements. While it would be more accurate to determine actual 

resting energy expenditure before flight for each astronaut, this testing has not been 

possible except in cases where these data were required for specific experiments. 

The aforementioned studies of energy expenditure on Space Shuttle missions were 

performed on flights of 10 to 14 days’ duration. The objective of a European Space 

Agency (ESA)-sponsored experiment initiated in 2012 is to determine energy expenditure 

during 6-month missions to the International Space Station. These data will help us 

understand whether any adaptation effect occurs on these longer missions, and thus 

may be important in estimating energy requirements for exploration missions (missions 

beyond low Earth orbit). 

Energy Intake 

Historically, inadequate energy intake and subsequent body mass loss have been 

considered hallmarks of space flight, and have occurred on many missions and 

programs (2, 4, 6, 11, 19-27). From Apollo through the Shuttle program, crewmember 

dietary intakes during flight averaged about 70% of predicted requirements (6), and ISS 

intakes have averaged about 80% of requirements (Figure 13). There are exceptions to 

this finding, including the Skylab missions of the early 1970s (28, 29), European flights 

to the Mir space station (30), and more recently some of the ISS missions (31). In the 

Skylab and Mir examples, crew participation in metabolic experiments has required 

consumption of balanced, 

controlled, eucaloric diets. 

As a result, crewmembers met 

their recommended energy 

intake requirements. It is 

difficult to determine whether 

the intakes on Skylab were 

related more to the requirement 

to consume the food or to 

the fact that the food was 

more palatable because of 

the additional variety available 

with frozen foods; however, 

increased palatability is 

obviously beneficial. 

The International Space Station in different space programs. Data are expressed as 

has accommodated 4- to percentage of energy requirements predicted by the 

6-month missions dating back World Health Organization (17) and are mean ± SD. 

Apollo N = 33, Skylab N = 9, Shuttle N = 32, Mir N = 7, to 2000. During this time, many ISS E1-18 N = 26, E19-36 N =31. E = expedition. aspects of these missions Apollo and Skylab data are from Bourland et al (32). have evolved and new exercise Figure is adapted from earlier publications (2, 33), equipment, reformulations of with additional published data included (6, 31, 34). 

Figure 13. In-flight dietary intake of crewmembers 
9

many space food items, and international foods from all partner agencies have debuted. 

These factors, coupled with the passage of lessons learned from one crew to the next, 

may have been responsible for our observation that many ISS crewmembers now 

consume recommended dietary intakes of energy, and also maintain body mass (6, 31). 

In cases where energy intakes do not meet requirements, absent definitive causes, 

many potential explanations have been proposed (8, 23, 35). Anecdotal reports 

of appetite vary significantly, as indicated in a Russian study in which 40% of Mir 

crewmembers reported decreased appetite, 40% reported no change, and 20% 

reported increased appetite (36). 

Food palatability is occasionally reported as a cause of reduced in-flight intake, and 

many anecdotal reports exist of changes in taste and aroma of food during flight 

(37-39). One hypothesis is that fluid shifts and congestion associated with the first 

days of microgravity can alter taste and odor perception. Other possibilities exist as 

well, including effects of atmospheric contaminants, stress, radiation, and psychological 

factors (37). Experimental research has not been able to clearly document changes in 

taste or olfaction during space flight or head-down-tilt bed rest (37, 40, 41), but it is 

hoped that ongoing ground-based and in-flight research on ISS will provide clarity. 

When tongue taste perception was measured before, during, and after a 30-day 

–6° head-down bed rest period, subjects reported decreased appetite and lack of 

taste early in the bed rest phase (41, 42). By day 13 of the bed rest phase, for all tastes 

(sweet, salt, acidic, bitter), the threshold for taste sensitivity had increased. In contrast, 

a bed rest study in the 1990s found no changes in odor or taste perception after 14 days 

of head-down bed rest (43), suggesting that multiple factors are likely involved in this 

process. Additional studies of taste and smell changes during bed rest are ongoing, 

and will help expand the evidence base for this area. 

Flight-related changes in gastrointestinal function may also occur. Fluid shifts, in 

combination with reduced fluid intake, would tend to decrease gastrointestinal motility. 

Gastrointestinal transit time has not been systematically studied during flight, but during 

10 days of –6° head-down bed rest, mouth-to-cecum transit time was significantly 

longer than it was during ambulatory control periods (44). However, because the Skylab 

astronauts and others were able to maintain a eucaloric diet in space, hypotheses about 

inability to consume the requisite amount of food because of stomach fullness or other 

factors are not likely to fully explain decreased dietary intake during flight. Russian 

studies of gastrointestinal function during actual and simulated space flight, in humans 

and in animal models, have previously been reviewed (45). A common cause of reduced 

dietary intake during the first days of a mission (46) is space motion sickness (39, 46-49). 

The effects of space motion sickness typically pass after the first several days of flight, 

but the decreased dietary intake can extend well beyond the first week (35). 

Implications for Inadequate Energy Intake 

The obvious and immediate reason for concern about reduced dietary intake is the risk 

of body mass loss, and more specifically, loss of lean mass and bone tissue. Body mass 

losses of 1% to 5% of preflight body mass have been a typical finding in the history 

of space flight, although some crewmembers have been able to maintain body mass 

(6, 31, 50). In-flight and postflight losses of body mass are compiled in Figure 14 and 

Figure 15. Documented weight losses have occurred on short- and long-duration flights 

in both the US and Russian space programs (23, 51-53). Indeed, all crewmembers 

on Gemini, Apollo, Skylab, and Apollo-Soyuz Test Project missions lost body mass 
10

(54); thus, ingestion of 

the prescribed energy 

intake on the US Skylab 

missions did not ensure 

maintenance of body 

mass (28). In one study 

of 13 male Shuttle 

crewmembers, body 

mass losses ranged from 

0 to 3.9 kg (11). Body 

mass loss has been 

observed to reach 10% 

to 15% of preflight body 

mass (55). Crewmembers 

on ISS have shown 

similar patterns of mass 

loss during and after flight. Figure 14. In-flight body mass measurement data from 

55 ISS crewmembers. Pre = preflight, FDx = flight day x, 

R+x = x days after landing. Data are expressed as percent 

change from preflight values and are mean ± SD. Figure 

and data adapted from Zwart et al (7). 

Figure 15. Changes in body weight on the day of landing relative to before flight. Data are 

expressed as percent change from preflight values. Each symbol represents 1 crewmember from 

a mission on the Space Shuttle (green, N=25), Skylab (red, N=9), Mir (blue, N=19), ISS Expeditions 

1-18 (dark purple, N=26), or Expeditions 19-36 (light purple, N=31). Duration data have been 

adjusted slightly to ensure anonymity. Data updated from initial publications (2, 56). 

Data from Apollo missions clearly document the relationship between energy intake 

and weight loss (Figure 16). Data relating reduced dietary intake during semi-starvation 

to loss of body mass were collected in 2 ground-based studies, not related to space 

flight. In the first study (57), subjects who consumed 580 kcal/d lost 7% of their body 

mass in 12 days and subjects who consumed 1010 kcal/d lost 11% of their body mass 

in 24 days. In the other study, starved subjects lost 9% of their body mass after 11 days, 

15% by day 18, and 18% by day 43 (58). 
11

Figure 16. Relationship between energy intake (kcal/kg 

body mass/d) and weight loss (change in body mass/d, 

kg) during Apollo missions. N = 33. Data are courtesy 

of William Carpentier. 

Only about 1% of the loss of 

body mass can be explained 

by loss of body water (19); 

most of the observed loss 

of body mass is accounted 

for by loss of muscle and 

fat tissue (10, 59). The water 

loss may be confounded by 

lean tissue loss, as metabolic 

water loss will be associated 

with depletion of glycogen 

stores and protein catabolism, 

both of which occur with 

inadequate intake. Inadequate 

energy intake is associated 

not only with loss of fat tissue, 

but also with decreased 

protein synthesis (60) (during 

space flight), increased protein 

catabolism (61) (during bed 

rest), and subsequent loss 

of lean tissue mass. 

Besides the obvious concerns about body mass loss and dehydration (62), existing 

data suggest that many systems are affected by inadequate nutrient intake, including 

the muscle, bone, cardiovascular, and immune systems. The German Institute of 

Aerospace Medicine at the German Aerospace Center conducted a study jointly with 

the European Space Agency (ESA) to evaluate the impact of hypocaloric nutrition on 

multiple systems. A crossover design was used, with hypocaloric and eucaloric phases, 

and bed rest and ambulatory phases. Data from this study document the fact that 

undernutrition exacerbates the negative effects of bed rest on human physiology (61). 

Undernutrition has also been found to impair cardiovascular performance (orthostatic 

tolerance) in controlled bed rest settings (63) and after space flight (William Carpentier, 

personal communication; see additional information in section 6, “Cardiovascular 

Health”). The mechanism for this energy-cardiovascular connection has been 

hypothesized to involve multiple functions of many endocrine factors, including insulin, 

leptin, and growth hormone (64). 

Anecdotal reports from crewmembers on long-duration missions indicate that 

crewmembers who had lost a significant amount of body mass on orbit had an excess 

amount of rebound weight gain after landing, but in general, the data do not support 

this finding (2). 

It is imperative that adequate resources be provided to support food consumption on 

exploration missions. A reliable food system must include a variety of palatable foods and 

the means (such as rehydration, heating, and cooling) to process them for consumption. 

Time (for meal preparation, consumption, and cleanup) is another limited resource that 

often hinders dietary intake during space flight. The use of freezers and refrigerators for 

food storage and preparation would provide a more palatable food system, which would 

increase dietary intake as well as provide added psychological support. 

Deficiency of dietary energy intake leads to wasting and ultimately tissue breakdown, 

or even death. The loss of lean body mass during space flight is significant, and is 
12

associated with increased proteolysis and catabolism related to metabolic stress (65). 

Inadequate energy intake can also have negative effects on bone, exacerbated by 

exercise (66, 67). This highlights the interaction between systems, and the fact that 

exercise regimens must be coordinated with energy provision. 

It is difficult to predict the impact of suboptimal (or lack of) energy intake on otherwise 

healthy individuals. One issue is that the energy equivalent of the lost mass changes 

with time, as different body fuels are used at different times during semi-starvation 

(57, 68). With partial rations available (1000 cal/d), it is reasonable to expect that a 

person could survive for more than 4 to 6 months, potentially longer if the metabolic 

rate were to decrease because of decreased intake. If energy availability were restricted 

further, survivability would range between this amount of time and the 1 to 2 months 

possible with no food. These projections obviously include many assumptions, 

unknowns, and extrapolations. Data from 10 Irish Republican Army hunger strikers, 

who consumed water ad libitum but no energy, vitamins, or minerals, indicate that an 

average 25-year-old male could survive no longer than 60 days without energy (69, 70). 

Other possible effects of long-term low intake of calories include decreased motor and 

cognitive function, each of which could impair an astronaut’s ability to perform workrelated tasks necessary for landing. According to military survival studies, astronauts 

would be expected to experience decreased endurance early on, and the decrease 

in strength would parallel the decrease in lean body mass (71). During total fasting, 

degradation of coordination, speed, and cognitive function would be evident within 

the first 2 weeks (71). 

The metabolic condition of ketosis, which would be expected to result from starvation, 

not only would have metabolic effects (including decreased appetite), but might also 

affect other aspects of the mission (for example, the life-support systems might not 

be able to remove the ketones from the air). Ketoacidosis can obviously have negative 

effects on acid-base balance, which in turn can affect bone, muscle, and other systems. 

It is speculated that a crew could survive on a spacecraft or planetary base for 

40 to 60 days without food. With limited rations (1000 cal/d), a crew could survive 

4 to 6 months (although physical performance capability might be severely degraded). 

The high-stress environment of a contingency during transit or on a planetary surface 

would likely exacerbate the basic effects of limited rations, and would shorten 

projections of survivability estimated from ground-based studies. 

Insufficient dietary intake and subsequent loss of body mass are significant not only 

for crew health but also for medical operations and research studies, in which clear 

interpretation of essentially all other physiological data is impossible when subjects are 

malnourished. That is to say that virtually all space flight data collected on Shuttle, Mir, 

and many ISS missions are confounded by inadequate dietary intake. Investigators who 

have studied bone and muscle, cardiovascular function, immune response, and other 

systems during space flight cannot say to what degree undernutrition affected their 

findings. 

While research may be warranted to better understand why astronauts typically 

do not consume 100% of their recommended intakes, recent data from ISS 

crewmembers clearly document that intakes can be met during space flight (31). 

In addition to maintaining energy intake and vitamin D status, in conjunction with 

exercise, these crewmembers maintained body mass; came home leaner, with less 

fat; and maintained bone mineral density at preflight levels (31). Additional details 

are provided in section 4, “Bone.” 
13

Studies of energy expenditure have been conducted only on short-duration (Shuttle) 

flights (11, 12). Whether the same trends continue on longer flights is not known 

(an ESA-sponsored study of energy expenditure on ISS missions is currently in 

development). The health implications of decreased energy expenditure need to 

be determined, and ways to prevent both in-flight loss and postflight gain of body 

mass need to be evaluated. 

At least 2 approaches exist to controlling body mass and composition while studying 

human adaptation to bed rest: maintaining body mass (as is typically done in the US) 

or allowing subjects to lose total mass while keeping fat mass constant (and thus losing 

lean tissue). While this latter approach sounds intriguing, implementing it has proven 

very difficult, given the difficulties in measuring fat mass and adapting intake in a timely 

manner. Nonetheless, Biolo and colleagues have recently reported data suggesting that 

the more the fat mass increases during bed rest, the more lean tissue is lost, and that 

this loss is confounded by increases in oxidative and inflammatory damage markers (72). 

Altered fuel homeostasis has been documented in other bed rest studies (73) and animal 

studies (74, 75), but remains to be fully elucidated, in bed rest or space flight (75, 76). 

Carbohydrate 

Carbohydrates play an important role in the body because they supply the primary 

source of energy as well as a readily available source. This energy is oxidized and used 

by various organs and cells in the body, particularly the brain and red blood cells, which 

depend solely on carbohydrate for energy. The human body stores about 150 to 500 g 

of carbohydrate as glycogen, in the liver and skeletal muscle (77). Most of the body’s 

glycogen is in skeletal muscle. Muscle glycogen stores are used mainly by muscle, 

whereas the smaller glycogen stores in the liver are used to maintain, store, and export 

blood glucose. Glycogen stores, especially those in the liver, fluctuate greatly during the 

day in response to food intake, and these fluctuations may be involved in the regulation 

of food intake (78). Liver stores of glycogen are depleted after 12 to 18 hours of fasting 

(77). In skeletal muscle, glycogen synthesis is triggered by a rise in insulin after the 

consumption of carbohydrates. De novo synthesis of glucose from non-carbohydrate 

precursors can and does occur in the body, if needed. This allows the liver to maintain 

adequate blood glucose concentrations. Insulin is required for the uptake of glucose 

into cells, and various transporter systems are found in different types of tissues that 

utilize glucose. 

Requirements for carbohydrate in space are thought to be similar to those on Earth. 

However, to date, few investigations have been conducted on the effects of microgravity 

on the metabolism of dietary carbohydrate, and those studies have had conflicting 

results. 

On the Shuttle, studies by German investigators showed no impact of 7 days of flight 

on glucose tolerance tests (79). A Russian study documented a reduction in fasting 

plasma glucose after 60 or 88 days of flight on a Salyut-Soyuz spacecraft complex, 

and a reduced peak of blood glucose in glucose tolerance tests (80, 81). Insulin 

resistance (lack of sensitivity to insulin) has been found to result from simulated 

weightlessness (bed rest) (73, 82-84). Using C-peptide excretion as a proxy, Stein 

et al found evidence of insulin resistance during actual and simulated space flight (85). 

Heer et al. documented that after 3 weeks of bed rest, glucose tolerance was altered 

for more than 4 days after reambulation (378). Efforts to maintain muscle mass 

(and presumably correct the insulin resistance) continue, but little research has 

been done to pursue this as a nutritional issue. 
14

Suboptimal carbohydrate intake before and during space flight may have consequences 

for the crew’s productivity and impede their ability to respond in emergency situations 

(86). Deficiency of carbohydrate leads to ketosis. A ketotic state would likely impair 

performance of crewmembers, as seen in studies conducted by the military (71), as 

well as increase renal stone risk secondary to reduced urinary pH (87-89). Other aspects 

of the mission would also be at risk (for example, the life-support systems may not be 

able to remove exhaled ketones from the air). Toxicity of carbohydrate has not been 

well studied, and would likely be an issue only because it would displace other nutrients 

(protein and fat) from the diet. 

Few data are currently available to assess the impact of space flight on carbohydrate 

metabolism. Observations from space flight as well as ground-based bed rest studies 

show subtle changes in insulin secretion, insulin resistance, and glucose intolerance 

(83, 84, 90, 91). Even subtle changes in such important metabolic processes make 

it critically important to consider the likelihood, nature, and consequences of altered 

carbohydrate and insulin metabolism for exploration missions. 

Fat (and Fatty Acids) 

Fat is the most energy-dense of all the nutrients, and therefore is a major energy 

source for the body. Chemically, dietary fat is mainly in the form of triacylglycerols, 

which contain a glycerol backbone with as many as 3 fatty acids attached. Many types 

of fatty acids exist, including saturated, monounsaturated, polyunsaturated, and trans. 

Dietary fat assists in the absorption of fat-soluble vitamins and supplies the body with 

the 2 essential fatty acids, linoleic acid and linolenic acid. These essential fatty acids 

are necessary for growth and development as well as many other biochemical 

processes, including production of eicosanoids (physiologically active substances 

derived from arachidonic acid). Lipids, in the form of phospholipids, make up a large 

proportion of the structural components of the cellular membrane bilayer. Energy stored 

as fat is released in the process of fatty acid oxidation, and fat supplies more energy 

than any other macronutrient because of its higher content of carbon-to-hydrogen 

bonds. According to case studies, people following fat-free diets can exhibit symptoms 

of essential fatty acid deficiencies after only 1 month (92, 93). 

While few, if any, studies have been conducted to look at dietary fat, plasma lipid 

levels, and related factors in space flight, voluminous data exist from routine medical 

examinations conducted before and after flight, along with annual medical exams, 

as reviewed previously (2). Contrary to the typical lipoprotein response to weight 

loss, low-density lipoprotein concentrations tended to increase in long-duration 

crewmembers who lost weight during the flight. This relationship seemed to return 

to normal by the subsequent medical exam (2). 

Alterations in fuel homeostasis and regulatory hormones have been noted in space 

flight and ground-based studies. Bed rest studies have documented alterations in fuel 

homeostasis (94), including gender differences (73). Specifically, lipogenesis increased 

during bed rest, to a greater extent in women than in men. Additionally, men had 

increased carbohydrate oxidation (73). Other studies have shown inflammatory changes 

during bed rest, along with insulin resistance, leading to increased body fat and altered 

fatty acid metabolism (95). Given these data, and the insulin, leptin, and other endocrine 

changes noted in bed rest and space flight (15, 96, 97), changes in fuel homeostasis in 

bed rest clearly warrant additional investigation. 
15

The role of omega-3 fatty acids in cancer prevention has been investigated in animal 

models of space flight radiation effects (98, 99). Not only do omega-3 fatty acids 

(in combination with pectin) show promise in alleviating cancer risk (98-102), but these 

fatty acids also have well-documented cardiovascular benefits. Abundant data show 

that eicosapentaenoic acid can successfully prevent muscle atrophy in other musclewasting circumstances, such as cancer or sepsis (103-112), indicating the likelihood 

is high that eicosapentaenoic acid will have the same beneficial effects on muscle 

atrophy during space flight or in ground-based analogs including bed rest. 

Omega-3 fatty acids also have been shown to protect bone, in the general population 

(113-115) as well as in space flight analog studies, including bed rest and cell culture 

(116). Although omega-3 fatty acids have not been studied in a controlled fashion during 

actual space flight, a positive correlation was found between fish intake and bone loss 

in astronauts (116). That 

is, those who ate more fish 

lost less bone (Figure 17). 

These data provide additional 

evidence of the potential 

importance of fish oils as a 

countermeasure for muscle, 

bone, and radiation risks of 

space flight. Studies showing 

positive effects of omega-3 

fatty acids typically look at 

intake of fish or other food 

sources of these nutrients 

(117-119). Studies of fish oil 

supplements added to typical 

diets often fail to document 

any benefit (120-122), thus 

highlighting the need for 

dietary modification, and not 

simply supplementation. 

Cofactors in Energy Metabolism 

Many micronutrients—vitamins and minerals—are involved in energy homeostasis, 

including thiamin, riboflavin, niacin, pantothenic acid, iodine, manganese, and even 

chromium. We previously reviewed sources, functions, deficiency symptoms, and 

concerns for space flight for these nutrients (2). In general, few data are available 

on these nutrients with respect to effects of space flight, or on availability and stability 

of these nutrients in the space food system. No specific concerns have been raised at 

this point, but ensuring adequacy of intake of these nutrients and understanding their 

metabolism, human requirements for them under space flight conditions, and their 

stability in the food system will be critical on exploration missions. Additionally, their 

nutritional status should continue to be monitored as we begin to embark on longer 

missions, including the initial 1-year mission on the International Space Station 

planned for 2015. 

Figure 17. Relationship between fish intake during longduration flight and whole-body bone mineral density loss 

after flight on ISS. Figure adapted from (116). 
16

Astronaut Sunita L. Williams, Expedition 14 flight engineer, 

uses chopsticks to eat a meal near the galley in the Zvezda 

Service Module of the International Space Station. 
17

As the major structural component of all cells in the body, protein includes molecules 

that perform many essential physiological functions, serving as enzymes, hormones, 

transporters, and other important molecules. The total energy contribution of protein 

to the average diet is about 15%. The nitrogen in its amino-acid building blocks makes 

protein, along with nucleic acids, one of the major nitrogen-containing macromolecules. 

Protein is one of the most important limiting factors when the body is deprived of 

energy, because essential amino acids are not stored in the body. A complete depletion 

of energy and protein reserves is said to be the cause of death from starvation. It is 

estimated that when 33% to 50% of total body protein is lost, death results (123). 

Loss of more than 40% to 50% of initial body mass is not compatible with life (71, 124). 

In one case report, individuals on a hunger strike lost 30% of their total body mass and 

19% of total body protein before they died (69, 70). 

Protein Intake 

Maintaining a proper protein intake is vital, as both low-protein and high-protein 

diets can cause harm (and, at the extreme, death). A low-protein diet (below the 

recommended dietary allowance) for up to 4 weeks can decrease calcium absorption 

and cause increased secretion of parathyroid hormone in otherwise healthy subjects 

(125, 126). The impact of chronically low protein intake is not well understood; 

however, several studies suggest that low-protein diets are associated with loss 

of bone density (127, 128). 

Actual intakes of protein 

during space flight 

typically exceed these 

recommendations (2), 

as shown in the table 

"Nutrient Intake Data" in 

the Appendices (page 134). 

European studies have 

shown that on long missions, 

reaching (or exceeding) 

3 Muscle and Protein 

nominal protein intakes is 

common, but that on short 

flights (Shuttle missions) 

protein intake is less than 

the recommended amount 

because of insufficient 

food intake (21). On ISS 

missions, on average, 

protein intake is more 

than adequate (Figure 18). 

Figure 18. Protein intake during space flight on ISS 

missions (N=56). Each point represents an individual 

crewmember, and is their reported average intake over the 

course of the mission. Dashed lines represent space flight 

(and ground-based) protein intake requirements of 0.8 g 

protein/kg body mass, and the range of protein intakes 

(1.2.-1.7 g/kg) recommended by the American Dietetic 

Association, Dietitians of Canada, and American College 

of Sports Medicine for high-intensity athletes (130). 
 

18

Some data suggest that during the recovery period after short-duration Shuttle 

flights, protein was a limiting nutrient, and that competition for substrate to replenish 

plasma proteins and muscle mass strains the system (129). This has not been 

tested experimentally, but it is clear that good nutrition is required for rapid return 

to optimal health. 

Vitamin B6 

Vitamin B6 comprises a group of 3 compounds and their 5-phosphates (P): pyridoxal 

(PL) and PLP, pyridoxine (PN) and PNP, and pyridoxamine (PM) and PMP (131). We 

previously reviewed vitamin B6 basics (2). Weightlessness has been shown to reduce 

the cross-sectional area of muscle fibers and is associated with a change from type I 

to type II muscle fibers (132). As vitamin B6 is stored mainly in muscle tissue (133), 

a decrease in muscle cross-sectional area could reduce the amount of the vitamin 

that is stored. Increased excretion of 4-pyridoxic acid (4-PA) during bed rest, a finding 

observed in short- (134) and long-duration bed rest studies (135), likely reflects this 

loss of muscle stores of vitamin B6. 

Given the changes observed in vitamin B6 metabolism during bed rest, vitamin B6 

status during and after long-duration space flight warrants further attention. A deficiency 

in vitamin B6 causes a decrease in the synthesis of serotonin and catecholamines, which 

has been shown to be associated with depression (136). Excess vitamin B6 can lead to 

neuropathy (137-139). 

There is no evidence that vitamin B6 status changes during long-duration space flight; 

however, we have shown that crewmembers who have experienced vision-related 

issues have higher blood concentrations of cystathionine and 2-methylcitric acid than 

crewmembers who did not experience such issues (140). Although increased levels 

of these metabolites do not point to a vitamin B6 deficiency, they do suggest there may 

be a perturbation in the 1-carbon metabolism pathway, which is dependent on folate, 

vitamin B12, and vitamin B6. 

Muscle Loss and Protein Turnover in Microgravity 

Exposure to microgravity reduces muscle mass, volume, and performance, especially 

in the legs, on both short (28, 141-146) and long flights (28, 141, 147-151). 

As with most physiological systems, a variety of techniques are used to quantify 

changes in different aspects of muscle. These include functional muscle performance/ 

exercise tests, muscle biopsies to evaluate cellular changes, magnetic resonance 

imaging (MRI) of muscle tissue, and tracer kinetic studies to evaluate changes in protein 

metabolism. Each technique provides a different perspective and unique information. 

The picture becomes more complex when results from models and analogs (including 

nonhuman ones) are compared with those from space flight. As a result, the topic 

of space flight effects on muscle is very complex, and defining and understanding 

the big picture is difficult, to say the least. This topic has been reviewed (142, 152-156), 

and the difficulties of interpreting findings across the literature have also been noted 

(146, 152, 155). 

Basic clinical testing includes determinations of circulating proteins. Blood 

concentrations of total protein and albumin were elevated at landing after Skylab 

missions. Urinary albumin has been shown to be reduced during space flight and 

bed rest (157-159). Measurements of urinary albumin excretion, which is typically 
19

low in healthy individuals, have not been reported after landing. Potassium and nitrogen 

balances became increasingly negative throughout the Skylab flights, but urinary 

creatinine (a measure of muscle mass) did not change (29, 160) despite losses of leg 

volume (28, 161). Nitrogen balance has also been shown to be negative during Shuttle 

flights (162). 

Evaluation of plasma and urinary amino acid levels often does not provide a clear picture 

of muscle or even protein metabolism, but in some cases, these are among the limited 

available data. An increase in plasma amino acids was noted in cosmonauts after short 

(2-d) and long-duration (63-d) flights (163, 164). Limited Shuttle flight data indicate a 

tendency for plasma branched-chain amino acids to be increased during flight, relative 

to preflight levels (165). Data from short-duration Shuttle flights have revealed little 

or no change in urinary amino acid profiles (26), but data from the Apollo and Skylab 

missions showed increases in urinary excretion of the amino acid metabolites creatinine, 

sarcosine, and 3-methylhistidine (166), suggesting that contractile proteins of skeletal 

muscle are degraded in weightlessness. 

The amount of protein in the body or individual tissues is affected by the balance 

of protein synthesis and protein catabolism. Studies to understand changes in body 

protein, ideally, will include measurement of both of these factors in addition to turnover. 

Measuring these factors is not easy, and the results are variable (151, 167), but having 

the complete picture is critical. For example, although both decreased protein synthesis 

and increased protein catabolism will yield a net loss of muscle, the mechanisms and 

countermeasures for the 2 processes are quite different. 

Turnover studies with stable isotopes indicate that during short-term space flight, 

whole-body protein turnover and protein synthesis increase, and a greater percentage 

increase occurs in protein breakdown (162, 168). The increase in synthesis is 

hypothesized by Stein et al (156, 169) to be related to physiological stress, as indicated 

by generally (but not consistently) increased urinary cortisol during flight (15, 19, 97). 

These findings are similar to those in catabolic patients undergoing metabolic 

breakdown. Decreased prostaglandin secretion has also been implicated in the loss 

of muscle tissue during space flight, secondary to decreased mechanical stress on 

muscle (97). On long-duration Mir flights, conversely, investigators noted decreased 

rates of protein synthesis (12), secondary to reduced dietary energy intake (60). 

Ground Analog Studies 

In humans, bed rest is the most common model used for studying changes in muscle 

and protein during disuse. Some studies have also used unilateral limb suspension 

(ULLS) as a model for these changes, allowing subjects more freedom, while significantly 

reducing the costs of such studies. Other human models have also been tested, 

including “dry immersion,” as reviewed by Navasiolava et al (170). All of these models 

provide a means to collect data that would be difficult if not impossible to collect during 

actual space flight, on a larger group of subjects. However, it is important to remember 

that the model system may not, and likely does not, provide an exact replica of the 

physiological changes during space flight. Untreated bed rest and ULLS both result 

in loss of muscle mass and strength, as nicely reviewed by Narici and de Boer (152). 

As mentioned earlier, muscle loss in space may be related to changes in turnover 

of protein in the whole body. Many studies document a decrease in protein synthesis 

during bed rest (171-178). Evidence of increased protein catabolism during bed rest 

is limited, as reviewed by Bodine (155). There are 2 key confounding, and intertwined, 

issues (among others) with this area of research: energy intake and inflammation. 
20

Energy consumption during space flight is generally not controlled; that is, astronauts 

are allowed to consume as much, or as little, as they desire. Historically, this lack of 

control has resulted in underconsumption, as described in section 2, “Energy and Fuel 

Metabolism.” The prospect of comparing the controllable ground-analog model with 

actual space flight, in which underconsumption can occur, presents a study design 

issue. Investigators typically either seek to maintain energy intake and body mass of 

the subjects during bed rest, or restrict calories to recreate the hypocaloric intakes 

of space flight. Other studies allow subjects to regulate energy intake voluntarily. 

It should be noted that, because energy expenditure decreases (about 15%) during 

bed rest secondary to decreased activity (14, 179), essentially all studies reduce energy 

intake during bed rest to some degree. Furthermore, because protein is an energycontaining nutrient, it is impossible to alter caloric intake without altering either protein 

intake or the relative contribution of protein to overall energy intake. This becomes 

critical in nutritional countermeasure studies, in which provision of additional protein 

and/or amino acids is often a focus. 

Restricting energy consumption to match earlier findings from space flight concedes 

the point that astronauts may be able to maintain dietary energy intake during flight. 

This was observed during Skylab and EuroMir missions, albeit in a controlled fashion 

(ie, these crewmembers were participating in metabolic studies and were required to 

consume the planned menu) (180, 181). More recently on ISS, crewmembers have been 

able to maintain energy intake at >90% of predicted requirements. Coupling this energy 

intake with heavy resistance and other forms of exercise, crewmembers maintained 

body mass, returned with an increased percentage of lean body mass, and maintained 

bone mineral density (31). This study is described in more detail later in this book, but 

it is mentioned here because it documents that crewmembers can consume adequate 

energy intake without their diet being strictly controlled. 

Inadequate caloric intake results in 2 significant effects on protein and muscle. Initially, 

breakdown of glycogen in muscle will reduce muscle volume, in large part because of 

the water content associated with stored glycogen. Second, the body will catabolize 

protein for energy. Thus, studies of hypocaloric subjects will inherently be confounded 

by this situation. As detailed below, provision of supplemental protein or amino acids is 

an oft-tested countermeasure for muscle loss, but it is plausible that provision of energy 

in any form will help maintain lean (and other) tissues in this metabolic environment. 

Muscle loss has been observed in bed rest and other analog studies, even when energy 

intake and body mass are maintained. It has been argued that in individuals who lose 

muscle under those conditions, maintenance of body mass during bed rest leads to 

a chronic, low-level inflammatory response, which can subsequently lead to increased 

fat mass along with muscle loss (72, 182, 183). This argument has led to attempts to 

reduce caloric intake to the point where fat mass is not changing (although body mass 

decreases, related to loss of muscle tissue). This is a difficult prospect at best, given 

that body composition is measured (typically by dual-energy x-ray absorptiometry) 

infrequently (eg, every 2 wk), and thus any corrections lag behind, or may overshoot, 

before the next determination. 

The ideal answer for how to control body (or fat) mass during bed rest remains under 

debate. Nonetheless, as with any scientific study, it is critical to know the limitations 

and implications of every study design and, if possible, to relate studies with these 

limitations to other studies in the literature. One necessity for making the findings 

of such studies useful to readers of scientific papers is body mass data, which we 

suggest should be reported in every published bed rest study. 
21

Beyond the controllable factors, other issues exist that can affect bed rest study results. 

Variability in stress levels can be a key factor for many physiological systems, but 

particularly muscle. An increase in stress level, as indicated by increased concentrations 

of cortisol in blood plasma and urine, is typically associated with the initial days of 

space flight. In many cases, urinary cortisol excretion returns to preflight levels after 

5 to 9 days, although this phenomenon has yet to be fully characterized or generalized 

to all crews. Ground-based studies have the potential for stress to increase, but this 

is not an entirely consistent finding. Some studies have shown no change, or even 

a downward trend, in cortisol excretion during bed rest (184). As seen with studies 

of energy metabolism, administration of exogenous cortisol or thyroid hormone induces 

metabolic stress, which may produce a more accurate ground-based model of protein 

metabolism during space flight (16, 185-187). Ground-based rodent studies generally 

show increased proteolysis along with reduced protein synthesis (151), a pattern similar 

to that seen in studies of humans during flight, described above. 

Muscle Loss Countermeasures 

Mechanical 

Exercise is the most common first-pass approach to maintaining muscle mass and 

strength (142, 154, 188-190). The exercise regimens tested as countermeasures to 

date have generally not succeeded in maintaining muscle mass or strength (or bone 

mass) during space flight (190). On Mir flights, crewmembers differed significantly with 

respect to in-flight exercise frequency and intensity (related to such factors as mission 

requirements and personal habits). However, losses of leg muscle volume, detected 

immediately after flight by magnetic resonance imaging, were almost 20% in all subjects 

(191). Similar findings (wide variations in exercise, lack of difference in bone loss) have 

also been documented for bone loss (55). 

Many types of exercise protocols have been proposed to aid in the maintenance of 

both muscle and bone during flight (175, 192-203), but these have yet to be fully tested 

on orbit. Given that ISS crews use multiple exercise devices, studies ideally will look 

at exercise protocols that use more than 1 device. Combined resistance and aerobic 

exercise protocols have shown promise for protecting muscle (and the cardiovascular 

system) in bed rest (204, 205); space flight testing is underway. 

Whole-body vibration alone or superimposed on resistive exercise is a rather new 

concept and has received much attention recently in the hope that it can provide 

a viable musculoskeletal countermeasure, particularly when strenuous exercise might 

increase the risk for injury, for instance in older people (206-213). However, the efficacy, 

vibration dose, frequency, and duration of whole-body vibration exercise need further 

research. As mentioned earlier, mechanical stimulation is a prerequisite to avoid muscle 

and bone degradation. According to the mechanostat theory by Frost (214, 215), to 

keep up muscle and bone mass and strength, a certain individual level of mechanical 

stimulation has to be achieved. Lowering or increasing that level leads to respective 

degradation or increase. For whole-body vibration training, the vibration magnitude 

seems to be one of the key factors. Vibration magnitude is vibration frequency (Hz) 

times the amplitude or displacement (mm) (211). This is most likely the reason why 

mere whole-body vibration training to the musculoskeletal system with a frequency 

of 20 Hz has not always been demonstrated to be effective for muscle and bone (216). 

Superimposing whole-body vibration training on resistive exercise seemed to be more 

effective. This was done in 2 bed rest studies with durations of 56 and 60 days and 
22

young, healthy male subjects (210, 213). When whole-body vibration training was added 

to resistive exercise, it was able to attenuate muscle and bone atrophy and lumbar spine 

deconditioning, and to prevent fat accumulation in vertebral marrow (210, 217-220). 

Pharmacological 

Testosterone has also been suggested as a muscle (and/or bone) loss countermeasure, 

on the basis of data from animal models (221, 222) and data documenting a reduction 

in testosterone concentrations during flight in humans, animals, and cellular models of 

space flight (223-230). A potential confounding factor is the drop in testosterone that 

has been observed in exercising bed rest subjects (but not controls) (231). 

The first in-flight testosterone data from human space flight were from 4 astronauts on 

Space Shuttle mission STS-55, which flew in 1993, and from 3 astronauts on Skylab 4, 

an 84-day mission (232). On the Shuttle mission, after 4 or 5 days of flight, circulating 

testosterone levels were decreased relative to preflight levels, when measured in serum, 

saliva, and urine. Serum cortisol, cortisol biorhythms, and dehydroepiandrosteronesulfate concentrations in these 4 astronauts were unchanged during flight (228, 233). 

A significant confounding issue is that these crewmembers were consuming only about 

60-85% of their basal metabolic energy requirements during the flight (21). Estimates 

of space flight energy requirements typically use an activity factor of 1.7 (that is, 

1.7 x basal metabolic rate) (2). This factor is based on data documenting that total 

energy requirements are unchanged during flight (11), or in some cases are even 

increased with heavy exercise (12), relative to before flight. Even if lower estimates of 

activity were used, the result would reveal significant energy deficit in crewmembers 

on the STS-55 mission, especially during the days of sample collections (21). Indeed, 

energy intake, which was very carefully documented on these missions, was below even 

basal requirements. Energy deficits, both short-term and long-term, are associated with 

lower circulating testosterone (free and total) (234-236). Thus, the discrepancy between 

the long-duration data presented here (Figure 19) and the earlier reports of effects 

observed during the first week of flight could be explained simply by inadequate 

energy intake. 

Recent data show 

that testosterone 

and related hormones 

are unchanged by 

real or simulated 

weightlessness, apart 

from transient effects 

on landing day (237). 

The Skylab data include 

urinary testosterone 

from 3 crewmembers; 

at 2 in-flight data points, 

excretion was increased 

relative to the preflight 

period (232, 237). 

Plasma data from 

the 3 Skylab missions 

(N=9) are reported to 

have shown 

Figure 19. Total serum testosterone concentrations (mean ± SD) 

before (L-x), during (FDx), and after flight on ISS. Although 

circulating concentrations decreased significantly after flight 

(at R+0), no other time point differed significantly from the 

preflight mean. N=15. Data are from Smith et al (237). 
23

“a trend toward lower values after the mission” (232). Although we do not have urinary 

testosterone data on all crewmembers, these reports from the 1970s confirm the 

findings from ISS (237). 

Bed rest analog data from a number of studies demonstrate that bed rest has no effect 

on circulating testosterone concentrations (174, 231, 237-239). Consistent decreases 

in serum testosterone were observed after subjects had been in the bed rest facility 

for 7 days (while they were still ambulatory) and then another decrease occurred when 

testosterone was measured 5 days after re-ambulation. The pre-bed rest change is likely 

related to stress and decreased ambulation while subjects were in the bed rest facility, 

and the post-bed rest change was probably related to body fluid shifts during and after 

bed rest. No changes in testosterone occurred during bed rest (237). 

Bed rest studies have generally shown no effect of bed rest on circulating total or 

free testosterone (174, 231, 237-239) in sedentary subjects. Bed rest subjects, in most 

models, are required to consume energy at a level to maintain body mass. If energy 

deficits are indeed part of the observed decrease in testosterone during the Shuttle 

flights previously reported, this may also explain the difference between those flight 

data and bed rest study data, reported herein and elsewhere. Although we showed an 

intermittent decrease in total and free testosterone in bed rest subjects with or without 

an artificial gravity 

(ie, centrifugation) 

protocol (134), this 

study had combined 

the 2 pre-bed rest 

collection sessions. 

When these sessions 

were analyzed separately 

because of our results 

in the later bed rest 

study, it turned out 

that testosterone 

concentrations were 

indeed higher only at 

the first data collection 

point (BR-10) than during 

or after bed rest (237) Figure 20. Total serum testosterone concentrations (mean (Figure 20). ± SD) in subjects before, during, and after bed rest with 

One criticism of (dashed line and open circles) or without (solid line and filled 

sedentary bed rest squares) an artificial gravity countermeasure. These data 

studies as an analog have been published (134), although the pre-bed rest data 

(BR-9, BR-1) were combined as an average in the original for space flight is that manuscript. In this figure, the 2 pre-bed rest data points are astronauts are not graphed separately, as reported in (237). sedentary, especially 

on long-duration 

missions, when they exercise extensively. Wade et al reported that 

in a 4-week study, bed rest subjects with intensive exercise protocols had lower nonfasting circulating plasma testosterone concentrations than non-exercising bed rested 

controls (231). They reported a small loss of (non-fasting) post-breakfast body mass 

(231, 240), and reported that caloric and liquid intakes were designed to maintain 

body mass. Despite the exercise, which was described as including an expenditure 

of 214 or 446 kcal/d (5 times a week), actual intakes in the exercise groups were only 

155 or 212 kcal/d greater than those of the no-exercise group (240). 
24

In an earlier 30-day study, Zorbas et al showed that serum testosterone decreased 

during bed rest only in trained subjects, whereas it did not change in untrained subjects 

during bed rest (241). Interestingly, when conditioned subjects were “hyperhydrated” 

by saline ingestion during bed rest, testosterone did not change relative to the pre-bed 

rest period. In a shorter, 3-day bed rest, no differences in plasma testosterone were 

observed before or after exercise, in typically untrained or trained individuals, either 

cyclists or weight trainers (242). The astronauts in the study reported herein were 

relatively fit before flight and exercised heavily during flight, using treadmill, cycle, 

and resistive exercise devices. Clearly, the interrelationship of energy balance, exercise, 

stress response, and endocrine function warrants further evaluation to better understand 

the adaptive responses to space flight. 

As reviewed by Tou (243), in studies of rats with sample collections after space 

flight, serum (244) and urinary (245) testosterone were generally decreased relative 

to the preflight period (246). Unfortunately, in-flight biological samples are typically 

not available, given the difficulties with collection procedures in the microgravity 

environment. These postflight conclusions are consistent with data reported on 

landing day after a short-duration space flight (237). In ground-based rodent models, 

short-duration (7-12 d) unloading generally results in reduced circulating testosterone 

concentrations and an associated loss of bone and muscle mass (221, 222, 247). 

Unloading of longer duration (6 wk) in rats resulted in impaired spermatogenesis, but 

had no effect on circulating testosterone concentrations (248). Similarly, the production 

of testosterone by rat testes after actual space flight is diminished, as is response to 

stimulation by luteinizing hormone (249). Contradicting these findings, another study 

showed no change in circulating testosterone in suspended rats after suspension for 

1 or 3 weeks, but an increase in testosterone of suspended animals after 8 weeks, 

despite reduced testicular weight (250). One critical confounding factor in the hindlimbsuspended rat model is that not all studies take (surgical) precautions to prevent 

ascension of the testicles into the abdominal cavity, which can significantly affect 

testosterone production, and interpretation of the study. Some, but not all, studies have 

accounted for this, and this limitation contributes to inconsistencies in the literature. 

Rotating cell culture vessels have also been used as an analog of weightlessness, 

with some limitations, as with all analogs. Cultured testicular fragments exposed to 

this environment, compared with static 1g cultures, have maintained cellular architecture 

and have increased both proliferation and testosterone secretion (251), but with altered 

testicular physiology (252), including impaired Leydig cell responsiveness to luteinizing 

hormone stimulation. Whether the lack of change in circulating testosterone observed 

in the studies reported herein obscured alterations in testicular physiology is unclear, 

but it seems imprudent to make that leap without additional data. 

Exogenous testosterone administration in humans during bed rest studies has 

maintained muscle mass and protein balance, but with no effect on muscle strength 

(239). Administration of testosterone to suspended rats mitigates muscle and bone 

losses (221). The bone data in rats are confounded by differential effects on growing 

and adult rats (253), but these results are of interest nonetheless. Testosterone 

administration to elderly individuals has shown that the bone response to testosterone 

depended on the initial circulating testosterone concentrations (254). That is, subjects 

who had normal blood concentrations of testosterone had less or no response to 

testosterone administration. Given these data, there is little rationale for providing 

testosterone during flight to mitigate bone loss. 
25

Hypergravity, induced by centrifugation, has been shown to result in increased urinary 

testosterone excretion in monkeys (225), as well as in rats (245, 247). Hypergravity 

has also been found to affect tissues of rats and some other endocrine variables, but 

increased gravitational force had no effect on circulating testosterone (244). On the basis 

of these data, authors have suggested that the response to gravity is roughly linear, from 

hypergravity (increased), to unit gravity, to microgravity (decreased) (225, 244). Intriguing 

as this concept may be, the data presented herein do not support it. 

As is understandable, the proposed use of exogenous steroids is somewhat 

controversial. Muscle physiologists argue that despite the lack of change in endogenous 

steroids, exogenous androgens may prove a viable countermeasure nonetheless. 

Treatments with such androgens have been reported to improve physical performance, 

muscle mass, and muscle strength in both young athletes and older sedentary men 

(255). The interaction of endocrine factors, aging (including middle age), the space 

flight environment, and the use of exercise to replace loading is not well understood. 

In summary, circulating testosterone and related hormones are unchanged by real or 

simulated weightlessness, apart from transient effects after flight. Nonetheless, as we 

contemplate space exploration beyond low Earth orbit, endocrine data will be critical 

for understanding human adaptation in this unique environment, and potentially for 

helping to counteract the negative effects of space flight on the human body. 

Nutritional 

Use of protein and amino acid supplementation has long been studied as a potential 

means to mitigate muscle loss associated with space flight (82, 187, 204, 256-260), but 

results have been inconclusive at best. Oral doses of branched-chain amino acids had 

little effect on leg-muscle protein kinetics in ambulatory male subjects (261), whereas 

feeding a bed rest group adequate energy with excess protein reversed nitrogen losses 

(176). However, feeding Skylab crewmen energy and protein equivalent to those given 

to the bed rest group did not prevent negative nitrogen balance and loss of leg muscle 

strength during flight (148, 160, 161). In another bed rest study, a leucine-enriched, 

high-protein diet failed to mitigate muscle loss, and in some sites exacerbated loss 

(258). It remains unclear whether nutritional measures beyond the consumption of 

adequate energy and protein would be beneficial in reducing muscle atrophy. 

In a 2011 review, Stein and Blanc evaluated the literature from bed rest studies (262), 

and found that the effect (or lack thereof) of amino acids on muscle depended greatly 

on protein intake and energy provision. Specifically, if nominal protein intake (ie, in both 

treatment and control groups) was at levels greater than 1.1 to 1.2 g protein/kg body 

mass/d, then supplemental amino acids had no effect. If control subjects were provided 

with ≤0.8 g protein/kg body mass/d while the supplemented group consumed >1.0 g 

protein/kg body mass/d, then the supplement appeared to have an effect. If further 

research is conducted in this area, the issue of study design, with particular reference 

to protein and energy intake of all groups, needs to be carefully assessed. The bottom 

line seems to be that if crewmembers consume enough protein and energy, then 

supplemental amino acids (or other variants of protein supplementation) provide no 

benefit (and as reviewed in the next subsection, “Protein and Bone,” they may actually 

be detrimental). Thus, the primary countermeasure against muscle loss remains 

adequate energy intake, which will no doubt include protein, but protein supplements 

are not required. In fact, protein can have negative implications for other systems, 

bone in particular. 
26

Debate continues between nutritionists and exercise physiologists on the importance 

and benefit of protein intake, amount and type of protein intake, and even timing of 

protein intake on muscle. It is our contention that these studies are often too short 

(if not simply a single bout of exercise) to allow an understanding of long-term effects 

and adaptation, and are often not completely controlled with respect to treatment 

groups. Protein (or amino acids) provides not only a nitrogen source, but moreover, 

an energy source. Studies often compare protein consumption with fasting, without 

evaluating whether a balanced meal would offer similar benefit. The summary above, 

regarding the view of amino acid supplementation during bed rest, certainly gives reason 

for concern about studies of protein and exercise, with or without bed rest. Thus, fully 

balanced, long-term studies are required in order to conclusively define the effect of 

protein intake and its timing on muscle. Unfortunately, these studies require multiple 

treatment groups, which drives study costs to levels that are typically not available. 

We will not review this field of literature here, but we refer interested readers to the 

Muscle Space Flight Evidence Book. 

Protein and Bone 

The interrelationship of protein and bone health is complex, and often seemingly 

contradictory. In certain populations (such as growing children), protein is essential 

for bone growth. However, in some cases, high protein intakes can be detrimental 

to bone (263), a fact confounded by the type of protein (and amino acids) consumed 

and by their relation to other dietary factors (264, 265). 

High-protein diets lead to hypercalciuria, and increase the risk of bone fracture and 

the risk of renal stone formation (265, 266). In one 5-year study of 120 men, the relative 

risk of stone formation on a restricted protein (52 g/d) and salt (50 mEq/d) diet was 

found to be half that of men on a calcium-restricted diet (400 mg/d) (267). The reason 

for the decreased risk of renal stones on a low-protein diet is not well understood, but 

several potential mechanisms have been postulated. It is generally well accepted that 

high-protein diets induce hypercalciuria, and this can contribute to formation of calcium 

oxalate or calcium phosphate stones. One hypothesis to explain protein-induced 

hypercalciuria is related to the “acid-ash” hypothesis that excessive intake of animal 

protein provides excess sulfur-containing amino acids that are metabolized to sulfuric 

acid (268, 269). Since bone is a large reservoir of base, it can be broken down to provide 

carbonate or phosphate to neutralize fixed acid loads. Furthermore, low urinary pH 

decreases urinary excretion of citrate, which is a potent inhibitor of stone formation. 

In addition, dietary animal protein represents a rich source of purines that may raise 

uric acid excretion, which could increase the risk of forming uric acid stones (270). 

Protein-induced hypercalciuria may also be detrimental to bone (271). Some studies 

show that high-protein diets increase intestinal absorption of calcium (272), but this has 

not been widely accepted. The key to understanding the interrelationship of protein and 

bone may lie in understanding the complexities of these types of studies, which may 

require a full accounting of many nutrients and environmental factors (273, 274). 

Several studies show that animal protein increases acid load more than does vegetable 

protein because of the higher sulfur content per serving of food. Vegetable protein itself 

does not necessarily have less sulfur per gram of protein, but a larger mass of foods 

containing vegetable protein would have to be consumed to get the same amount of 

protein as from foods containing animal protein. It can be assumed that foods containing 

vegetable protein contain less sulfur than foods containing animal protein. In studies 
27

with controlled dietary intakes with varying sulfur content, diets consisting of animal 

protein yielded greater urinary calcium excretion and lower urinary pH than similar diets 

consisting mainly of vegetable protein (275). The results of another study comparing 

the effects of 2 sources of protein (meat and soy protein), with and without additional 

supplementation with sulfur amino acids, indicated that dietary meat elicited a greater 

positive association between protein intake and urinary calcium, sulfur, ammonia, and 

titratable acids than dietary soy elicited (276). When the soy diet was supplemented 

with sulfur amino acids, urinary calcium and acid excretion increased. Conversely, the 

addition of dietary potassium (as fruit or K+ supplement) to both diets decreased urinary 

calcium and acid excretion (276). Other studies have shown that greater amounts of 

protein or higher ratios of animal protein to potassium are more detrimental when bone 

health is already compromised (such as during bed rest, and potentially during space 

flight) (277, 278). 

Dietary intake of protein, specific types of protein, and patterns of acid and base 

precursors have recently been associated with the concentrations of urinary markers 

of bone resorption during bed rest (277, 279, 280). In one study with male identical 

twins, the relationships between acid and base precursors in the diet and markers of 

bone and calcium metabolism during bed rest were investigated (281). With respect 

to dietary intake patterns, a strong positive correlation existed between markers of 

bone resorption (n-telopeptide [NTX], deoxypyridinoline, and pyridinoline) and the ratio 

of animal protein to potassium intake during bed rest. A positive correlation existed 

between urinary NTX excretion and the ratio of animal protein to potassium intake 

during 4 weeks of bed rest (277). No relationship was found between the ratio of 

vegetable protein to potassium and markers of bone metabolism. There tended to be 

a positive association between these variables before bed rest and during weeks 1 and 

2 of bed rest, but the relationship was not significant, likely because of high variability 

among the population and small sample size (277). These results document that the 

ratio of animal protein to potassium intake was less related to bone metabolism markers 

in the group of subjects who exercised and more related to bone markers at the end of 

bed rest, when calcium excretion was highest. The results support the argument that 

calcium status could have an important role in determining the effect of protein on bone. 

If calcium is being resorbed from bone, then acid load can have a more detrimental 

effect on bone, similar to what has been observed in other studies of the effect of 

high-protein diets on bone (263, 265). 

The idea that the levels of acid and base precursors in the diet can affect bone and 

calcium metabolism is supported by the results of studies testing the ability of a 

supplement containing essential amino acids and carbohydrate (45 g/d essential amino 

acids and 90 g/d sucrose) to mitigate muscle loss (278). The supplement contained 1.5 g 

methionine, which is about 1.13 times the recommended daily intake (supplementing the 

amount of methionine provided in the diet). The sulfur in methionine is converted in the 

body to sulfuric acid, and thus methionine is an acid precursor in the diet. It was evident 

that more methionine was broken down than was used by the body because urine pH 

decreased in the amino acid-supplemented group (Figure 21). It was hypothesized that 

this low-grade metabolic acidosis (282) contributed to the higher urinary concentrations 

of bone resorption markers and calcium excretion in the supplemented group. 

In a separate study, 13 volunteers were subjected to 60 to 90 days of 6° head-down-tilt 

bed rest (283). Net acid excretion, as determined by dietary acid and base components, 

was positively correlated with NTX during but not before bed rest (Zwart et al, 

unpublished data). Net acid excretion has also been associated with calcium loss 

using meta-analysis techniques (284). 
28

Figure 21. Urine pH (mean ± SD) of amino 

acid-supplemented (red ) and placebo 

(blue) groups during 4 weeks of bed rest. 

*Significantly different from before bed 

rest (Pre), P < 0.05. #Significant difference 

between groups, P < 0.05. Figure is from 

Zwart et al, J Appl Physiol 2005 (278). 

The risks associated with protein intake come from deficiency or excess. Deficiency of 

protein leads to muscle loss, weakness, wasting, tissue breakdown, inability to perform 

the job (including getting out of the spacecraft), and ultimately death. Low-protein diets 

can have negative consequences for bone (125, 285-287). Excess protein exacerbates 

increased excretion of calcium and the risk of renal stone formation, and is detrimental 

to bone. Specific amino acids may additionally increase these risks. 

View of Expedition 30 Flight Engineer Don Pettit in the ISS Node 1 with an edible “friend” 

composed of flat bread, peanut butter and chocolate-covered espresso beans. 
29

4 Bone 

Whereas nutrition is critical for virtually all systems, the interaction of nutrition with bone 

is perhaps more extensive and complex than most of its interactions with body systems. 

Bone is the body’s reservoir of calcium, which provides structure and strength to bone, 

but also provides a ready resource to maintain blood calcium levels during periods with 

insufficient dietary provision of calcium. Several nutrients are required for the synthesis 

of bone, including protein and vitamins D, K, and C. 

Multiple risks are associated with bone loss during space flight. Almost immediately 

upon entering weightlessness, bone resorption increases, and calcium (and other 

minerals) are released into the blood and urine. This increases kidney stone risk on 

short missions, and on longer missions, chronic bone and calcium loss can increase 

risks to bone health both in the near term (eg, risk of fractures) and in the long term 

(eg, risk of osteoporosis-like bone degradation). The International Space Station 

provides the opportunity for relatively large numbers of crewmembers to fly on 

missions of 4 to 6 months, with even longer missions being planned. At current 

capacity, 6 crewmembers are on board at a time: 3 Russian cosmonauts and 

3 astronauts from the other partner agencies (Canadian, European, Japanese, 

and US space agencies). This number of crewmembers has allowed the effects 

of space flight on bone physiology to be documented, along with the testing of 

countermeasures aimed at counteracting bone loss. 

We provide here an overview of bone and calcium changes that occur during space 

flight, and we review the research efforts made to understand bone loss, including 

more detail about specific nutrients. We provide an update on recent countermeasure 

studies, which have started to show progress toward meeting the challenge of 

mitigating risks associated with bone and calcium changes during flight. 

Bone Loss 

Bone is lost during space flight, primarily from weight-bearing bones. This was first 

documented on Skylab missions (288, 289), and later on Mir (290-293) and ISS missions 

(31, 50, 294). Attempts to come up with a simple way to express the amount of loss 

usually estimate it at about 0.5% to 1% per month (291, 292, 295, 296), roughly similar 

to postmenopausal bone loss rates over a year. Bone mineral density losses at landing 

after 6-month ISS missions are estimated to be 2% to 9% for different bone sites 

(297), with significant site-to-site and individual-to-individual variability. The bone loss 

and mineral shedding are accompanied by an increased risk of renal stone formation 

during and after flight (88, 89, 270, 298, 299). The subject-to-subject variability seems 

a characteristic of space flight-induced bone loss (300), and as more data are accrued, 

may provide insight to find a means to mitigate this loss (301). 

Long-term follow-up data on bone recovery are far from complete (302, 303). However, 

if the rate of postflight recovery estimated from biochemical data is also assumed to be 

constant (reasonable according to ground-based (304) and flight (20, 55) data), then the 

rate of recovery is about +100 mg/d (20, 55). By these estimates, on flights up to about 
30

6 months, it takes 2 to 3 times the mission duration to recover the lost bone. Analysis 

of bone recovery data from dual-energy x-ray absorptiometry (DXA) analyses suggests 

that while regional differences exist, the half-life of bone recovery after flight is on the 

order of 5-9 months after flight (301, 302). For longer exploration missions, however, 

the usefulness of these assumptions comes into question, as space flight data are not 

available for these durations. Although more data clearly are required to validate this 

hypothesis of bone mineral recovery, it nevertheless has significant implications as 

mission durations increase. Beyond bone mineral density, questions of changes to, 

and recovery of, bone architecture and strength also remain unanswered. 

Bone Metabolism 

Bone is a metabolically active tissue, constantly undergoing turnover through 

breakdown (resorption) and formation processes. When these 2 processes are 

in balance, no net loss (or gain) of bone occurs. Alterations in either, of both, 

of these processes can be problematic. 

Bone resorption increases during space flight. This wasn’t clearly documented until 

the 1990s, when markers specific to resorption were identified and analytical capability 

was commercialized. Collagen crosslinks are chemical linkages that give collagen 

its strength. These were found to be released during the resorption process and not 

metabolized before renal excretion. Many assay variants are available commercially, and 

are based on immunoassay techniques that bind to different portions of the molecules. 

Increased collagen crosslink excretion, and thus bone resorption, has been shown to be 

clearly increased during space flight (20, 55, 293, 305-308). Earlier studies showed that 

plasma concentrations of a similar but more easily confounded marker, hydroxyproline, 

were elevated during short-duration Shuttle flights (307) and longer duration Skylab 

flights (29, 160, 166). Calcium tracer kinetic studies also provided data indicating that 

bone resorption increases about 50% during flight relative to preflight (20, 55) levels. 

Bone formation either remains unchanged or decreases during space flight (20, 55, 295). 

As indicated by serum concentrations of bone-specific alkaline phosphatase and 

osteocalcin, bone formation was unchanged during Mir flights, but increased 2 to 3 

months after landing (20, 55). Trends toward decreased levels of bone formation markers 

were noted in 2 Mir studies with 1 subject each (293, 306). The results of studies, using 

calcium tracer techniques, of bone formation in 3 Mir crewmembers (20, 55) were 

equivocal (formation was unchanged or decreased). Together, increased resorption and 

decreased or unchanged formation yield an overall negative calcium balance (20, 55). 

The exact triggering mechanism for these changes in bone metabolism during 

space flight has yet to be identified, but the physiological and endocrine responses 

to the changes are as expected. The release of calcium from bone suppresses 

parathyroid hormone (PTH), which results in lower levels of activated vitamin D 

(1,25-dihydroxyvitamin D), which leads to a reduction in calcium absorption from the 

gastrointestinal tract. Studies of calcium metabolism were conducted on Mir astronauts, 

and indeed, PTH, 1,25-dihydroxyvitamin D, and calcium absorption were all decreased 

(20, 55, 308). Although it remains important to maintain calcium intake during flight, 

the lower calcium absorption during flight suggests that increasing calcium intake 

is not a viable countermeasure for weightlessness-induced bone loss, a fact proven 

in bed rest studies (309, 310). 

Space flight analog studies (such as bed rest) with humans have shown qualitative 

effects on bone and calcium homeostasis similar to those shown in flight studies 

(300, 311), with quantitative effects generally being of smaller magnitude. Effects 
31

include loss of bone mass (304, 312-314), decreased calcium absorption (315), 

increased urinary excretion of calcium and biochemical markers of resorption 

(283, 314-326), increased risk of renal stone formation (87, 322, 326), and decreased 

serum concentrations of parathyroid hormone (281, 283, 316, 317, 319, 327) and 

1,25 dihydroxyvitamin D (281, 315, 316, 319, 328). 

That bone resorption increases during bed rest has been shown by histomorphometry 

(313, 329) and measurement of biochemical markers. Excretion of hydroxyproline 

(315, 323, 325) increases during bed rest, and excretion of collagen crosslinks 

(281, 283, 305, 314-317, 327) is elevated about 50% above control levels, compared 

with the increase of greater than 100% during space flight (20, 55, 305). 

The concentrations of biochemical markers indicate that bone formation is unchanged 

during bed rest (281, 283, 312, 315-317, 327), but histomorphometry data from bone 

biopsies show that bone formation decreases (313, 319, 329). This difference likely 

reflects the difference between site-specific (biopsy) and systemic (biochemical markers) 

indices of bone formation. After ambulation begins following bed rest, bone formation 

generally increases (312, 315). Recent evidence indicates that with bed rest studies 

of longer duration (eg, 90 d), bone formation markers tend to increase (283, 314). 

In these initial publications, bone-specific alkaline phosphatase (BSAP} did not 

change significantly over time, but a retrospective analysis of data from multiple 

studies does show a change in BSAP that reaches statistical significance (Smith 

et al, unpublished observations). 

Although early animal studies suggested that the primary change in bone metabolism 

was related to bone formation, the identification of markers specific to bone resorption 

in the late 1980s (330, 331) and the availability of commercial immunoassays in 

the 1990s (332, 333) allowed resolution of this matter: bone resorption increases 

during space flight and bone formation decreases or does not change significantly, 

substantiating that space flight disrupts the balance between bone resorption and 

formation, which can lead to a net loss in bone mass. 

Bone Loss Countermeasures 

Exercise 

In-flight exercise was first implemented on Skylab missions, largely because this 

was the first vehicle to allow enough room for exercise. In addition to in-flight exercise, 

ground-based research has been conducted to help document the effectiveness of 

exercise as a countermeasure for muscle, bone, and cardiovascular maladaptations that 

occur during space flight (202, 311, 334, 335). With respect to bone specifically, treadmill 

and cycle exercise devices available on Mir did not prevent bone and calcium loss (20, 

55, 292, 336, 337). This result has been attributed to the lack of resistance provided by 

these devices, and for the treadmill, the inability to generate sufficient ground-reaction 

force in weightlessness (338). 

Bed rest studies evaluated many types of exercises and devices, alone or in rare 

cases in combination, demonstrating positive effects of treadmill, flywheel, and weight 

stacks on maintenance of bone (as assessed by various means from densitometry 

to biochemistry) (192, 279, 281, 339-343). Most evidence, from the general scientific 

literature as well as space flight analogs, pointed to resistance exercise as the most 

likely means of counteracting bone loss during space flight. Heavy resistance exercise 

in bed rest protected bone mineral density, but did so not by suppressing the bed 

rest-induced bone resorption, but rather by increasing bone formation (Figure 22). 
32

Figure 22. Bone resorption (as indicated by urinary n-telopeptide, left panel) and bone formation 

(as evaluated by serum bone-specific alkaline phosphatase [BSAP], right panel) during 17 weeks 

of bed rest with (solid line) or without (dashed line) heavy resistance exercise. Data are expressed 

as percentage of pre-bed rest values, and are mean ± SD. The vertical line separates the bed rest 

and post bed rest periods. Data adapted from (192). 

Bed rest studies combining resistance exercise (with a flywheel device) and supine 

treadmill exercise while in a lower-body negative pressure (LBNP) chamber on 

alternating days yielded results similar to those shown above, but with about half 

the response of bone-specific alkaline phosphatase (317) (Figure 23). 

Figure 23. Bone resorption (as indicated by urinary n-telopeptide, left panel) and bone formation 

(as indicated by serum bone-specific alkaline phosphatase, right panel) during 60 days of bed rest 

with (solid line) or without (dashed line) a combination of resistance exercise and supine treadmill/ 

LBNP exercise. Data are expressed as percentage of pre-bed rest values, and are mean ± SD. 

Data adapted from (317). 

Attempts to provide resistance exercise on early ISS missions were unsuccessful, 

with the interim resistance exercise device (iRED) providing no additional benefit over 

the Mir equipment (31, 294). This initial device was deployed on the inaugural ISS 

expedition, and time and other constraints did not permit development of all desired 

hardware requirements before this expedition was launched. Thus, the appropriately 

named interim resistance exercise device was to be used until a more advanced device 

capable of allowing heavier loads could be developed, tested, and launched to ISS. 
33

Figure 24. Garrett Reisman shown using the interim Resistance Exercise Device (iRED, left), while 

Sandy Magnus is shown using the Advanced Resistance Exercise Device (ARED, right) on ISS. 

The iRED was launched in 2000 with the first ISS crew, and the ARED replaced it in late 2008. 

In 2008, the Advanced Resistance Exercise Device (ARED) was launched to ISS. 

This device accommodated additional exercise protocols and had almost twice the 

loading capability of iRED (31, 344). Comparing crewmembers exercising with each 

device (Figure 24) has been somewhat difficult, given that more recent crews have 

maintained their energy intake at levels >90% of estimated requirements, and have 

had better vitamin D status than earlier crews. These better nourished crewmembers 

exercising with the ARED maintained body mass during flight (and came back leaner, 

with less body fat) (Figure 25), and as assessed by DXA (31, 345), maintained bone 

mineral density in most regions and in whole-body scans (Figure 26). 

Figure 25. Body composition changes (left panel, lean body mass; right panel, total body fat) 

in astronauts on Mir and ISS missions. ISS crews had access to either iRED or ARED exercise 

devices. Data are expressed as percent change per month of flight and are mean ± SD. 

Figure adapted from (31). 

Figure 26. Bone mineral density loss 

in astronauts on Mir and ISS missions. 

ISS crews had access to either iRED 

or ARED exercise devices. Data are 

expressed as percent change per month 

of flight. Figure adapted from (31). 
34

The number of ISS subjects whose data had been analyzed when the comparison 

of exercise devices was published in 2012 was small, in part because many 

crewmembers participated in other countermeasure studies and were not included 

in this initial analysis. In the time since that initial comparison of crewmembers with 

access to the iRED or the ARED was published, another study has confirmed these 

findings with a much larger set of data. In 42 astronauts (33 male, 9 female), the bone 

mineral density response to flight was the same for men and women (50) (Figure 27), 

and those with access to the ARED did not have the typical decrease in bone mineral 

density that was observed in early ISS crewmembers with access to the iRED (31). 

Biochemical markers of bone formation and resorption responded similarly in men 

and women. These recent data are encouraging, and represent the first in-flight 

evidence in the history of human space flight that diet and exercise can maintain 

bone mineral density on long-duration missions. 

Figure 27. Bone mineral 

density (BMD) loss after flight 

in men (N=33, open bars) and 

women (N=9, solid bars) who 

used either the iRED or ARED 

exercise device. Data are 

expressed as percent change 

per month of flight and are 

mean ± SD. Figure adapted 

from (50). 

The bone biochemical changes in crewmembers exercising with the ARED were very 

similar to what had been observed in bed rest studies testing resistance exercise. 

That is, the exercise did not affect bone resorption, but did increase bone formation 

(31). In the flight study, as published, a significant (P < 0.001) increase occurred (31), 

and in bed rest, heavy resistance exercise 6 days a week led to dramatic increases in 

bone formation markers (192). In another study with resistance exercise every other 

day (combined with a treadmill protocol), subjects had roughly half the bone formation 

response (317) of subjects in the first study (192). The exercise also did not have a 

significant effect on serum total calcium or urinary calcium. When data from additional 

crewmembers became available, these solidified the “trend” into statistical significance 

(50). The slow increase in bone resorption over time during flight is likely related to the 

fact that the astronaut conditioning and strength trainers were initially reluctant to have 

crewmembers exercise too hard with the ARED, to minimize the risk of injury. This slow 

and steady increase over time (31) is different from results of the bed rest study, where 

formation markers plateaued at the first determination during bed rest (6 weeks of 

bed rest) (192). 

Although this mode of bone remodeling, with increases in biochemical markers of both 

resorption and formation, maintained bone mineral density, it may yield a bone with 

strength characteristics different from those that existed before space flight. Studies 

to assess bone strength after flight are underway at NASA, to better understand the 

results of bone remodeling. Studies are also underway to evaluate optimized exercise 

protocols and nutritional countermeasures. 
35

Gravity 

Other physical countermeasures, including artificial gravity and vibration, have also 

been tested in ground-based (bed rest) settings. A more recent and complex study 

used centrifugation to create artificial gravity transients during a 21-day bed rest study 

(346). One hour per day of 1 Gz exposure at the level of the heart and 2.5 Gz at the feet 

was beneficial for some systems (eg, cardiovascular, muscle) (177, 346, 347), but this 

regimen did not have any effect on bone or calcium metabolism (327, 348). Although 

greater durations, increased g forces, or combining centrifugations with exercise 

protocols have been proposed, these have not yet been extensively tested (346, 349). 

The assumption that it is lack of gravity that stimulates bone loss during space flight 

provided the impetus for proposing replacement of gravity by centrifugation (“artificial 

gravity”) as a countermeasure for multiple body systems (350, 351), particularly for bone. 

The statement has been made that it is known that unit gravity maintains bone, whereas 

microgravity does not. A key question is, “How much gravity do we need?” (W. Paloski, 

personal communication). In early bed rest studies, 2- or 4-hour intervals of standing 

or walking mitigated the increase in urinary calcium excretion associated with bed rest 

(352). Some of the artificial-gravity studies have relied on short-radius centrifuges (353), 

others on rotating exercise devices (349, 354) intended to provide gravitational impact 

as well as physical exercise. Artificial gravity or hypergravity has been shown to 

positively affect bone, in human and some animal studies (355-357). As noted above, 

1 hour per day of centrifugation resulting in 1 Gz at the heart and 2.5 Gz at the feet was 

ineffective for bone (327). The optimal artificial gravity prescription for bone, including 

dose, duration, and frequency of centrifugation, remains to be clarified (358), along 

with its potential impact on nutrition and related systems (359). 

Vibration 

Protocols for exposure to vibration, of both high and low frequency, have also been 

proposed and tested in space flight analogs. Although low-frequency protocols showed 

promise for protecting bone in both animal and ambulatory human studies (360-362), 

the beneficial findings were more limited when testing occurred during head-downtilt bed rest (363). Higher frequency vibration, often referred to as resistance vibration 

exercise, has generally shown positive effects on bone and muscle during bed rest 

(206, 364-366). Debate continues over the potential of this countermeasure protocol, 

amid safety concerns about neuromuscular issues with repeated vibration exposures. 

Pharmacological Agents 

Pharmacological agents, the most common being the bisphosphonates, have also 

been tested for their ability to mitigate weightlessness-induced bone loss. Many ground 

analog studies of bisphosphonates (including bed rest studies and studies of patients 

immobilized because of spinal cord injury or other reasons) have been conducted, with 

generally positive findings (313, 335, 340-342, 367-374). However, ongoing discussion 

and debate surround the relative safety of these compounds for use in otherwise healthy 

individuals (astronauts), as opposed to the target population for whom the drugs were 

developed (patients with disorders such as osteoporosis). In addition to resolving safety 

concerns, investigators have yet to determine the optimal drug, dose, and schedule 

of administration during space flight. As noted above with exercise, given that the 

bone loss of bed rest is about half that of space flight, there is little reason to believe 

that the same dose of drug will have the same effectiveness in both environments. 
36

Moreover, data from animal studies suggest that the disuse- or space flight-induced 

increase in bone resorption cannot fully, or chronically, be mitigated by bisphosphonates 

(375, 376). 

Endocrine therapies, including exogenous calcitonin administration (325, 335), have 

also been attempted, albeit unsuccessfully. In animal models, testosterone has also 

been suggested as a bone loss countermeasure (221, 222) on the basis of limited data 

showing a reduction in testosterone concentrations during flight in human, animal, and 

cellular models (223-229). Reduction of testosterone, however, has recently been shown 

to likely not be a concern during space flight. See section 3, “Muscle and Protein,” for 

a more detailed discussion of these data. 

Nutritional Countermeasures 

One of the most obvious nutritional countermeasures—providing calcium—does not 

protect against bone loss (377). This result is likely related to the decreased calcium 

absorption seen during bed rest (315, 348) and space flight (20, 55, 308), likely related 

to reduced circulating parathyroid hormone and 1,25-dihydroxyvitamin D. Phosphate 

supplementation, used in an attempt to reduce calcium excretion, was also ineffective 

(379). Combination therapy with calcium and phosphorus was also unsuccessful at 

mitigating bone loss and hypercalciuria (325). 

Omega-3 fatty acids have been shown to protect bone, in the general population 

(113-115) and animal studies (380), as well as in space flight analog studies, including 

bed rest (Figure 28) and cell culture (116). While omega-3 fatty acids have not been 

studied in a controlled fashion during actual space flight, a positive correlation was 

found between fish intake and bone loss in astronauts (Figure 28) (116). That is, those 

who ate more fish lost less bone. These data provide additional evidence of the potential 

importance of fish oils as a countermeasure for muscle, bone, and radiation risks of 

space flight. Studies showing positive effects of omega-3 fatty acids typically look 

at intake of fish or other food sources of these nutrients (117-119). Studies of fish oil 

supplements added to typical diets often fail to document any benefit (120-122), thus 

highlighting the need for dietary modification, and not simply supplementation. 

Figure 28. Relationship between omega-3 fatty acid consumption and bone resorption during 

bed rest (left panel), and relationship between fish consumption and percent change in wholebody bone mineral density at landing in astronauts (right panel). Figures adapted from (116). 
37

Other nutrients, specifically sodium, protein, potassium, and vitamin K, have 

been documented to have effects on bone, and/or have been proposed or tested 

as countermeasures to bone loss (280). These are discussed in detail in other 

sections of this book. 

Nutrients and Bone Health 

Calcium 

Calcium metabolism is of critical importance in bone health, and many studies use 

calcium determinations as one measure of bone health; however, it has been well 

documented that excess dietary calcium will not mitigate bone loss during space flight 

(310, 381). Nonetheless, ensuring adequate calcium intake remains important, and the 

role of calcium excretion in kidney stone risk must be considered. Two other facets of 

calcium chemistry and biochemistry have been researched, with intriguing findings. 

One is related to the impact of calcium in the reclamation of water from urine on 

spacecraft, and the other relates to naturally occurring stable isotopes of calcium 

and their ability to document changes in bone metabolism. 

Calcium Balance 

Negative calcium balance was observed during the Skylab (29, 160, 288, 321, 382­

384) and Mir (20, 55) missions. During the 84-day Skylab 4 mission, calcium balance 

was –200 mg/d (160, 384, 385). Increased urinary and fecal calcium excretion accounts 

for most of the calcium deficit (20, 29, 55, 160, 288, 299, 321, 383). Estimates of bone 

calcium loss from multiple studies and techniques converge on the suggestion that 

about 250 mg of bone calcium is lost per day during flight (20, 55, 160, 386). When this 

rate of loss may slow down is not yet known, but it does not appear to be within the first 

6 months of flight. For comparison, bone loss after spinal cord injury seems to stabilize 

after about 6 to 12 months (387-389), around the duration of many ISS missions. 

Vitamin D 

The best-understood role of vitamin D is its involvement in calcium metabolism. 

One of the major functions of this vitamin is to maintain normal blood levels of calcium 

and phosphorus. The liver converts vitamin D to 25-hydroxyvitamin D. Typically the 

gold-standard measurement for determining vitamin D status, 25-hydroxyvitamin D 

is converted to 1,25-dihydroxyvitamin D in the kidney, and from there it is transported 

systemically to target organs. Classic target organs include bone, intestine, and kidney. 

The Institute of Medicine (IOM) conducted an extensive review of the literature in 2010 

and increased the recommended dietary allowance (RDA) for vitamin D to 600 IU/d for 

healthy males and females 9-70 years old, and 800 IU/d for those older than 70 years 

(390). The main factors that were taken into account were changes in bone density and 

fracture risk. The IOM Committee did not find conclusive evidence that vitamin D plays 

a role in extraskeletal health outcomes. Even though a considerable number of papers 

published in the past 10 to 15 years have shown associations between vitamin D status 

and adverse health outcomes such as cancer, autoimmune diseases, diabetes, cognitive 

decline, and all-cause mortality, the evidence that vitamin D status causes any of 

these outcomes is inconclusive, and insufficient to inform nutritional recommendations 

(391-396). The noncalcitropic functions of vitamin D may help explain why a robust 

set of reproducible data shows an inverse correlation between sun exposure and 
 

38

several types of cancer (397-399). Although 1,25-dihydroxyvitamin D is the biologically 

active form for the noncalcitropic functions also, 25-hydroxyvitamin D must be 

available in sufficient quantities for the 1-hydroxylase enzyme in nonrenal tissues to 

synthesize 1,25-dihydroxyvitamin D. Besides kidney cells, other cell types, including 

bone cells, epithelial cells, monocytes, and antigen-presenting cells, also synthesize 

1,25-dihydroxyvitamin D (400). Numerous tissues are affected by vitamin D status 

because their cell nuclei contain receptors for 1,25-dihydroxyvitamin D (394). Some 

of these tissues are adipose tissue, bone marrow, brain, breast, cancer cells, cartilage, 

lung, muscle, ovary, placenta, prostate, stomach, testis, thymus, and uterus (401). 

People who are normally exposed to sunlight make vitamin D in their skin. Ultraviolet B 

light, a component of sunlight, converts 7-dehydrocholesterol to 25-hydroxyvitamin D3 

in the skin (402). Although sunlight has a positive effect on health through its role in 

making vitamin D, caution must still be exercised to avoid too much sun exposure 

(403-405). 

Before 2006, when ISS crews were first provided 400 IU vitamin D/d, it was well 

documented that vitamin D status (serum 25-hydroxyvitamin D) decreased after 

long-duration space flight (6, 20, 29, 55). The absence of ultraviolet light during space 

flight diminishes vitamin D stores in the body, as observed during the 84-day Skylab 

mission (29), space station Mir missions (20, 55), and early ISS expeditions (6). Despite 

the reported use of vitamin D supplements by some of the astronauts on early ISS 

expeditions (average supplement use was 3.0 ± 2.8 per week of a 400-IU vitamin 

D supplement), the mean serum concentration of 25-hydroxyvitamin D for the ISS 

crewmembers was about 25% less after landing than before launch. 

In 2006, vitamin D recommendations to crews increased from 400 IU vitamin D/d 

to 800 IU vitamin D /d. In-flight 25-hydroxyvitamin D data from the Nutritional Status 

Assessment Supplemental Medical Objective provide evidence that 800 IU vitamin D/d 

is enough to maintain vitamin D status during long-duration space flight (Figure 29) 

(31). Furthermore, recent data show that crewmembers who exercised with the ARED, 

maintained energy intake at recommended levels, and took 800 IU/d vitamin D during 

flight maintained bone mineral density during 4- to 6-month space flights (31). 

An ideal ground-based model for individuals lacking ultraviolet light exposure is the 

Antarctic, where winter levels of ultraviolet B radiation are essentially zero. Two studies 

have been conducted at McMurdo Station, Antarctica, to determine the dose of vitamin 

D needed to sustain serum levels of 25-hydroxyvitamin D during a 5- to 6-month 

period when there is little to no ultraviolet B exposure, without increasing risks 

of hypercalcemia (406, 407). 

Several ground-based studies (performed in Antarctica and at the Johnson Space 

Center) provide evidence that a vitamin D dose in the range of 800-2000 IU/d is tolerable 

and safe, and can maintain vitamin D status for 3 to 6 months even in environments with 

no UV light exposure (406-408). This recommendation is in line with the recent Institute 

of Medicine recommendations for vitamin D intake for North Americans (390). One 

study conducted at McMurdo Station in Antarctica showed an interactive effect of 

serum cortisol and vitamin D status on immune function (407). In that study, subjects 

with higher serum cortisol and lower vitamin D status presented with more latent virus 

reactivation in their saliva. It is clear that vitamin D may have an effect on other systems 

besides bone, but further research is required before evidence-based recommendations 

can be made for other systems. 
39

Figure 29. Pre- and postflight data 

(mean ± SD) from testing required for 

medical operations show that vitamin D 

status decreased after long-duration space 

flight, despite vitamin D supplementation with 

400 IU/d (black dashed line, N=16). In-flight 

data (purple line) show that 800 IU/d 

is enough vitamin D3 to maintain status 

during long-duration space flight (N=26). 

Red lines depict Institute of Medicine-defined 

lower acceptable limits (with respect to bone 

health), and upper advised limit (390). The 

green line at 80 nmol/L reflects what many 

perceive as an optimal level with respect 

to parathyroid hormone suppression and 

non-bone health outcomes. Figure adapted, 

and data updated, from Smith et al (409). 

As noted above, vitamin D deficiency is linked to calcium metabolism, and in severe 

cases leads to osteomalacia and osteoporosis in adults (and rickets in children). 

Throughout the ISS program, supplemental vitamin D has been provided to astronauts 

to ensure optimal vitamin D status. 

Efforts to provide vitamin D supplements are misinterpreted to infer that this might 

be a viable bone loss countermeasure, but this is not the case. Even when vitamin D 

stores during flight are adequate, the circulating concentration of the active form of 

vitamin D, 1,25-dihydroxyvitamin D, is decreased (20, 55). As described in the previous 

subsection, “Calcium,” this is likely the result of the increased release of calcium from 

resorbed bone, and results in decreased intestinal absorption of calcium. Adequate 

stores of 25-hydroxyvitamin D will not affect this. Any attempt to directly provide the 

1,25-dihydroxyvitamin D, or as in some cases on Earth, excess 25-hydroxyvitamin 

D levels, may lead to hypercalcemia, renal stones, soft-tissue calcification, and even 

death. Controlled trials in bedridden subjects have also proven that several months 

of supplementation fail to affect bone metabolism. In one trial, bedridden elderly people 

took supplemental vitamin D (400 or 1200 IU/d compared to placebo) for 6 months. 

Little effect was found on parathyroid hormone, and no effect on bone markers (410). 

In a similar 40-week trial of 1000 IU of vitamin D2 or D3 (2 groups), neither had an effect 

on bone markers (309). The problem of weightlessness-induced bone loss must be 

solved, but vitamin D is not the answer. Nevertheless, even if bone loss is not stemmed, 

ensuring an adequate amount of vitamin D will remain important. 

As the current space food system includes very few dietary sources of vitamin D, 

and vitamin D cannot be synthesized endogenously due to lack of UV light, decreased 

vitamin D status is a serious concern for exploration missions that could last 1000 days. 

Toxicity of vitamin D is typically less likely to occur than a deficiency (411-414), but 

use of supplements would increase its likelihood. Excessive blood levels of vitamin 

D can lead to hypercalcemia, which can lead to nephrocalcinosis, arteriosclerosis, 

and irreversible soft tissue calcification. In one study conducted in Houston in healthy 

individuals, a 50,000 IU/wk dose of vitamin D for 4 weeks and then monthly increased 

mean urinary calcium excretion to numbers higher than the normal range (408). In that 

study, a daily dose of 2000 IU or a single weekly dose of 10000 IU did not increase 

the incidence of hypercalciuria. 
40

Vitamin K 

The function of vitamin K was originally assumed to be strictly limited to involvement 

in blood coagulation, but an increasing amount of evidence indicates that this vitamin 

affects multiple physiological systems. Vitamin K is a cofactor in the posttranslational 

synthesis of ɣ-carboxyglutamic acid (GLA). ɣ-Carboxyglutamic acid is a constituent of 

all vitamin K-dependent proteins, and its role is related to increasing the affinity of the 

proteins for calcium (415). Vitamin K-dependent proteins include blood coagulation 

proteins and bone proteins (eg, osteocalcin, matrix GLA protein, protein S). On the 

basis of findings from Mir in the 1990s and from early ISS missions (described below), 

vitamin K had been proposed as a bone countermeasure (416). 

Data from 11 US astronauts on ISS Expeditions 1 to 8 (mission durations of 128 to 

195 d during 2000-2004) revealed that on landing day their serum phylloquinone 

(vitamin K1) was 42% lower than it was before flight, whereas urinary GLA did not 

change (6). In another study, undercarboxylated osteocalcin was elevated (a sign of 

vitamin K insufficiency) as early as the eighth day of space flight, and remained high 

during 21- and 180-day missions (417). Studies on the EuroMir 95 mission showed 

that markers of vitamin K status were decreased after 12.5 weeks of space flight, and 

vitamin K supplementation (10 mg/d for 6 wk) reversed these effects (418). Vitamin 

K supplementation elevated GLA and decreased undercarboxylated osteocalcin, 

suggesting that vitamin K status was lower during space flight and was improved by 

supplementation (417, 418). Despite the changes on landing day, the monitoring of 

vitamin K status during flight has documented no evidence that vitamin K status is 

decreased during space flight. In-flight data from 15 crewmembers on Expeditions 

14 to 22 showed no major 

changes in phylloquinone, 

urinary GLA, or 

undercarboxylated osteocalcin 

(419). Phylloquinone data 

from those 15 crewmembers 

(419) plus an additional 11 

crewmembers on Expeditions 

23 through 31 are shown 

in Figure 30. Even with the 

additional data, vitamin K 

status did not significantly 

decrease during flight. 

This is an important finding, 

to document that a vitamin K Figure 30. Serum phylloquinone before launch (launch 

countermeasure to mitigate minus 180 d, or L-180, L-45, and L-10), during flight 

a vitamin K deficiency during (flight days 15, 30, 60, 120, and 180), on landing day 

space flight is not needed (R+0), and 30 days post landing (R+30). The dashed 

lines indicate the normal range for phylloquinone. and therefore likely would Data are mean ± SD, N=26. Data and N are expanded not have an effect on bone. 

from the original publication of these findings (419). 

Phosphorus 

Phosphorus is an important component of cell membranes and bone mineral. 

Phosphate accounts for about 60% of bone mineral (420), and most (85%) of the 

body’s extracellular phosphorus is in bone (421). After ISS missions, urinary phosphorus 

excretion was about 45% lower than preflight excretion (6). 
41

Excretion of phosphorus during untreated bed rest was not changed (283) from 

ambulatory conditions. An earlier study of 3 subjects revealed increased urinary 

phosphorus and negative phosphorus balance (323). In bed rest studies, investigators 

have attempted to use combination therapy with calcium and phosphorus to mitigate 

bone loss and hypercalciuria, with trends in the right direction but no significant 

changes (325). 

High phosphorus intake, relative to calcium in particular, can have effects on many 

systems, including bone, kidney, and cardiovascular (422-424). Ideally, the ratio of 

calcium:phosphorus should be around 1.0, or higher. The ISS requirements match this, 

with a notation that the phosphorus:calcium ratio should not exceed 1.5 (2, 425, 426), 

based on evidence that a dietary phosphorus:calcium ratio greater than 1.5 is known 

to decrease calcium absorption, 

which could further impair skeletal 

integrity. Serum phosphorus rises 

with increasing phosphorus 

intake, and if hyperphosphatemia 

occurs, it can result in 

calcification of the kidney. 

For this reason, ensuring optimal 

phosphorus intake during flight 

becomes very important (295). 

To date, phosphorus intakes 

have been higher than desired. 

The standard menu has a P:Ca 

ratio of 1.8 (2); actual intakes have 

been slightly lower than that, but 

still greater than 1.5 (Figure 31). 

Bed rest studies have tended 

to have P:Ca ratios closer to 

1.0 (unpublished observations). 

Magnesium 

Magnesium is the fourth most 

abundant cation in the body, and more than half of the body’s magnesium is in bone 

(427). Adequate intake of magnesium is necessary to prevent hypocalcemia, resistance 

to vitamin D, and resistance to parathyroid hormone. Excessive magnesium intake from 

supplements has been shown to impair calcium absorption (427). 

Decreased urinary magnesium after flight, relative to before flight, seems to be a 

hallmark of space flight (29, 428). Serum magnesium trends downward during and after 

flight, as seen with Skylab and Shuttle crews (2, 6, 29, 429, 430). After crewmembers 

had spent 4 to 6 months in space on ISS, their urinary magnesium was about 45% less 

than it was before flight (6). 

Autopsy results after the tragic end of the 24-day Salyut-1 mission documented 

that, relative to control subjects, the Salyut-1 cosmonauts had 12% to 32% lower 

concentrations of magnesium in the compact layer of the femoral epiphysis and 

diaphysis, vertebral body, and sternum (431). These changes were reported as 

appearing “with a high degree of certainty.” Magnesium balance was slightly negative 

during extended-duration bed rest studies conducted in Russia (341), with little effect 

Figure 31. Dietary phosphorus:calcium 

ratio in ISS crewmembers during flight 

(approximate flight days 15, 30, 60, 120, 

and 180). The dashed line indicates a ratio 

of 1.5. Data are mean ± SD, N=9. 
42

of exercise or bisphosphonate. Recent studies conducted in the US have shown 

a decrease in magnesium excretion in short- and long-duration bed rest (134, 283). 

Magnesium shows promise for reducing the risk of renal stone formation (432). 

In ground-based studies, potassium-magnesium citrate has proven effective in 

reducing bed rest-induced risk (87). Potassium citrate (KCit) has been successfully 

tested during ISS missions (433), and has been “transitioned to operations.” This means 

that KCit is part of the flight surgeon’s toolbox for helping crewmembers mitigate renal 

stone risk. That is, it is available on ISS for use based on flight surgeon discretion if 

clinically indicated. However, given that fluid intake for optimum hydration is a preferred 

countermeasure, and some residual concerns exist about side effects of potassium 

supplementation, it was decided not to routinely provide KCit to crewmembers. 

Zinc (and Lead) 

Zinc status of astronauts, as assessed by serum zinc and urinary zinc excretion, did 

not change after long-duration space flight (6). Circulating zinc levels are an imperfect 

tool to evaluate zinc status, as other physiological factors may affect them (434). 

However, to increase the reliability of zinc status evaluation, more intensive and/or 

invasive techniques would be required. 

The release of zinc from bones (as a result of demineralization) has been noted in 

bed rest studies (435, 436), and a similar increase in excretion of zinc was noted 

in Wistar rats flown during COSMOS 1129 (a 20-d space flight) (437). This release 

of zinc associated with demineralization has raised concern that other metals, including 

lead, could also be released secondary to weightlessness-induced bone resorption 

(438, 439). Garcia et al developed a computer-based model that predicted that blood 

lead levels would actually decrease during microgravity exposure. The model predicted, 

for the majority of astronauts, that any increase in circulating lead would be more than 

offset by decreases in ingested or inhaled lead during the mission (440). Postflight data 

supported this model (440). 

Unique Aspects of Calcium and Space Flight 

Urine Processing and Water Reclamation 

The ability to reclaim water from urine will be a pivotal factor in implementing 

exploration-class missions. When the prototype Urine Processor Assembly (UPA) on ISS 

clogged due to an unknown precipitate in 2009, the available in-flight urine volume and 

calcium excretion data from 24-hour pools and single voids were closely examined (441). 

Urine 24-hour volume was about 17% lower during flight than before flight, and urinary 

calcium concentration was 50% greater during flight than before flight. The increased 

urinary calcium concentration during flight was identified as a primary reason for UPA 

failure, and new recommendations for percentage of water to be recovered were made 

as a result of those findings. In 2012, when the data were re-evaluated with data from 

an additional 10 subjects in a Supplemental Medical Objective study of nutritional status 

assessment, it was clear that crewmembers in recent years are drinking more fluid than 

crewmembers did in the past, and as a result urinary calcium concentration is lower. 

The UPA extracts water by vacuum distillation, and as a result, the primary concern 

for the UPA is the concentration of calcium in the urine, not the amount of calcium 

excreted (441). Suggestions for ways to mitigate high calcium concentrations included 

exclusion of the first morning void from the system, based on the assumption that this 
43

is typically more concentrated than other voids throughout the day. Another suggestion 

was administration of bisphosphonates to all crewmembers to reduce calcium excretion. 

Calcium concentration data from the in-flight single-void data showed that neither of 

these suggestions looked promising for lowering the calcium concentration of urine that 

enters the UPA (441). Regarding bisphosphonate treatment, people sometimes have 

the impression that the pharmacological blocking of bone resorption will reduce urine 

calcium to near zero, but this is not the case. In fact, although urinary calcium 

decreased in ISS crewmembers treated with bisphosphonate, it had started higher 

in these individuals, so the daily excretion was about the same as for individuals who 

did not receive bisphosphonate (345). 

When in-flight urinary calcium concentration was examined in light of in-flight mean 

fluid intake estimated from the food frequency questionnaire, all but one 24-hour 

pool had calcium concentrations below 23.7 mg/dL (the cutoff point for a 75% water 

recovery) when fluid consumption was greater than 32 mL fluid/kg body weight. 

For the 23 crewmembers in that analysis, that would average to a fluid consumption 

(from food and beverages) of 2.5 L/d. In September 2012, a decision was made to 

increase recovery from 70% to 74%, which allows a savings of >80 L H2O/year. 

Natural Calcium Isotope Composition of Bone 

Analytical techniques to assess bone health, bone loss, and bone metabolism continue 

to evolve with technology. Although densitometry techniques (such as DXA and 

quantitative computerized tomography) provide valuable assessment of specific bones, 

these techniques detect only relatively large changes in bone, which can take months 

to occur, despite the likelihood that biochemical changes are initiated within hours of 

exposure to space flight. Studying calcium metabolism requires either intensive balance 

studies or tracer kinetic studies, as calcium excretion alone is confounded by too many 

factors to be useful in non-controlled studies. Bone formation and resorption markers 

provide the ability to assess changes in bone biochemistry, but assessing the relative 

association of these 2 factors has not been possible to date, and thus it is difficult to 

assess net changes in bone mineral. 

A new technique to rapidly detect and quantitatively predict changes in whole-body 

bone mineral balance was recently validated in bed rest (442). This technique is based 

on natural, biologically induced variations in the presence of the naturally occurring 

stable (nonradioactive) calcium isotopes (40Ca, 42Ca, 43Ca, 44Ca, 46Ca, and 48Ca), which 

react at different rates depending on their mass (443). These variations exist because 

bone formation favors the lighter isotopes, and thus the blood and urine (and other 

“soft tissue”) tends to have the “heavier” calcium isotopes. Bone resorption, however, 

releases whatever calcium is in bone, and that tends to be the lighter isotopes. 

Thus, when bone is being resorbed (as it is during bed rest), the urine contains greater 

amounts of the lighter calcium isotopes than it did before the increased resorption. 

When the isotope ratio technique was applied to a bed rest study, it was shown that 

the calcium isotope ratio shifted in a direction consistent with bone loss after just 

7 days of bed rest, long before detectable changes in bone density occur. Consistent 

with this interpretation, the calcium isotope variation accompanied changes observed 

in n-telopeptide, whereas bone-specific alkaline phosphatase, a bone-formation 

biomarker, was unchanged (442). 

As the relationship between calcium isotopes and whole-body bone mineral balance is 

well established (444-446), this relationship can be used to quantitatively translate the 

changes in the calcium isotope ratio in urine to changes in bone mineral density using 
44

a simple model. Using this model it was estimated that subjects lost 0.25 ± 0.07% 

(1 SD) of their bone mass from day 7 to day 30 of bed rest (442). This rate of loss 

extrapolates to a loss of 1.36 ± 0.38% of skeletal mass over 119 days, which is 

equivalent, within error, to bone loss rates determined by DXA scans in long-term 

(119-d) bed rest studies (314). 

Given the rapidity with which calcium isotope measurements detect change and their 

potential for use in assessing bone loss, this technique is ideally suited for space flight 

studies in which changes in bone formation and resorption are not only being altered 

by space flight itself but are being manipulated by various countermeasures. 

ISS flight control team celebrating and toasting the first drink of the on-board water reclamation 

system on the space station during Expedition 19. 
45

5 Iron and Hematology 

Iron 

Iron is an essential element involved in oxygen transport, oxidative phosphorylation 

in carbohydrate and lipid metabolism, and electron transport in cytochromes and 

cytochrome oxidase (447, 448). Adequate iron is crucial for meeting the needs of organs 

and tissues, but excess iron is detrimental to cells and can cause oxidative damage. 

The body achieves iron balance through regulation of absorption by enterocytes in 

the intestine and regulation of iron export from cells. Once iron is absorbed into the 

enterocyte, it can be bound to ferritin and stored. Serum ferritin has been shown to be 

a sensitive indicator of iron stores (449). Ferritin is exponentially correlated with storage 

iron, as determined by quantitative phlebotomy in patients with iron overload (450). 

Iron deficiency is the most common nutritional deficiency worldwide, but iron toxicity 

is also worthy of concern. Deficiency of iron leads to anemia, fatigue, reduced work 

capacity, impaired behavior and intellectual performance, cognitive deficits and memory 

loss, heart palpitations, impaired thermoregulation, and decreased immune function 

(451-453). Toxicity of iron may lead to tissue damage or cancer. High iron intakes 

have also been related to gastrointestinal distress. The toxic potential of iron derives 

from its ability to exist in 2 oxidative states (ferrous and ferric forms). Iron serves as a 

catalyst in redox reactions; however, when these reactions are not properly modulated 

by antioxidants or iron-binding proteins, cellular damage can occur (454). Adaptation 

of iron metabolism in humans typically allows the maintenance of normal body iron 

concentrations in spite of disparate physiological requirements and dietary supply (455). 

Body iron, about 4 g in the adult human, is determined by physiological iron demands, 

dietary supply, and adaptation (447, 455, 456). Dietary iron is a function of both content 

and bioavailability of total food iron; bioavailability is lower in non-heme than in heme 

iron sources. Dietary factors that inhibit iron absorption include tea, coffee, bran, 

calcium, phosphate, egg yolk, polyphenols, and certain forms of dietary fiber (447). 

Conversely, meat, fish, poultry, and ascorbic acid will enhance the bioavailability 

of non-heme iron. 

Iron is known to be involved in immune system function—specifically, adaptive and 

innate immune response—and both iron overload and iron deficiency affect immune 

function. As reviewed by Dao and Meydani, iron overload can affect susceptibility to 

infection (457). On the other hand, iron deficiency affects the function of certain immune 

cells, including neutrophils and natural killer cells, and production of cytokines (457). 

As with any nutrient, supplementation must be used with caution, as some have found 

that in areas of the world where infection rates are high, such as malaria-endemic 

regions, iron supplementation can actually increase risk of infection, suggesting that 

the supplemented iron provides an environment for pathogens to thrive (458). Others 

have shown that iron deficiency can help protect against some types of infections (459). 

Evidence from short- (weeks) and long-duration (months) space missions shows that 

red blood cell (RBC) mass decreases during flight because of neocytolysis (460, 461). 

An early hypothesis for the cause of decreased RBC mass was that RBC synthesis 
46

in space was understimulated relative to synthesis on the ground (462). Decreased 

release of mature RBCs into the circulation is associated with a decrease in circulating 

erythropoietin concentrations. Serum erythropoietin decreases in the first few days of 

space flight, but it returns to preflight levels later and iron turnover is unchanged during 

flight (461, 463), indicating that synthesis of RBCs and hemoglobin is unchanged. 

A consequence of the decreased red blood cell mass is the subsequent transfer of 

the iron from newly synthesized cells into storage proteins and processes. Evidence 

of this includes increased circulating concentrations of serum ferritin, an index of iron 

storage, after short- and long-duration space flights (6, 430, 464). In addition to these 

physiologic changes that can affect tissue iron stores, dietary iron content is very high 

in the International Space Station (ISS) food system, largely because many of the 

commercial food items in the ISS menu are fortified with iron (2). The mean iron content 

of the standard ISS menu is 20 ± 6 mg/d, and individual crewmembers have had intakes 

in excess of 47 mg/d for some weeks during long-duration missions. For reference, 

the defined space flight requirement for iron is 8-10 mg/d for both men and women 

(2, 425), and the current US Dietary Reference Intake (DRI) for men is 8 mg/d and the 

DRI for women is 10 mg/d (465). The tolerable upper intake limit for iron as defined 

by the Institute of Medicine is 45 mg/d (465). 

Indices of iron metabolism and erythropoiesis return toward normal relatively quickly 

(days) after landing, although the replenishment of RBC mass may take several weeks. 

The repletion of RBCs usually occurs after the disproportionate return of plasma volume, 

so that a dilutional “anemia” often occurs after flight (466). For example, a 3% to 5% 

decrease in hematocrit between landing (R+0 d) and R+3 days is common after both 

short- and long-duration flights (466). 

Although the in-flight decrease in RBC mass is substantial, the efficient postflight 

recovery suggests that the change represents an adaptation to weightlessness. After the 

first weeks of flight, RBC mass and body fluid volumes reach new plateaus (lower than 

on Earth), as shown by data from long-duration flights (35, 428, 467, 468). The triggering 

mechanism for these changes is unknown. One hypothesis is that the body senses a 

decreased requirement for blood volume and adapts accordingly. This may be related to 

changes in fluid (circulatory) dynamics and reduced gravitational strain on the circulatory 

system during flight, which may result in easier delivery of oxygen to tissues, or to the 

decreased plasma volume and increased concentration of RBCs in the first few days of 

space flight. The decrease in RBC mass has no documented functional consequences. 

In-flight data show that iron stores increase early during a mission (within 15 d) and then 

return to preflight concentrations by the end of a 6-month mission (469). In a recent 

study with 23 crewmembers of missions 50 to 247 days in duration, ferritin increased 

about 220% in women and 70% in men by flight day 15 (469). At several time points, 

the transferrin index exceeded 1 μmol iron/μmol transferrin, which provides evidence 

that iron overload occurred (470). Other acute-phase proteins (C-reactive protein and 

ceruloplasmin) were not changed during flight, indicating that the ferritin response was 

likely not just an inflammatory response. In this study the amount of increase in ferritin 

(area under the curve) was associated with the change in bone mineral density after 

flight, which was supported by the association between ferritin and other markers of iron 

status and markers of bone resorption. The greater the increase in ferritin during flight 

(or the longer it was elevated; either case would result in a greater area under the curve), 

the greater the decrease in bone mineral density in the hip, trochanter, hip neck, and 

pelvis after long-duration space flight (469). The change in ferritin over the course of 
47

a 6-month mission is presented 

in Figure 32, and is very 

similar to the change in urinary 

8-hydroxy-2'-deoxyguanosine 

(8OHdG, a marker for oxidative 

damage) during space flight). 

These data are important 

to show that mean ferritin 

concentrations during flight 

that were not outside the 

normal clinical range were 

associated with evidence Figure 32. The percent change from preflight of oxidative damage and (mean ± SD) in serum ferritin (red circles, solid line) bone resorption, and this is and urinary 8OHdG (blue squares, dashed line) 

supported by other studies before, during, and after long-duration space flight 

in healthy ground-based (N=23). “Pre” was determined from the mean of 

populations (471-473). preflight data points (3 for ferritin, 4 for 8OHdG), 

and percent change was calculated from that Bed rest studies have not average. (469) 

proven to be consistently 

reliable models for the 

hematological changes of space flight. Early bed rest studies showed a decrease in 

RBC mass during bed rest, but erythropoietin was unchanged and hematocrit increased 

(474), suggesting that the mechanisms that bring about hematological changes during 

bed rest are different from those that act during flight. If the reduced RBC mass 

during flight is caused by the reduced gravitational load on the circulatory system, it 

is reasonable to assume that bed rest alone would not alleviate these forces, but would 

only reposition them. More recent studies have shown small changes in iron status 

measurements, the most consistent being a drop in hematocrit and hemoglobin after 

reambulation (134, 283), suggesting an impact of plasma volume replacement, with 

a smaller role of hematopoiesis. 

Another model is provided in studies involving changes in altitude, where the descent 

from high to low altitude induces changes similar to those observed for space flight 

(decreased red cell mass, increased iron storage) (475). Exogenous erythropoietin 

prevented the changes (475), suggesting that it is involved in the regulating mechanism, 

as it may be in the initial change in space flight. 

The NASA Extreme Environment Mission Operations (NEEMO) undersea environment 

provides an excellent space flight analog for changes in iron status, specifically with 

respect to the environment in the NEEMO habitat (476). Because of the increased air 

pressure in the habitat, crewmembers are exposed to higher oxygen pressures, which 

increase their risk for oxidative damage to DNA, proteins, and lipids (477-480). Probably 

because of the increased pressure and greater oxygen availability, body iron stores 

are elevated during the saturation dive (6, 464). Serum ferritin is routinely increased 

during the NEEMO saturation dives (10-14 d long), and evidence for oxidative damage 

and stress is also observed (476). On a recent NEEMO mission, red blood cell folate 

was decreased during the dive, and plasma folate status was inversely correlated with 

serum ferritin (481). Decreased superoxide dismutase activity and peripheral blood 

mononuclear cell poly(ADP-ribose) were also evident during the dive, indicating a 

DNA repair response occurred (481). 
48

The implications of moderately increased iron stores in the body include exacerbated 

bone loss, oxidative stress, cardiovascular disease, and cataracts or other ophthalmic 

issues. For example, ground studies show that in healthy subjects, increased body 

iron stores (assessed by measuring serum ferritin) were related to the rate of change 

in regional bone loss over a 3-year period (482). This finding supports what we have 

observed during space flight (469). Other risks with iron overload are retinal degeneration 

and cataract risk (483). We have seen that, in rat studies of iron loading plus radiation 

exposure, increased oxidative damage occurs in the retina as well as systemically and 

in the liver (484). Furthermore, the formation of free radicals subsequent to elevation of 

iron stores has been linked on Earth to cardiovascular disease and cancer. Although 

aspects of some of the evidence supporting this thesis contradict each other (485, 486), 

a correlation between coronary heart disease and iron status has been described in a 

number of recent studies (487-489), and an association between increased incidence 

of myocardial infarction and increased iron stores (as measured by serum ferritin) has 

been observed (489, 490). In a prospective Finnish study, increased risk of all cancer 

types combined and colorectal cancer in particular was associated with high iron stores 

(491). The relationship between iron, lipids, and cancer has also been documented in 

the Framingham study (492). A relationship has also been indicated between excessive 

iron stores and ascorbic acid deficiency; when reductions in ascorbic acid occur, vitamin 

A and selenium tend to exacerbate iron-induced peroxidation processes (493). These 

data suggest that the alterations in erythropoiesis and iron metabolism that occur in 

microgravity could cause significant changes affecting crew health. 

Better characterization of iron metabolism during space flight with respect to other 

systems is warranted because of the high levels of dietary iron, the increase in iron 

stores early during flight, and the potential for iron to act as an oxidizing agent during 

space flight, complicated by increased radiation levels. It is known that bacterial 

virulence increases upon exposure to microgravity (494), and ground studies also 

show that increased iron status can increase risk for infection (495). Investigating the 

increase in iron status during flight with respect to changes in immune function will be 

an important next step in understanding the implications of elevated iron status during 

space flight. Furthermore, iron absorption has yet to be determined during flight. 

Copper 

Copper is an essential cofactor for enzymes involved in energy production, metabolism 

of oxygen and iron, maturation of the extracellular matrix and neuropeptides, and 

neuroendocrine signaling (496). Deficiencies in copper have implications for bone health, 

the nervous system, immune function, the cardiovascular system, and lipid metabolism 

(496). The involvement of copper in bone health is specifically related to lysyl oxidase 

function and collagen synthesis (271, 496). 

Copper is not usually stored in tissues, but liver, brain, and kidney typically contain 

the largest amounts per unit tissue mass (496). Total body copper is about 50 to 

120 mg (0.79-1.9 mmol) (497). Copper transport and regulation involve the blood 

protein ceruloplasmin. 

Frank copper deficiency is rare in human populations consuming a normal diet; 

however, copper deficiencies have been noted in infants fed milk formulas, infants 

recovering from malnutrition and fed cow’s milk, and patients receiving total parenteral 
49

nutrition for a prolonged period (498). Six patients fed (through the gastrointestinal 

tract) a diet containing 15 μg copper/100 kcal for 12 to 66 months (499) developed 

a copper deficiency. 

When copper deficiency occurs it leads to normocytic, hypochromic anemia; decreased 

production of leukocytes and neutrophils; and defects in connective tissue (specifically 

in collagen synthesis) that can lead to vascular and skeletal problems and central 

nervous system dysfunction, or even death (498). Heartbeat irregularities have also been 

reported in cases of copper deficiency (500). Deficiency symptoms, including macrocytic 

anemia, bone abnormalities, and decreased neutrophil production, have been reported 

in subjects with serum copper concentrations ranging from 0.9 to 7.2 μmol/L (499). 

Toxic concentrations of copper lead to oxidative damage, gastrointestinal distress, 

liver damage, or even death (496). 

Serum copper and ceruloplasmin of ISS crews have been determined as part of the 

medical requirement to assess nutritional status in long-duration crewmembers, with 

no significant changes observed after flight (6). In-flight determinations of copper status 

have not yet been reported. 

One Russian report on the effect of space flight on copper content of bones (431) 

documented “non-uniform changes” in copper content of bone from different regions 

after flight relative to nonflight controls. Copper content of the femoral epiphysis was 

81% to 159% greater, while the amounts of copper in the vertebral body and sternum 

were 36% and 58% less, respectively. (This study reported on the autopsy results after 

the tragic end of the 24-day Salyut-1 mission, relative to controls.) 

During a 17-week bed rest study, copper balance was unchanged, but after 

re-ambulation it increased (435). During and after 3 weeks of bed rest, serum copper 

and ceruloplasmin were unchanged (134). After 90 days of bed rest, serum copper 

was slightly elevated, but the change was statistically significant (283). In 60- and 

90-day bed rest studies, ceruloplasmin was unchanged (283). 

Changes in copper status could contribute to the effects of space flight on bone, red 

blood cells, and iron status. The changes in bone during space flight, described in this 

volume, could be exacerbated by copper deficiency and impaired collagen synthesis. 

Anemia of space flight is manifested as a reduction in circulating red blood cell mass 

with elevations in serum ferritin and iron concentrations (35, 461). Since copper is 

required for iron mobilization and absorption, alterations in copper status may affect 

iron and red blood cell changes during flight. 

Appropriate amounts of certain nutrients, copper in particular (501), are vital for 

maintaining normal immune function. The immune system seems to be altered during 

space flight (77, 502-505), and this may have direct or indirect (when alterations are 

induced by stress or radiation) implications for nutrition and nutritional status as 

possible causes or effects (503, 506). 

No information about copper absorption and metabolism during space flight is available, 

but, given the available ground data, obtaining such information is not a high priority at 

this point. Ensuring adequate copper content of the diet and verifying that the flight data 

on copper status follow ground trends are important monitoring steps. 
50

Josef F. Schmid, NASA flight surgeon, comes face-to-face with a grouper through the observation 

window of his undersea habitat during the 12th NASA Extreme Environment Mission Operations 

(NEEMO) mission aboard the National Oceanic and Atmospheric Administration's Aquarius 

Underwater Laboratory, which is operated by the University of North Carolina at Wilmington 

and located off the coast of Key Largo, Florida. 
51

6 Cardiovascular Health 

Cardiovascular issues are a key concern for space travelers (507-513), but the role of 

nutrition in cardiovascular adaptation has not (yet) been well characterized. In a few 

areas there is some degree of evidence (described below), but many more have yet to 

be examined. It is worth noting that multiple studies are being planned or are underway 

on ISS and in bed rest models that may help shed light on this area in the near future. 

Energy 

As discussed in section 2, “Energy and Fuel Metabolism,” striking data show that 

cardiovascular deconditioning and loss of plasma volume are negatively correlated 

with energy consumption during bed rest (63) and after space flight (Figure 33). 

That is, insufficient energy intake is associated with greater plasma volume loss and 

cardiovascular deconditioning. The space flight data came from the work of Dr William 

Carpentier, who evaluated crewmember medical records from the Mercury, Gemini, 

and Apollo programs. These data have yet to be published in the scientific literature, 

but a book in which they are compiled is in final development. 

Dr Carpentier’s data from astronauts in the early US space programs have been 

integrated and modeled to predict postflight heart rate response to lower body negative 

pressure (LBNP), standing, and tilt from factors including flight duration, plasma volume 

loss or energy intake, and preflight resting heart rate. The plots in Figure 33 and Figure 

34 are based on Gemini and Apollo mission data. Combined with the data presented 

in Figure 35, these data clearly link energy intake and plasma volume loss with 

cardiovascular health during and after space flight. 

Figure 33. Relationship between energy intake 

(kcal/kg body mass/d) and plasma volume loss 

(mL/d) during Apollo missions. N = 21. Data are 

courtesy of William Carpentier. 
52

Figure 34. Postflight heart rate under 3 conditions, predicted (from factors including 

plasma volume loss) versus actual measured heart rate. These data were calculated from 

the following equations: 

LBNP: 29.00 – (0.064 x flight duration) + (1.45 x resting preflight heart rate) + (4.2553 x plasma 

volume loss, mL/h flight) 

Standing: 77.23 – (0.52 x flight duration) + (1.9098 x preflight resting heart rate) + (17.4758 x 

plasma volume loss, mL/h flight) 

Tilt: 330 – (2.53 x preflight resting heart rate) – (7.20 x plasma volume loss, mL/h flight) – 

(0.078 x flight duration, h) 

Heart rates for LBNP and tilt were those recorded at LBNP = -50 mm Hg and at 70° tilt. 

For the stand test, crewmembers stood relaxed against a wall with their heels 6 inches 

away from the wall. Data are courtesy of William Carpentier. 

Figure 35. Postflight heart rate, predicted (from factors including energy intake) versus 

actual measured heart rate. These data were calculated from the following equations: 

LBNP: 63.02 + (1.18 x preflight resting heart rate) + (0.48 x energy intake, kcal/d/kg) – 

(0.17 x flight duration)
 
 

Standing: 115.87 + (1.12 x preflight resting heart rate) – (2.17 x energy intake, kcal/d/kg) – 
 


(0.17 x flight duration)
 
 

Tilt: 239.48 – (0.75 x preflight resting heart rate) – (1.86 x energy intake, kcal/d/kg) – 
 


(0.052 x flight duration)
 
 
53

Similar findings relating energy intake and cardiovascular deficits were obtained from 

bed rest studies led by Dr Martina Heer to evaluate the effects of hypocaloric diets on 

many physiological systems (61, 183). The cardiovascular data showed that caloric 

restriction during bed rest led to decrements in cardiovascular physiology (specifically, 

performance on a stand test), exceeding the decrements that occurred during bed rest 

when subjects received adequate calories (unpublished results). 

One piece of the space 

flight puzzle that is still 

missing is the effect of 

flight duration. That is, the 

data presented in Figure 

33, Figure 34, and Figure 

35 were generated from 

short-term flights, and this 

relationship may change on 

longer ISS missions. Given 

the inference from Figure 33 

that energy intake should 

be greater than 33 kcal/kg 

body mass to avoid plasma Figure 36. Energy intake during space flight on ISS 

volume loss, we evaluated ISS missions (N=56). Each point represents an individual 

intake data (Figure 36) and crewmember, and is their reported average daily energy 

intake over the course of the mission. The dashed line found that few crewmembers 

represents 33 kcal/kg body mass, the energy intake are meeting this threshold. needed to avoid plasma volume loss. Solid lines 

represent the mean ± 1SD. 

Magnesium 

As mentioned in other sections, magnesium has benefits for metabolism of bone and 

calcium and for reduction of renal stone risk. In addition, magnesium has been shown 

to have effects on the cardiovascular system (514-516). Consistently decreased 

magnesium excretion after flight is a concern for many reasons (see section 4, “Bone,” 

for details), and warrants further investigation with respect to cardiovascular 

deconditioning during and after flight. 

Ongoing and Future Research 

Oxidative Stress 

The role of oxidative stress and antioxidants in cardiovascular adaptation to space flight 

is currently being studied on ISS. Although astronauts are exposed to several oxidative 

stressors during flight (eg, radiation, EVA, iron stores, exercise), abundant evidence 

exists of additional oxidative stress during space flight (reviewed later), and a wealth 

of literature reports effects of oxidative stress and damage on the cardiovascular system 

on Earth, these three areas have never been evaluated. A study initiated on ISS in 2013 

aims to accomplish this goal, evaluating oxidative damage during flight, as well as 

evaluating long-term effects after flight. 

The findings from these studies may have significant implications for future missions. 

As described later in this book, oxidative stress is a multifaceted issue that affects many 

systems, and given the radiation concerns of exploration-class missions, it will draw 

greater attention in the future. 
54

Omega-3 Fatty Acids 

Omega-3 fatty acids have a clear beneficial impact on cardiovascular health on Earth, 

but such effects have not been evaluated during space flight. Nonetheless, the initial 

efforts being made to increase fish and omega-3 fatty acid intake in astronauts for the 

benefit of other systems (bone, muscle) will likely have positive effects here as well. 

Healthier Diets 

While individual nutrients are easier to study in a controlled, experimental fashion, 

the effect of overall dietary quality is one topic continuing to gain ground, particularly 

as studies of individual supplements fail to produce the “magic” supplement. Overall 

dietary quality, including fruit and vegetable intake, fish (omega-3 and vitamin D) intake, 

and intake of foods rich in phytochemicals and lower in sodium, has broad health effects 

(517-522). Many of these effects clearly have an impact on the cardiovascular system, 

along with benefits for bone, muscle, kidney, and other systems. 

In 2014, NASA embarked on an “Integrated Nutrition” study, aimed at providing healthier 

food options for astronauts on ISS missions. As of this writing, it is hoped that the first 

flight of this experiment will be in 2015. Bone is a primary end point, but clearly this 

study will have implications for many other physiological systems (eg, cardiovascular, 

muscle, immune) and other risks of space flight (eg, radiation and oxidative stress), 

not to mention potential benefits for crew performance and morale. The outcome of 

this study will provide a solid backdrop for the definition of food system and nutrient 

requirements for exploration missions, and may also have significant implications for 

understanding the role of nutrition in disease prevention here on Earth. 

NASA astronaut Karen Nyberg, Expedition 36 flight engineer, eats a meal at the galley in the Unity 

node of the International Space Station. 
55

7 Sodium, Potassium, Fluid, 

and Renal Stone Risk 

Sodium and Chloride 

Sodium is the major cation of extracellular fluid (523). Together with chloride, sodium 

is used by the body to maintain normal water distribution, osmotic pressure, and anioncation balance in the extracellular fluid compartment (524). Electrolyte concentrations 

in the body are essential for proper cardiovascular function and are under renal and 

hormonal control (525). Increases in blood sodium levels can be caused by diabetes, 

renal polyuria, diarrhea, insufficient water intake, excessive sweating, or increased 

dietary sodium intake. Sodium levels decrease with edema, excessive water intake, 

vomiting, diarrhea, diuretic therapy, renal tubular damage, hyperaldosteronism, or 

lower dietary intake. 

For the normal adult, total body sodium averages about 60 mmol/kg body weight. 

Forty to 45 percent of total sodium resides in bone, with the balance found in 

extracellular and intracellular fluid. These sodium stores are classified as either 

exchangeable (42 mmol/kg body weight) or nonexchangeable, the exchangeable 

stores being composed of all cellular sodium and less than half of bone sodium (526). 

Exchangeable sodium becomes available by diffusion when plasma sodium levels 

become low, and in states of edema, the exchangeable sodium stores absorb sodium. 

Animal studies show that symptoms of sodium deficiency occur after 3 to 4 weeks 

of dietary sodium restriction (527). During acute starvation, urinary sodium excretion 

decreases to less than 0.2 g within 10 days (528), and can be affected by the amount 

of sweat (529). Plasma sodium levels are maintained fairly well during acute starvation: 

an initial decrease is followed by a return toward normal values (530). Maintenance 

of blood sodium is also observed during semi-starvation. During the Minnesota 

Experiment, plasma sodium levels in samples taken after the 6-month semistarvation period were 0.6 ± 7.3% higher than baseline levels (N = 4) (529). 

Six days of undernutrition resulted in large negative balances of sodium chloride 

(–12.8 ± 3.6 g/d), likely related to changes in water balance (529). 

Pre-, in-, and postflight plasma sodium and chloride data are available from Apollo, 

Skylab, and Shuttle flights, and have been reviewed extensively (2, 21, 531-534). 

In-flight daily sodium intakes during Skylab and Shuttle missions averaged 4 to 5 g, 

and were similar to the astronauts’ preflight intakes (32). The current food system is 

high in dietary sodium, and typical intakes on ISS have been in excess of 4.5 g, even 

with suboptimal food intake (6). Intakes as high as 7 to 10 g of sodium per day have 

been observed. Sodium homeostasis and blood sodium levels are maintained during 

real and simulated space flight (535), but the high sodium content of the current 

space food system makes it important to monitor and restrict dietary sodium intake 

of astronauts to maintain their bone and renal health. 

European studies with Mir crewmembers documented positive sodium balance during 

space flight, in a non-osmotic fashion (that is, without a concomitant increase in fluid 

compartments) (21, 30, 533, 534, 536, 537). These data were confirmed in a series of 

ground-based studies, documenting an increase in messenger RNA expression of some 
56

of the enzymes required for glycosaminoglycan syntheses in the skin, the displacement 

of sodium by hydrogen in the glycosaminoglycans, and a subsequent acidosis 

(537-543). These findings of increased sodium-proteoglycan binding had already 

been observed in animal studies by Ivanova in the 1970s (544). The skin here functions 

as a reservoir that stores sodium in case of overconsumption and releases sodium 

when it is insufficiently supplied. In recent studies carried out by the group of Titze et 

al (538, 539, 541) in animals and summarized in a mini-review (541), they demonstrated 

the involvement of the lymph capillary system in the clearance of sodium and chloride 

from the skin. Increasing density of lymph capillaries in the skin seemed to play one of 

the key roles in this clearance, and when hyperplasia of the cutaneous lymph capillary 

system was inhibited, skin sodium and chloride retention was augmented and led to 

increased blood pressure. 

Sodium is also stored in bone; however, sodium stored in bone does not seem to 

be exchangeable and therefore does not take part in day-to-day sodium regulation. 

However, on Earth, excessive sodium intake has been associated with increased bone 

turnover (545-547). Dietary sodium is known to affect calcium homeostasis (548-553). 

A predictable relationship exists between urinary sodium and calcium; that is, for 

each 100 mmol of sodium excreted in urine, 1 mmol of calcium is excreted (554). 

This phenomenon is expressed at high levels of dietary sodium. More than 90% of 

dietary sodium is absorbed, even when intake is high (555). Sodium is excreted mostly 

in the urine, but about two-thirds of the sodium filtered by the kidney is reabsorbed 

by mechanisms thought to involve solvent drag and electrochemical gradients. 

The sodium-dependent calcium transport system uses the energy stored in the 

electrochemical gradient of sodium to drive calcium into the lumen of the proximal 

renal tubule, and ultimately the presence of calcium in this location leads to increased 

calcium loss secondary to increased sodium excretion. In the distal tubule, calcium is 

preferentially reabsorbed, an event stimulated by parathyroid hormone (PTH) and cyclic 

adenosine monophosphate (cyclic AMP) (556). Cyclic AMP also influences reabsorption 

of sodium (557). 

A small amount of sodium is excreted in feces. When 550 mmol sodium was ingested 

each day for 7 d, the average fecal excretion was 1.8 ± 0.4% of the total dose, and 

when smaller amounts of sodium were ingested (50 mmol/d), an average of 6.0 ± 1.0% 

was excreted in the feces (2). 

Salt loading alone increases intestinal calcium absorption. In hypoparathyroid patients, 

dietary salt increased intestinal calcium absorption in one study by Meyer (558) but not 

in another study by Breslau (559). In Breslau’s study, calcium absorption correlated with 

serum 1,25-dihydroxyvitamin D. Thus, conclusions about the role of PTH in the increase 

in intestinal calcium absorption after a sodium load are speculative. 

Studies in premenopausal women suggest that increased intestinal calcium absorption, 

rather than increased bone resorption, compensates for sodium-induced hypercalciuria 

in subjects whose adaptive processes related to bone metabolism are intact (560, 561). 

Ginty et al (560) examined the effects of 7 days of high or low dietary sodium 

on bone markers in young women. Although urinary calcium was increased with high 

(180 mmol/d) sodium intakes, the effect of high sodium on markers of bone resorption 

was not different from the effect of low (80 mmol/d) sodium intakes. Lietz et al (561) also 

found no effect of intakes of 170 mmol/d or 60 mmol/d of sodium for 8 days on bone 

resorption markers in postmenopausal women. However, Evans et al (547) reported that 

postmenopausal women ingesting 300 mmol sodium per day for 7 days had greater 

excretion of bone resorption markers than those ingesting 50 mmol sodium per day. 
57

These differences were not observed in a premenopausal group (547). These results 

suggest that bone resorption is increased in situations where the adaptive responses 

of bone are limited or altered, as they are after menopause. It might also suggest that 

at levels above 200 mmol sodium intake per day, the regulatory processes are different. 

Data from human and animal studies suggest that high dietary sodium chloride leads 

to bone loss due to increased bone resorption (562-568), and even that restriction of 

dietary sodium will reduce bone resorption (569). In a review of the interactions between 

dietary salt, calcium, and bone, Massey and Whiting (567) suggested that habitual 

excessive salt intake contributes to bone loss. Other reviewers have come to the 

conclusion that increased dietary sodium chloride intake negatively affects acid-base 

balance, with subsequent loss of calcium (570, 571). 

Massey and Whiting (567) found that the effect of excessive salt intake on bone loss 

is modulated in specific subpopulations. For example, people who tend to form renal 

calcium stones are more responsive to changes in dietary salt than are non-stone 

formers. Although sodium intakes of stone formers are typically similar to those 

of controls (572, 573), the detrimental effects of high sodium intakes on renal stone 

risk have been well documented (562, 568, 571). Increasing sodium intake from 

50 mmol/d to 300 mmol/d increased renal stone risk by elevating urinary saturation 

of calcium phosphate and monosodium urate, and reducing inhibition of calcium 

oxalate crystallization (574). 

Work by Goulding (545, 546) and Matkovic (575) has generated interest in the effect 

of dietary sodium on bone mass. High levels of dietary sodium are not only major 

predictors of urinary calcium and hydroxyproline excretion, but are also associated 

with greater loss of bone with age, unless dietary calcium is supplemented (576). 

Work by Dr Heer’s group has also documented the resorptive response to high dietary 

sodium, and the role of acid-base balance in this process (577). 

Dietary sodium also seems to exacerbate the calciuric responses to musculoskeletal 

unloading in weightlessness (Figure 37). Bed rest subjects consuming a low-sodium 

diet (100 mmol/d) had no change in urinary calcium, while those on a high-sodium 

diet (190 mmol/d) had hypercalciuria (578). A more recent bed rest study by Heer et 

al documented that the high-sodium-induced increase in bone resorption exceeded 

the bed rest-induced increase, through a mechanism mediated by acid-base balance 

(577, 579, 580). One mechanism could be that metabolic acidosis causes an increase 

in urinary corticosterone (581). Increased sodium intake and consequent low-grade 

metabolic acidosis caused an increase in bioactive glucocorticoids (540, 582). 

In turn, the increase in glucocorticoid concentration caused rapid bone loss (583-586). 

Applying an alkaline salt together with high sodium intake (582) reduced bioactive 

glucocorticoid excretion, which result supports the idea that acid-base balance plays 

a role in the effects of high sodium intake. A symposium was held recently in Germany 

(its proceedings were published in 2008) regarding the impact of acid-base balance 

on health issues (282), including the role of sodium in bone loss (587-589). 

High sodium intakes during space flight can exacerbate bone loss and lead to increased 

risk of renal stone formation. In and of itself, excess sodium can lead to hypernatremia, 

hypertension, and even death. Although it has not been a concern to date, too little 

dietary sodium or a deficiency of this electrolyte during flight could lead to hyponatremia, 

hypotension, and even death. 
58

Figure 37. Proposed mechanism of the effects of high dietary sodium on bone loss. 

Further research is required to investigate potential effects of high sodium intake during 

space flight, as the space food system currently has very high sodium levels. The impact 

of high sodium intake on bone, calcium, and pH is not well understood, and adjustments 

in sodium intake may serve as a viable countermeasure to bone loss. Furthermore, 

the role of a high-sodium diet in potassium homeostasis is not well understood. 

This may prove to be an area where nutrition and cardiovascular effects of space flight 

may interact, and study of the interaction may produce a dietary countermeasure. 

Potassium 

As the most plentiful intracellular cation, potassium has a significant role in several 

physiological processes (525). It is crucial to regulation of acid-base balance, energy 

metabolism, blood pressure, membrane transport, and distribution of fluid within the 

body. It is also involved in the transmission of nerve impulses and cardiac function (590). 

Potassium metabolism that is disordered because of excessive or deficient circulating 

levels has negative consequences for cardiac, muscle, and neurological function. 

Deficiency of potassium leads to hypokalemia, muscle weakness, constipation, and 

fatigue, or even death. No evidence of adverse effects is associated with toxicity of 

potassium from naturally occurring sources. However, supplemental intake may cause 

hyperkalemia (and associated weakness, cardiac arrest, and paralysis), metabolic 

acidosis (282), decreased neuromuscular function, or even death. 

Serum and urinary levels of potassium were both decreased after space flight in 

Apollo crewmembers (591), and evidence exists that a similar decrease occurred 

in Skylab crewmembers (29). 
59

Loss of both total body potassium and exchangeable potassium was observed in 

Apollo crewmembers (591). Increased levels of urinary potassium during space flight 

may be related to muscle disuse atrophy and inadequate intake (535). In the initial days 

of bed rest, excess dietary sodium was shown to be potassium-depleting (Heer et al, 

unpublished observations). Loss of lean body mass, along with high sodium intake, 

may also result in potassium depletion. 

The relationship between bone health and the protein:potassium ratio in the diet 

needs to be further investigated, along with the role of potassium in cardiovascular 

health during flight. 

Fluid 

Adequate fluid intake is necessary to maintain the body’s normal hemodynamic state 

and normal fluid osmolality, which are important for cardiovascular health and for 

maintenance of fluid and electrolyte homeostasis. Water is a structural component of 

the body and the solvent for transportation of nutrients and waste. Fluid and electrolytes 

may be lost from the body by a variety of routes and for a variety of reasons. They are 

excreted in sweat, urine, and feces, and in abnormal situations excessive amounts 

can be lost by these routes and others. Significant losses may occur through the 

gastrointestinal tract as a result of diarrhea, vomiting, or gastric drainage. Loss through 

the skin increases with fever, increased metabolism, sweating, and burns (523). 

Total body water makes up about 50% to 70% of body mass (592). Fluid requirements 

increase with metabolic rate and heat stress. Death from dehydration can occur within 

weeks of depriving the body of all water (593). 

Fluid and electrolyte homeostasis is significantly altered during space flight, and this 

has been extensively reviewed (19, 30, 531, 594-600). The hypothesis originally 

proposed to explain this was that upon entering weightlessness, the human body 

would experience a headward shift of fluids, with subsequent diuresis and dehydration. 

A series of flight experiments was conducted to assess fluid and electrolyte homeostasis 

during space flight; the most comprehensive of these took place on the 2 Spacelab Life 

Sciences missions in the early 1990s. Despite much research, the hypothesis of diuresis 

and subsequent dehydration secondary to the headward fluid shifts has never been 

confirmed during actual space flight (19, 531, 533, 534, 599, 601). 

Within hours of the onset of weightlessness (the earliest available data point), a reduction 

in both plasma volume and extracellular fluid volume had occurred, accompanied by 

the “puffy” faces typically observed early in flight (19, 602). Initially, the decrement in 

plasma volume (~17%) was larger than the decrement in extracellular fluid volume 

(~10%), suggesting that interstitial fluid volume (the other four-fifths of extracellular 

fluid) is conserved proportionally more than plasma volume (19). The idea that interstitial 

fluid volume is conserved is supported by rapid decreases in total circulating protein, 

specifically albumin (19), indicating that protein, and associated oncotic pressure, shifted 

from the intravascular to the extravascular space. This would have facilitated the initial 

changes in plasma volume (19). 

Following the initial adaptation, extracellular fluid volume further decreased between 

the first days of flight and 8 to 12 days after launch, from the initial ~10% below preflight 

levels to ~15% below preflight levels (19). Plasma volume was partially restored during 

this period, from the initial ~17% below preflight levels to ~11% below preflight levels 

(19), and it has been found to remain 10% to 15% below preflight levels even for 

extended-duration flights (603). 
60

Leach et al (19) and Norsk et al (534) have hypothesized that the shift of protein and fluid 

to the extravascular space represents an adaptation to weightlessness, and that after 

several days, some of the extravascular albumin has been metabolized, with a loss of 

oncotic force and a resulting decreased extracellular fluid volume and increased plasma 

volume. This loss of extracellular protein (intra- and extravascular) and the associated 

decreased oncotic potential probably play a role in postflight orthostatic intolerance, 

which has been considered to result partly from reduced plasma volume at landing 

(604). Furthermore, the loss of protein may in part explain why fluid loading alone does 

not restore circulatory volume (605, 606), as no additional solute load exists to maintain 

the fluid volume. Another potential (or perhaps partial) explanation for the failure of 

fluid loading is that because astronauts’ diets are high in sodium, additional salt cannot 

help increase plasma volume or extracellular fluid volume. This explanation has been 

documented in bed rest (537). 

The effect of space flight on total body water has been evaluated to assess hydration. 

Shuttle and Skylab astronauts had decreases of about 1% in total body water during 

flight (19, 607, 608), and the percentage of body mass represented by water did not 

change. Thus, the often-proposed weightlessness-induced dehydration does not exist. 

This has also been shown by European investigators on Shuttle and Mir missions 

(534, 536, 599, 601, 609). 

Diuresis is also typically not observed during flight (59, 531, 534, 536, 594, 601, 

610-612), for a number of possible reasons. Operational constraints have made it 

difficult to document urine volume accurately on the first day of space flight. However, 

on the Spacelab Life Sciences missions, urine volume on the first 3 days of flight was 

significantly less than preflight volume, and urine volume tended to be less than preflight 

volume throughout the flight (19). Urine volumes on a week-long flight to Mir were also 

less than preflight volumes (611). During the first week of the 59- and 84-day Skylab 

flights (29), urine volume was less than it was before flight, and for the remainder of the 

missions it was unchanged from preflight levels. Decreased fluid intake likely accounts 

for the decreased urine volume, which was accompanied by little or no change in total 

body water. Adequate urine volume during flight is important for reducing the risk of 

renal stone formation (298, 299, 613, 614). 

As mentioned above, the percentage of body mass represented by total body water 

is relatively unchanged during flight (19). However, on a volume basis, the change in 

extracellular fluid volume was found to be greater than the change (or lack of change) 

in total body water (19). Thus, by difference, intracellular fluid volume increased during 

space flight. This had been previously hypothesized from ground-based studies (615) 

and observed in postflight studies of Apollo crewmembers (59). The mechanism for 

a space flight-induced increase in intracellular fluid volume is unknown. One possible 

explanation is that a shift in fuel utilization results in increased glycogen storage, 

a condition known to increase cellular water content. 

Diuresis has been documented to occur in bed rest studies (616-618). Urinary albumin, 

a marker of kidney function, has been shown to be reduced in both space flight (relative 

to before flight) and bed rest (relative to the ambulatory state) (157-159). However, space 

flight, but not bed rest, results in reduced urine flow rates (601). Taken together, these 

data suggest that differences in fluid metabolism exist between analog studies and 

actual space flight (533, 534, 599, 601, 609, 612, 618). Such differences do not seem to 

be a simple effect of abnormal renal function, and thus require further investigation (619). 

Although no space flight-induced dehydration occurs, care must be taken to ensure 

adequate fluid intake and hydration status. Inadequate fluid intake increases the risk 
61

of dehydration and renal stone formation. Fluid intake during flight is typically less than 

preflight intake, and often below the recommended quantity. In closed flight vehicles, 

water is often a limiting resource, but rationing of water should be avoided. 

Deficiency of fluid leads to dehydration and ultimately death. Likewise, an excess of 

fluid intake leads to water intoxication and ultimately death. Obviously, the risk of this 

occurring during space flight, where water is a limited commodity, is extremely low. 

Decreased fluid intake during space flight may be a consequence of reduced thirst 

during flight (86), but the reason for reduced thirst is unknown. 

Studies described above have documented that total body water is unchanged 

during flight, but apparently a shift of fluid from the extracellular to the intracellular 

compartment occurs. The effect of this shift on cell size and cell function (such as 

the effect of a change in the density of receptors on cell membranes) has not been 

evaluated. A change in cell size and function might be responsible for some of 

the microgravity-induced changes noted in other systems (such as the endocrine, 

cardiovascular, and immune systems). 

Renal Stone Risk 

A renal stone risk profile is determined from measured urinary oxalate, uric acid, 

citrate, calcium, sodium, magnesium, sulfate, potassium, pH, phosphorus, and 

total volume, and calculated supersaturation of calcium oxalate, brushite (calcium 

phosphate), sodium urate, uric acid, and struvite (magnesium ammonium phosphate). 

Generally, renal stone risk is elevated during space flight (299). As with any space flight 

effect, some crewmembers are more affected than others. Figure 38 is an important 

illustration of this. Some crewmembers had very high elevations in brushite or calcium 

oxalate supersaturation during space flight, whereas others had very low levels of 

supersaturation. During space flight, environmental and dietary factors all greatly affect 

renal stone risk. Crewmembers who have an increased risk before space flight and 

then are exposed to microgravity with concomitant bone loss, hypercalciuria, increased 

urinary sodium, and decreased urinary output may have a further increased risk of renal 

stone formation during space flight. 

Figure 38. Estimated supersaturation risk from the Renal Stone Risk Profile during space 

flight on ISS. Each symbol represents a 24-h urine pool, and the solid black line represents 

the group mean. The red dashed line is the point above which the risk is greater than in the 

non-stone forming population. 
62

As of 2013, 25 renal stone events have been reported among 19 crewmembers with 

the majority of these stone events occurring post flight (Robert Pietrzyk, personal 

communication). To minimize the risk of stone formation, potassium citrate (KCit) 

has been successfully tested during ISS missions (433), and has been “transitioned 

to operations.” This means that KCit is part of the flight surgeon’s toolbox for mitigating 

the renal stone risk of crewmembers and is available on the ISS for use at flight surgeon 

discretion if clinically indicated. The use of KCit was shown to increase the urinary pH, 

increasing the solubility of uric acid and thereby decreasing the risk of uric acid stone 

formation. The dosage of KCit must 

be carefully prescribed so as not to 

increase the risk of brushite stone 

development due to elevated urinary 

pH. However, given that fluid intake 

for maintenance of hydration is 

a preferred countermeasure, and 

some residual concerns exist about 

potassium supplementation side 

effects, it was decided not 

to routinely provide KCit 

to crewmembers. 

Magnesium and citrate are both 

considered protective when it comes 

to calcium-containing renal stone Figure 39. Relationship between urinary citrate risk. However, they do not minimize and sodium urate supersaturation. The dashed risk for all forms of kidney stones; line at 2 represents the average risk of sodium 

specifically, these urinary stone urate stone formation for the general population, 

inhibitors do not reduce the risk with numbers above 2 being higher-than-average 

of sodium urate kidney stones. risk. Citrate, which typically protects against renal 

Thus, taking KCit or KMgCit should stone risk, is actually positively correlated with 

sodium urate supersaturation (Pearson r = 0.065; not be perceived as a panacea P < 0.016). The red dashed line is the point above for stone risk. In fact, in our data, which the risk is greater than in the non-stone­ sodium urate supersaturation is forming population. Data are from 1399 twenty- significantly correlated with urinary four-hour urine collections. 

citrate excretion (Figure 39). 

Liquid salt and pepper dispensers floating on Space Shuttle Columbia’s middeck during STS-94. 
63

8 Space Flight Ophthalmic 

Changes and Nutrition 

Ophthalmic health among astronauts has gotten attention in recent years because 

of a newly identified issue for some crewmembers. In addition to a general increase 

in cataract risk (620-622), some crewmembers have experienced vision-related 

changes after long-duration space flight. These changes include optic disc edema, 

globe flattening, hyperopic shifts, choroidal folds, and cotton wool spots (623). 

The etiology of the refractive and structural ophthalmic changes is currently not known 

and continues to be researched, but biochemical evidence indicates that the folate- and 

vitamin B12-dependent 1-carbon transfer pathway may be involved. Nutrition is known 

to be an important factor for ophthalmic health in general. This section will review the 

available literature on this topic and general nutrition in ophthalmic health, along with 

ongoing research to understand and counteract these effects of space flight. 

Ophthalmic Changes 

Mader and colleagues first described 7 cases among long-duration crewmembers on 

ISS who had evidence of ophthalmic changes after flight, including optic disc edema, 

globe flattening, choroidal folds, and hyperopic shifts (623). Since then, additional cases 

have been identified, with the prevalence approaching 20% to 30% of crewmembers. 

Myasnikov and Stepanova reported evidence of postflight edema of the optic nerve 

discs among Russian cosmonauts and 1 case (out of 10) with signs of intracranial 

hypertension, although they note the measurements were made before and after 

(not during) flight (624). The mechanism of the ophthalmic changes is not known, 

but microgravity-induced fluid shifts (2, 531), elevated cabin CO2 exposure, possible 

intraocular pressure or intracranial pressure changes, and local intraorbital (choridal 

and optic nerve sheath) changes have been suggested as possible contributing factors. 

It is not clear why some crewmembers on some missions may experience these 

issues and others on the same mission, exposed to the same environment, do not. 

Recent biochemical evidence that the folate- and vitamin B12-dependent 1-carbon 

transfer pathway may be involved could help explain individual susceptibility to 

these ocular changes. 

One-Carbon Metabolism 

One-carbon metabolism refers to a pathway that requires folate, vitamin B12, and 

vitamin B6, and is involved in purine and pyrimidine synthesis, amino acid metabolism, 

and synthesis of the methylating agent S-adenosylmethionine (SAM, Figure 40). 

Alterations in this pathway can lead to buildup of certain intermediates, including 

homocysteine. Factors such as genetic polymorphisms, pharmacological agents, 

and dietary intake and status of folate, vitamin B6, and vitamin B12 can influence 

plasma homocysteine concentration (625, 626). 

Much of the homocysteine literature has focused on its association with coronary artery 

disease, stroke, and migraines (625, 627, 628), but some studies show associations 
64

of homocysteine with ophthalmic health issues. Issues such as age-related macular 

degeneration result from lipid deposits under the retinal pigment epithelium (629) 

and decreased retinal vessel functionality (630), and some theorize that age-related 

macular degeneration is similar to the development of cardiovascular disease (631). 

Because homocysteine is a risk factor for cardiovascular disease, many have looked at 

relationships between homocysteine, or other metabolites and vitamins in the 1-carbon 

metabolism pathway, and ophthalmic health issues such as age-related macular 

degeneration, dry eye, glaucoma, and optic neuropathy (632-635). For instance, a 

meta-analysis showed that elevated plasma homocysteine was associated with an 

increased risk of primary open-angle glaucoma (636). Other meta-analyses have shown 

that increased serum homocysteine and low vitamin B12 status were independently 

associated with increased risk for age-related macular degeneration (634, 637). 

Daily supplementation with folate, vitamin B6, and vitamin B12 is associated with 

a 30% to 40% decreased risk for age-related macular degeneration (638). 

Figure 40. Overview of 1-carbon metabolism. AA, amino acids; CBS, cystathionine ß-synthase; 

CYS, cystathionine; FA, fatty acids; HCY, homocysteine; αKBT, α-ketobutyrate; MCA, methylcitric 

acid; MM-CoA, methylmalonyl coenzyme A (CoA); MMA, methylmalonic acid; MS, methionine 

synthase; 5-MTHF, 5-methyltetrahydrofolate; 5,10-MTHF, 5,10-methylenetetrahydrofolate; 

MTHFR, methylenetetrahydrofolate reductase; MTRR, 5-methyltetrahydrofolate homocysteine 

methyltransferase reductase; PRP-CoA, propionyl CoA; SAH, S-adenosylhomocysteine; SAM, 

S-adenosylmethionine; SUC-CoA, succinyl CoA; THF, tetrahydrofolate 
65

Figure 41. Artistic depiction of vision issues related to space flight, and the potential involvement 

of genetic influences in this problem. The depiction of a different DNA base pair (blue above and 

red below) in the 2 strands reflects genetic polymorphisms, or multiple forms, of the DNA. Inset 

imagery includes ISS and a Hubble Space Telescope image of the Cat’s Eye Nebula, to reflect 

that these issues are relevant not only for ISS missions, but exploration-class missions as well. 

Folate 

Folate is the general term used to describe the vitamin folic acid and compounds that 

have activity similar to that of folic acid (639). Folic acid is the form of the vitamin used 

in vitamin supplements and fortified food products, but it is rarely found to occur 

naturally in food. 

The reduction of folic acid and dihydrofolate by a cytosolic enzyme produces the 

active form of folate, tetrahydrofolate (THF). Tetrahydrofolate accepts single-carbon 

groups from reactions in amino acid metabolism to form active derivatives of THF 

(639). These derivatives function in amino acid metabolism, specifically in the reversible 

reactions of serine synthesis from glycine, methionine synthesis from homocysteine, and 

histidine metabolism. Folate is essential in cell division because the THF derivatives play 

important roles in purine and pyrimidine synthesis. Tetrahydrofolate derivatives play 

a major role in the formation of thymidylate, which is a substrate needed for 

DNA synthesis (639). 

Deficiency of folate leads to megaloblastic anemia. Low folate intake will cause red 

blood cell folate concentrations to diminish within 4 months. Bone marrow cells become 

megaloblastic (that is, they take on a nucleated, embryonic form), and anemia occurs 

after 4 to 5 months of low folate intake (640). Folate deficiency in humans has been 

described as a 4-stage process (641, 642), including changes in serum folate (Stage 1), 
66

changes in red blood cell folate (Stage 2), defective DNA synthesis and elevated 

homocysteine (Stage 3), and clinical folate deficiency (Stage 4), manifested by 

macroovalocytosis (many large, oval cells in the blood), elevated mean corpuscular 

(red blood cell) volume, and large, nucleated embryonic cells. 

Early space flight data showed a reduction in red blood cell folate after long-duration 

missions (4, 6). Serum folate is variable among crewmembers, but generally does not 

change during flight (Figure 42). Interestingly, serum folate was lower during space flight 

in crewmembers with vision-related issues (140). 

Figure 42. Red blood cell (RBC) (left) and serum folate (right) before, during, and after longduration space flight (data are mean ± SD). Note: RBC folate data are not available during flight 

because of sample processing requirements. 

As with many nutrients, folate deficiency on an exploration mission could be 

catastrophic. Animal studies have shown that low folate status increases chromosome 

damage resulting from radiation exposure (643-646); however, it should be noted that 

excessive folate supplementation provided no additional benefit (643). Similarly, cell 

models have shown that folic acid deficiency increases sensitivity to chromosome 

breakage from ionizing radiation (645). Antioxidant properties of folate have been 

studied, and folate was found to scavenge a diverse array of reactive oxygen species 

efficiently (647). Cell models also show the ability of folate to reduce iron toxicity in 

cases of iron overload, by oxidizing free or chelated iron (647). Evidence exists that 

in subjects living in saturation diving conditions with increased partial pressure of 

oxygen for 10 to 14 days, folate status decreases, which may be related to its 

antioxidant properties (481). Folate status may be even more important during 

exploration missions than on ISS because of known increases in iron storage 

during long-duration space flight (6) and exposure to ionizing radiation. 

Vitamin B12 

Vitamin B12 functions in many enzymatic reactions, and deficiencies result in anemia, as 

well as neurological disorders. Vitamin B12 works as a cofactor for 3 different enzymatic 

reactions: (1) the conversion of homocysteine to methionine, (2) the conversion 

of L-methylmalonyl-coenzyme A (CoA) to succinyl-CoA, and (3) the isomerization of 

L-leucine and ß-leucine. Vitamin B12 deficiency may cause the accumulation of folate 

in the serum because of a reduction in B12-dependent methyltransferase, also known 

as the methyl-folate trap (648). Vitamin B12 also functions in the synthesis of choline, 

which can be converted to the neurotransmitter acetylcholine. 
67

Unlike other water-soluble vitamins, vitamin B12 can be stored in the body for years. 
 


It is stored predominantly in the liver, but smaller amounts can be found in the muscles, 
 


kidneys, bones, heart, brain, and spleen. About 2 to 5 mg of vitamin B12 is stored in the 
 


body (649). The size of B12 stores remains relatively stable, partly because urinary and 
 


fecal excretion decrease in direct relationship to decreases in the body pools. The half-
 


life of vitamin B12 in humans is 350 to 400 days (649). 


No evidence of toxicity has been found with vitamin B12 supplementation in amounts 


greater than the RDA (649), and no adverse effects are reported to be caused by an 
 


excess of vitamin B12. If a person went for many years without adequate intake and/or 


supplementation, body stores could be depleted. Other factors that could contribute 
 


to a vitamin B12 deficiency include a decrease in gastric acidity, the presence of atrophic 
 


gastritis, and uncontrolled growth of bacteria accompanied by malabsorption of food-
 


bound B12 (650). Deficiency of vitamin B12 leads to pernicious anemia and demyelination 
 


of the central nervous system, and can lead to death (651). 


Methylmalonic acid is generally unchanged during space flight, suggesting that vitamin 


B12 deficiency is not a significant issue during flight. However, blood concentrations of 
 


methylmalonic acid were shown to be higher in crewmembers who experienced vision-
 


related issues after flight than in those who did not have such issues (140), and this 
 


difference was evident before, during, and after space flight. Several studies support 
 


the notion that perturbations in the vitamin B12 metabolic pathway can cause ophthalmic 
 


health issues such as optic neuropathy and age-related macular degeneration (632-634). 
 


Biotin 

Biotin is a required cofactor for pyruvate carboxylase, acetyl-CoA carboxylase isoforms 

1 and 2, propionyl-CoA carboxylase, and ß-methylcrotonyl-CoA carboxylase (652). 

The 5 biotin-dependent enzymes are involved in carbohydrate, fatty acid, and amino 

acid metabolism (652), and the primary role of biotin is to transfer CO2 units from one 

compound to another. Biotin exists in a free state or bound to proteins. About 81% of 

biotin in the human body is free biotin in serum, and 10% is free in tissues (653). 

Despite the observation that frank signs of deficiency are rare, there is growing 

appreciation of genetic, physiologic, and pharmacologic conditions that marginally impair 

biotin status (654-656). This suggests that the lack of physiologic manifestations of biotin 

deficiency may not be a reliable measure to gauge biotin status. Marginal changes in 

biotin status have been shown to affect a range of metabolic factors, from carboxylase 

activity to the expression of non-biotin-dependent enzymes such as glucokinase, 

ornithine transcarbamylase, and phosphoenolpyruvate carboxykinase (657-659). 

Biotin status has never been measured during or after long-duration space flight, but 

it is unlikely there are any frank biotin deficiencies, which would present as neurological 

and dermatological manifestations. 

Vitamin A 

Another important vitamin involved in vision health is vitamin A. Vitamin A is a general 

term that refers to a family of fat-soluble compounds that are structurally similar to 

retinol and share its biological activity. Among these are retinol, ɑ-carotene, ß-carotene, 

and retinyl palmitate. Vitamin A or carotenoids can be found in dark green leafy 

vegetables and in vegetables and fruits that are yellow, orange, or red. Vitamin A plays 

a fundamental role in the retinal response to light. Inadequate vitamin A can result in 

night blindness, delayed light and dark adaptation, and dry eye (660). 
68

Beyond its essential role in the visual process, vitamin A is directly involved in gene 

expression, reproduction, embryonic development, and immunity. Vitamin A and 

ß-carotene serve as biological antioxidants and have been shown in multiple studies to 

reduce the risk of cancer and coronary heart disease (661, 662). Vitamin A also plays a 

role, albeit sometimes indirectly, in the function of almost all of the body’s organs (663). 

Oxidative stress is increased during space flight, and this could affect cardiovascular 

health and cancer risk, as described in other sections of this book. Vitamin A status may 

play a critical role in maintaining antioxidant health during space flight; however, as with 

many antioxidants, the desire to supplement with high doses in the hope of staving off 

one disease is high, but unwarranted and potentially counterproductive. Excess vitamin 

A, in levels on the order of twice the recommended daily intake, has been shown to 

increase bone resorption and fracture risk (271, 664-666). Furthermore, supplementation 

with ß-carotene should be done with caution (either alone or with vitamin A or in 

combination with vitamin E), because of unanticipated outcomes of an increased risk of 

lung cancer in smokers (667, 668). This increased risk among smokers might be related 

to pro-oxidant actions of ß-carotene in the lung. 

When considering pre- and postflight space flight data, there is a significant interaction 

between the effects of landing site and space flight on serum levels of both retinol 

and retinol-binding protein (6). Russian landings are different from US landings in that 

blood samples are usually collected several hours later because of the logistics of 

the landing site. 

Serum retinol decreased from 0.73 ± 0.17 μg/mL to 0.59 ± 0.13 μg/mL when landings 

were in Russia, and increased from 0.52 ± 0.09 μg/mL to 0.63 ± 0.12 μg/mL when 

landings were in the US. Similarly, retinol-binding protein decreased from 61.4 ± 5.6 to 

50.92 ± 8.41 mg/L when landings were in Russia, and increased from 49.2 ± 9.2 to 

53.0 ± 8.7 mg/L when landings were in the US. These differences in landing sites could 

be related to the time delay in sample collection, the fact that crewmembers might have 

consumed food during the time delay, or even a difference in the stress response at 

different sites. These data, however, do not provide evidence that there is a deficiency 

of any sort for vitamin A. 

In-flight vitamin A data have been collected as part of Nutrition experiments on ISS, 

and Figure 43 shows that no significant changes in retinol or ß-carotene occur during 

space flight. 

Figure 43. Serum retinol (left) and ß-carotene (right) before, during, and after long-duration space 

flight. Dashed lines represent normal range. Data are mean ± SD. 
69

In addition to the vision-related changes observed in some astronauts after longduration space flight, several studies have confirmed that astronauts and cosmonauts 

have an increased risk of cataract formation after space flight (621, 622, 669, 670). 

Cucinotta et al (622) identified an increased risk of all types of cataracts (including 

posterior subcapsular, cortical, and nuclear) among astronauts with greater exposure 

to radiation. Longitudinal follow-up studies have been conducted and it was determined 

that progression of cortical cataracts, but not posterior subcapsular or nuclear cataracts, 

is related to space radiation exposure (620, 621). Although radiation exposure is a 

large driving force for the oxidative damage that leads to some types of cataracts, the 

longitudinal study provided evidence that intake of specific nutrients may provide some 

protective effects (621). In the first report of the NASA Study of Cataract in Astronauts, 

nutritional intake estimates were obtained from a questionnaire, and the data provided 

evidence that ß-carotene and lycopene intake had a protective effect for some types 

of cataracts in astronauts (621). As reviewed by Agte and Tarwadi, numerous groundbased studies have provided evidence for associations between micronutrients and 

antioxidants (either blood levels or estimated intakes) and cataracts (671). A recent 

meta-analysis provided similar results supporting an inverse association of ɑ-carotene 

(and vitamin E, lutein, and zeaxanthin) with age-related cataract (672). 

Although epidemiological data support the idea that lower vitamin A status is associated 

with cataract incidence, it is not known whether altered micronutrient and antioxidant 

intake during space flight could minimize cataract incidence related to space radiation, 

as this requires further interventional study and better estimates of in-flight nutrient 

intake along with nutrient status assessments. 

Expedition 35 Flight Engineers Chris Cassidy (pictured) and Tom Marshburn (out of frame) 
 


inspect and replace a pump controller box on the International Space Station. 
 

70

In the International Space Station's Harmony 

node, European Space Agency astronaut 

Alexander Gerst, Expedition 40 flight 

engineer, performs an eye exam for the 

Ocular Health experiment, which observes 

and seeks to understand vision changes 

during long-term space missions. 
71

9 Immune Function, 

Inflammation, and Nutrition 

Optimal function of the immune system is impaired in the presence of malnutrition. 

Without adequate nutrition, the immune system is clearly deprived of the components 

needed to generate an effective immune response (673, 674). Nutrients act as 

antioxidants and as cofactors (675). Historically, during space flight crews generally 

have lower dietary intake than they do under normal conditions on the ground (2). 

It is well known from ground research that a lack of macronutrients or selected 

micronutrients, like zinc, selenium, and the antioxidant vitamins, can have profound 

effects on immune function (676-679). Such a lack of nutrients also leads to deregulation 

of the balanced host response (680). Disruption of nutritional balance and dietary intake 

of astronauts and cosmonauts during space flight, which is often accompanied by a 

stress response, might influence their immune response (681, 682). However, detailed 

information on the effects of many micronutrients during space flight are mandatory 

before specific nutritional recommendations can be made, especially with respect 

to their relationship with immune system function. 

Energy Intake 

As discussed earlier, some crewmembers have an insufficient energy intake, which 

can lead to more extensive free radical propagation because of diminished proteinbased antioxidant defense mechanisms (2, 683). On the Mir station and the Life and 

Microgravity Science Shuttle mission, space travelers who consumed inadequate energy 

intake had significant increases in urinary excretion of 8-iso-prostaglandin-F2ɑ and 

8-oxo-7,8-dihydro-2 deoxyguanosine (8-OH-dG), markers for oxidative damage (684). 

Protein and Amino Acids 

Stein et al suggest that space flight triggers a stress response similar to the one 

triggered by stress induced by injury (685). Protein and amino acid deficiencies can 

have profound effects on a variety of immune system functions (686, 687). With respect 

to specific amino acids, arginine is necessary for normal T-cell function and may become 

essential in catabolic states. Supplementary dietary arginine has been shown to have 

useful effects on cellular immunity in animal studies, showing increased size of the 

thymus, enhanced lymphocyte proliferation in response to mitogen and alloantigen, 

augmented macrophage and killer cell lysis, and increased lymphocyte interleukin 2 

production and receptor activity (688). Supplementation of arginine led to improved 

wound healing and immune responses in elderly subjects (689). Judging by these 

observations it might seem promising to supplement arginine during long-term missions; 

however, up to now no studies have been carried out to test arginine as a measure to 

improve immune response during space missions. 

Another amino acid beneficial for the immune system is glutamine. Glutamine is the 

most abundant free amino acid in the body. It can inhibit NF-κB activation and cytokine 

expression after sepsis (690). Some of the beneficial effects of glutamine are its 
 

 

72

antioxidant effects and its actions as a precursor to glutathione, an energy substrate 

for lymphocytes and neutrophils, and as a stimulator of nucleotide synthesis (691, 692). 

Glutamine seems to have a significant beneficial effect on mortality, length of hospital 

stay, and infectious morbidity in critical illness (691). Positive results of glutamine 

supplementation have been shown in critically ill patients, in whom supplemental 

glutamine reduced complications and mortality rates in addition to having a stimulating 

action on the immune system (693, 694). However, up to now supplementation of 

glutamine as a pharmaconutrient has not been tested in space flight or space flight 

analogs (eg, bed rest). 

Vitamin D 

As already described in previous chapters, the classical function of vitamin D is t 

o regulate calcium homeostasis and thus bone formation and resorption. However, 

recent publications show that vitamin D also exerts other biological activities including 

immunomodulation. The latter seems to be mediated by the (nuclear) vitamin D receptor 

(VDR) expressed in antigen-presenting cells and activated T cells (695). Vitamin D and 

the VDR are required for the blood to have normal numbers of regulatory T cells. 

The discovery that VDR is inducible in lymphocytes suggests a role for 1,25(OH)2D3 in 

the immune system (696). Even the enzyme 25(OH)D3-1-ɑ-hydroxylase is expressed 

by active macrophages, making them able to synthesize and secrete 1,25(OH)2D3 (697). 

However, in macrophages the enzyme is mainly activated by immune signals such as 

interferon-ɣ rather than by parathyroid hormone, which is the activator in the kidney 

(695). Moreover, the active vitamin D metabolite 1,25(OH)2D3 can also be modulated 

by alternative mechanisms to increase the ability of peripheral blood mononuclear cells 

from sensitized human donors to resist microbes (here mycobacteria). Martineau et al 

found that 1,25(OH)2D3 suppressed both bacillus Calmette-Guérin and Mycobacterium 

tuberculosis in infected cell cultures, likely through “nonclassical” mechanisms including 

the induction of antimicrobial peptides (698, 699). Kondo et al found that vitamin D 

supplementation improved the sensitivity of the treatment response to pegylated 

interferon ɑ/ribavirin therapy in chronic hepatitis C patients (700). 

Studies during or after space flight have shown numerous changes in astronauts’ 

immune status, including altered distribution of circulating leukocytes, altered 

production of cytokines, decreased activity of natural killer cells, decreased function 

of granulocytes, decreased activation of T cells, altered levels of immunoglobulins, 

reactivation of latent viruses, altered virus-specific immunity, expression of EpsteinBarr virus immediate early and late genes, and altered neuroendocrine responses (701). 

Furthermore, evidence exists that among individuals wintering over in the Antarctic for 

6 months, who have high serum cortisol, a higher vitamin D status is related to a lower 

probability of viral shedding in saliva (407). In that study an interactive effect occurred 

between cortisol and vitamin D, and subjects with lower serum 25-hydroxyvitamin D 

and with the highest quartile of serum cortisol (22.5 μg/dL or higher) had more evidence 

for shedding of Epstein-Barr virus (EBV) in saliva than did individuals in the lowest 

quartile of cortisol (13.1 μg/dL or below, Figure 44). Thus, a low vitamin D status 

of astronauts during space missions might have an impact on their immune status. 

Further studies are mandatory to distinguish between the effects of vitamin D 

deficiency and mere microgravity effects. 
73

Figure 44. An interaction 

between serum cortisol, vitamin 

D status, and the probability of 

EBV shedding. Data from all 41 

participants in the Antarctic study 

are included in the graph, and the 

data were statistically analyzed 

using the continuous data set 

of cortisol data. The data are 

split into the 2 subgroups here 

for presentation purposes. 

The graph is from Zwart et al (407). 

Vitamin B12 

Vitamin B12 functions in many enzymatic reactions, and deficiencies result in anemia, 

as well as neurological disorders. Vitamin B12 functions as a coenzyme in 2 metabolic 

forms: adenosylcobalamin and methylcobalamin. Vitamin B12 works as a cofactor for 

3 different enzymatic reactions: (1) the conversion of homocysteine to methionine, 

(2) the conversion of L-methylmalonyl-coenzyme A (CoA) to succinyl-CoA, and (3) 

the isomerization of L-leucine and ß-leucine. Vitamin B12 deficiency may cause 

the accumulation of folate in the serum because of a reduction in B12-dependent 

methyltransferase, also known as the methyl-folate trap (648). Vitamin B12 also 

functions in the synthesis of choline, which can be converted to the neurotransmitter 

acetylcholine. Vitamin B12 deficiency may lead to alterations of immunological 

indicators, such as a reduction of lymphocytes and suppression of natural killer cell 

activity, both of which can be reversed by supplementation with vitamin B12 (702). 

In one study, for 4 months elderly subjects (aged 70 years) received, in addition to 

the regular diet, a special nutritional formula providing, among other nutrients, 120 IU 

vitamin E, 3.8 mg vitamin B12, and 400 mg folic acid. NK-cell cytotoxic activity 

increased in supplemented subjects, indicating increased innate immunity in elderly 

people (703). These few studies demonstrate the importance of a sufficient B vitamin 

status to maintain an adequate immune response (704). 

Sodium 

High sodium intake is correlated with development of hypertension in sodium-sensitive 

people. We have shown in space flight as well as in ambulatory conditions on Earth that 

at an intake level of about 4000 mg/d, sodium is retained without being accompanied by 

fluid retention (30, 537, 705). A hypothesis that might explain how sodium can be bound 

in an osmotically inactive way has been brought forward by Titze et al (543, 706) and 

proposes that sodium can be stored on proteoglycans in interstitial sites. This uniquely 

bound sodium can induce a state of local hypertonicity in the skin interstitium. 

In a further study they suggest that the local hypertonicity is sensed by macrophages, 

which then activate a transcription factor (tonicity enhanced binding protein), which 

in turn induces vascular endothelial growth factor C signaling (538, 539, 707). 

Macrophages play a key role in innate immunity, and therefore further studies in 

microgravity should distinguish between the effects of microgravity and high sodium 

intake on the immune system. 
74

Vitamin A 

In brief, vitamin A plays a well-known role in immune function and protection against 

infections (708-711). A vitamin A deficiency impairs mucosal barriers and diminishes 

the function of neutrophils, macrophages, and NK cells (712); it may affect host defenses 

directly (713) or indirectly through its role in epithelial cell differentiation and host barrier 

function (710). The considerable immunity benefits of vitamin A, which would contribute 

to reducing the risk of various pathogen-mediated diseases, warrant a recommendation 

to supplement individuals with minimal or poor vitamin A status. However, whether 

immunity benefits accrue from providing additional vitamin A to those with sufficient 

status is not known (714). 

Vitamin C 

Ascorbic acid (vitamin C) is an essential component of every living cell. The 

concentration of vitamin C is very high in leukocytes, and the vitamin is used rapidly 

during infection to prevent oxidative damage. Vitamin C is a regulator of redox and 

metabolic checkpoints that control activation and survival of immune cells (680). 

A deficiency in vitamin C status is associated with reduced immune function (715). 

Vitamin C has been shown to stimulate the immune system by enhancing T-lymphocyte 

proliferation in response to infection, and increasing cytokine production and synthesis 

of immunoglobulins (716). However, the antioxidant role during space flight of neither 

vitamin A nor vitamin C has been investigated up to now. In analog studies such as 

(short- or long-duration) bed rest, no significant change in vitamin C could be shown 

(283, 316), but a trend for an increase was apparent. This might be related to dietary 

vitamin C intake during the study relative to the intake before the study (283). 

Vitamin E 

Vitamin E is a strong antioxidant that can support monocyte/macrophage-mediated 

responses (717). Vitamin E and selenium have synergistic functions in tissues to reduce 

damage to lipid membranes by the formation of reactive oxygen species (ROS) during 

infections. The ability of vitamin E to scavenge lipid-soluble free radicals depends to 

some extent on the status of 2 other antioxidant compounds, vitamin C and glutathione, 

which are involved in reducing oxidized vitamin E back to a reusable (ie, able to be 

oxidized) form. Additionally, vitamin E may improve T-cell function by decreasing 

production of prostaglandin E2 by macrophages, by modulating the amino acid 

cascade initiated by lipoxygenase and/or cyclooxygenase (100). Furthermore, vitamin 

E influences lymphocyte maturation, possibly by stabilizing membranes and allowing 

enhanced binding of antigen-presenting cells to immature T cells through increased 

expression of intercellular adhesion molecule-1. 

Copper 

Copper has wide-ranging functions in the body, including many considered vital for 

space flight (77, 502-505). This fact might have direct or indirect (when alterations 

are induced by psychological stress or radiation stress) implications for nutrition and 

nutritional status being possible causes or effects of alterations in immune system 

function (2). Nonetheless, to date, little to no information is available about copper 

metabolism during space flight. 
75

Zinc 

In addition to its many essential functions in growth and development, zinc is 

essential for the function of cells of the immune system (718). It has an important role 

in promotion of wound healing and in maintenance of intestinal integrity. A deficiency 

of zinc is also associated with reduced concentrations of insulin-like growth factor 1 

and reduced rates of protein synthesis. Therefore, zinc deficiency could be especially 

detrimental during immobility. However, zinc status of astronauts, as assessed by mean 

serum zinc and urinary zinc excretion (admittedly, not the best markers of zinc status), 

did not change after long-duration space flight (2). No data are available on the use 

of zinc supplementation as a countermeasure during space flight. 

Polyphenols 

Naturally occurring polyphenols like resveratrol, quercetin, curcumin, and catechins 

have shown antioxidant and anti-inflammatory effects (719). These effects seem to 

be modulated through different pathways such as protein kinase-dependent pathways 

activated by NF-κB or mitogens, as well as through preventing the generation of reactive 

oxygen species by iron binding (720). Additionally, polyphenols seem to activate sirtuin 

1 directly or indirectly and thereby are beneficial—besides having other functions—for 

regulation of oxidative stress, inflammation, and autoimmunity. Accumulating evidence 

has shown that polyphenols such as resveratrol, curcumin, catechins, and quercetins 

have a regulatory role in immune function in vitro and in vivo (506, 721-726). Therefore, 

they might also have beneficial effects in prevention of immune dysfunction during 

long-term space missions, particularly because body iron stores are higher during space 

flight. However, the role of polyphenols in sirtuin-1–mediated or iron-related regulation 

in immune function remains to be studied. No results have been obtained during space 

flight yet; however, a recently selected flight study will examine the role of increasing 

polyphenol intake on bone health in particular. 

Iron 

As mentioned previously, the maintenance of iron homeostasis is extremely important 

for human health. During space flight, it is well established that iron homeostasis is 

altered (2, 469). The decreased red blood cell mass, increased serum ferritin, decreased 

transferrin receptors, and increased serum iron all provide evidence for increased iron 

storage during space flight. Furthermore, the space food system provides almost 3 times 

the recommended intake (2). Iron plays an ambiguous role in human health: not only do 

humans require it for survival, but also microorganisms (including pathogens) require iron 

acquisition from the environment for their survival. Cells of the innate immune system 

have genes that regulate proteins that can modulate iron homeostasis at the cellular and 

systemic level to restrict iron availability to invading microorganisms. One such protein 

is hepcidin, a key regulator of iron homeostasis and a critical factor in the anemia of 

inflammation (727, 728). Hepcidin has been shown to be endogenously expressed by 

innate immune cells—macrophages and neutrophils. It plays a role in making iron less 

available by increasing intracellular iron sequestration and decreasing circulating iron 

concentrations, and it is influenced by cytokines IL-6 and IL-1 (729, 730). Studies need 

to be done to determine the role of increased iron stores during space flight on immune 

function and reactivation of latent viruses. 
76

Polyunsaturated Fatty Acids 

Polyunsaturated fatty acids (PUFAs), such as omega-3 fatty acids, protect from 

oxidative damage and radiation-related risks (99, 731), both of which are concerns 

for space travelers. The mechanism of action of omega-3 fatty acids on these systems 

is likely related to multiple pathways, but evidence exists that the nuclear transcription 

factor NF-κB is affected differently by omega-3 and omega-6 fatty acids (732). This 

transcription factor affects transcription of genes involved in cell cycle regulation and 

inflammatory processes. NF-κB is activated by arachidonic acid and specifically by 

prostaglandin E2, but Camandola and colleagues have found that eicosapentaenoic 

acid (a PUFA) inhibits NF-κB activation (732). 

We have reported elevated NF-κB after short-duration space flight (116). The effects 

of omega-3 fatty acids on inflammatory cytokines, and specifically TNFɑ, are well 

documented on the ground (116, 733-735), but warrant further studies during 

space flight. 

In summary, astronauts in space are generally not optimally nourished. Dietary intake 

tailored to the astronauts’ needs may be beneficial for their immune system function. 

Furthermore, the environmental stress of space flight can lead to changes in immune 

response as well as in the nutritional needs of the individual astronaut. Nutrition for 

optimal immune response and other functions is required to support optimal astronaut 

health during long-duration missions. However, it is important to be aware that ‘‘one size 

does not fit all.’’ An immune nutrient intake profile that is appropriate for one astronaut 

or one condition may be of minimal benefit for another individual or condition, and could 

be harmful in other settings. Making evidence-based decisions in choosing the optimal 

diet or formula will minimize adverse effects. To reach that level of individualized diet 

for astronauts, further research is needed to assess whether diet can benefit immune 

function during space flight. The use of basic clinical pharmacology, molecular biology, 

and clinical research principles in the study of nutritional therapy during space flight and 

analog studies will lead to answers on how to administer the right nutrients, in the right 

amounts, at the right time during astronauts’ space missions. 

Members of the STS-124 and Expedition 17 crews share a meal in the Zvezda module of the ISS. 
77

10 Antioxidants and 

Oxidative Stress 

The space environment exposes astronauts to numerous sources of oxidative stress. 

Some of these sources are extravehicular activity (EVA) and EVA prebreathe protocols, 

exercise, diet, and radiation (469, 472, 736-738). After space flight, anabolic processes 

and substrate competition between muscle rebuilding and host defense mechanisms 

stress the antioxidant defense system and may contribute to postflight increases in 

oxidative damage (683). 

Hypoxic Conditions 

As exploration missions come closer to reality, we will need to better understand risks 

associated with the type of EVAs that are currently planned for those types of missions. 

Current mission designs could involve as many as 30,000 hours of EVA exploration time, 

which is far more than the 20 total hours spent conducting EVAs during the entire Apollo 

Program (739, 740). Future vehicles or exploration habitats will likely operate at 8.0 

to 8.2 psia and 32% to 34% oxygen with the balance nitrogen (740). EVAs at 4.3 psia 

would require some amount of oxygen prebreathing. As a comparison, ISS has operated 

at an Earth equivalent of 14.7 psia and 21% oxygen. The terrestrial altitude equivalent 

of 8 psia and 32% oxygen is about 6,000 to 8,000 feet (1829 to 2438 m), and would 

represent a hypobaric hypoxic scenario. Concerns associated with prolonged hypobaric 

hypoxic conditions include potential increased risk of vision impairment issues, acute 

mountain sickness, sensorimotor impairment, alterations in cardiovascular and immune 

function, and anorexia. 

Extravehicular Activity 

Extravehicular activity (EVA), or spacewalking, is a unique situation from a nutritional 

perspective, because the EVA suit does not easily allow food consumption. On early 

Shuttle missions, a 165-kcal fruit bar was custom-made to fit in the EVA suit, but it was 

typically not consumed, and is no longer included. Crewmembers can go without food 

for as long as 8 to 10 hours while they are preparing for and performing EVA. Nutritional 

recommendations for EVA were designed to help maximize crew performance and 

efficiency. When nutrition for EVA was reviewed in 1991, the recommendation for EVA 

crewmembers was that they should consume an additional 500 kcal on days of EVA 

(741). This was designed to account for the metabolic cost of EVA (~200 kcal/h). 

In 2000, another review of this situation was requested by NASA’s Flight Medicine 

Division. The resulting recommendation was to provide food items for consumption 

during EVA preparation (as close as possible to the donning of helmets). The food items 

should contain 300 to 500 kcal, with about 70 to 100 g of carbohydrate and a high 

content of soluble fiber. Candidate items are reviewed to ensure that, in the attempt to 

meet the basic criteria, undesirable nutrients or additives are not included, and that crew 

preferences are accounted for. It was also recommended that crewmembers reconsider 

use of the in-suit food bar, or that alternatives be sought. 
78

Fluid intake during EVA is also a topic of concern. Crewmembers lose 6 to 8 ounces 

(177 to 237 mL) of fluid per hour during an EVA. The current EVA suit contains a 

24- or 32-ounce (710 or 946 mL) drink bag. Only water is used (early EVAs included 

flavored beverages, but a problem during a lunar EVA resulted in a programmatic 

decision to include only water). Provision of in-suit fluid is an important factor in 

suit design. For the current suit, use of the 32-ounce drink bag is recommended. 

The development of a larger, disposable drink bag is highly encouraged. The disposable 

drink bag should be designed so that a flavored drink (such as the current Shuttle food 

system beverages) could be used to increase palatability and intake, assuming that the 

technical concerns can be eliminated. 

Because missions to explore a non-Earth surface (planetary or asteroid) will likely include 

EVAs similar in duration to current ISS EVAs (8 to 10 h) but more frequent (2-3 times per 

week, instead of 2-4 times/6 months), a clear need exists for development of nutrition 

support during EVAs. A nutrition support system will need to fit the lunar suit design. 

The definition of the optimal nutrition support system will need to be based on the 

results of ground studies designed to optimize performance, minimize fatigue, and 

minimize oxidative damage from a high partial pressure of oxygen in the suit. 

Reactive Oxygen Species and Exercise 

Exercise-induced fatigue and muscle atrophy are mediated in part by reactive oxygen 

species (ROS). Electron spin resonance spectroscopy technology confirmed earlier 

findings from the 1950s suggesting that short-lived reactive intermediate molecules like 

ROS are present in skeletal muscle after exercise (742). Since then, numerous studies 

have supported a role of ROS in skeletal muscle fatigue (742-744). Mitochondria are 

the major source of ROS in muscle cells, where a fine balance of ROS exists between 

maximizing force and minimizing fatigue (736). Antioxidant-mediated depletion of ROS 

from unfatigued muscle yields decreased production of skeletal muscle force (745). 

On the other hand, excess ROS can be detrimental in terms of fatigue. ROS can 

denature proteins directly associated with the sarcoplasmic reticulum Ca2+ release 

mechanism (746), thus compromising tension development. Also, rat studies show 

that xanthine oxidase-induced ROS yields increased diaphragm fatigue, and that the 

elevated ROS during intense exercise is implicated in the onset of muscle fatigue (747). 

Furthermore, decreased antioxidant status lowers exercise capacity and increases 

onset of fatigue in human and animal studies (742, 744). 

Astronauts perform prolonged upper-body exercise during EVA activity, and one of 

the limiting factors in completing EVA tasks is forearm and hand muscle fatigue due 

to extensive tool operation. The fatigue often requires crewmembers to stop and rest, 

thereby prolonging the duration of EVA, and limits the number of tasks performed during 

each EVA. To date, there is little evidence showing that antioxidant supplementation has 

a benefit for improving muscle performance and inhibiting fatigue. Given the nature of 

the requirement for homeostasis of redox systems, there is a potential for antioxidant 

overload to decrease muscle force potential instead of having a protective effect. 

Radiation Exposure 

Astronauts are exposed to highly ionizing radiation, in addition to secondary radiations 

resulting from interactions of ionizing radiation with shielding materials or the human 

body. Biological effects of radiation include damage to DNA from a direct hit from 

an ion track, oxidative damage from generated ROS, and oxidative damage induced 
79

by a bystander effect (99, 748, 749). A bystander effect occurs when cells damaged by 

radiation particles secrete cytokines or other proteins that can generate ROS in cells that 

are not destroyed (750). 

Oxidative Damage Markers During Space Flight and in Ground Analogs 

Evidence for oxidative stress resulting from space flight exposure exists in multiple 

tissues, including ocular tissue (670, 751), urinary and blood biomarkers of damage to 

DNA, lipids, and protein (6, 469, 684, 752), and in gene expression (753, 754). Plasma 

malondialdehyde (MDA), 8-iso-prostaglandin F2ɑ (PGF2ɑ), and urinary 8-hydroxy-2’­

deoxyguanosine (8OHdG) have been measured during and after flight as indicators of 

lipid peroxidation (MDA and PGF2ɑ) and DNA damage (8OHdG) (4, 684). A significant 

elevation of urinary 8OHdG has been noted after long-duration missions (Mir and ISS) 

(6). These data are supported by results from the ground-based analog NEEMO, in 

which crewmembers underwent 10- to 14-day saturation dives (476, 755). Urinary PGF2ɑ

was significantly decreased during flight but elevated about 2.5-fold after flight (684), 

and plasma MDA was increased both during and after flight (684). In a Russian 120-day 

bed rest study, increased concentrations of markers of lipid peroxidation were found in 

subjects, and this increase was mitigated with vitamin E (756). 

The apparent increases in oxidative damage observed during and after flight could be 

caused by a number of factors, including altered DNA repair mechanisms, decreased 

antioxidant defense systems, or simply increased oxidative stress. Microgravity does 

not affect the repair of double-strand chromosome breaks (757, 758), but evidence 

exists that downregulation of antioxidant defense systems occurs during space flight 

(759). Along with increases in markers of oxidative damage and decreases in antioxidant 

defense systems, a decrease in total antioxidant capacity also occurs. 

Antioxidants and Related Nutrients: Selenium, Vitamin E, Vitamin C 

Selenium has been shown to play a role in the maintenance or induction of cytochrome 

P450, pancreatic function, DNA repair, enzyme activation, immune system function, 

and detoxification of heavy metals (760). Selenium is also a cofactor for glutathione 

peroxidase, which plays a role in the reduction of organic peroxides and hydrogen 

peroxide. Selenium has also been shown to be necessary for iodine metabolism. 

Postflight reductions in serum selenium of more than 10% have been observed after 

ISS flights (6). Whether this is related to intake or metabolism is not known. 

Deficiency of selenium can lead to impaired immune function, illness (Keshan disease 

and Kashin-Beck’s disease), or even death. Excess selenium can lead to problems 

affecting gastrointestinal, neurological, cardiopulmonary, and renal systems (760). 

Toxicity is not likely to occur except when selenium is consumed in large amounts in 

dietary supplements, but care must be taken to avoid toxicity despite the relationship 

of selenium to cancer risk and antioxidant status. 

Vitamin E is a lipid-soluble, chain-breaking antioxidant found in body tissues, and 

is also the first line of defense against lipid peroxidation reactions. Eight naturally 

occurring compounds have vitamin E activity: 4 tocopherol derivatives (α-, γ-, δ-, and 

ß-tocopherol) and 4 tocotrienol derivatives (α-, γ-, δ-, and ß-tocotrienol) (761). The 

tocopherols that are most abundant in biological systems are α- and γ-tocopherol, but 

small amounts of δ-tocopherol and γ-tocopheryl quinine are also present. About 90% 

of the tocopherol found in human plasma is in the form of α-tocopherol (762). After ISS 

crewmembers had spent 4 to 6 months in space, their plasma γ-tocopherol was 50% 

less than preflight levels (6). No change in α-tocopherol occurred in these subjects. 
80

Figure 45. Serum ɑ-tocopherol (top left) 

and ɣ-tocopherol (top right), and total 

vitamin E:lipid ratio (left) before, during, 

and after long-duration space flight. 

All data are mean ± SD. 

Oxidative stress can increase in microgravity and high-radiation environments 

(683, 684, 763), and the antioxidant properties of vitamin E may help to counteract 

the free-radical damage caused by high-linear energy transfer radiation in space. 

Pretreatment with antioxidants may help decrease radiation damage during missions 

(764), and it may be necessary to provide enough vitamin E for astronauts’ blood levels 

of the vitamin to be higher during space flight than on Earth. However, knowledge gaps 

weaken the evidence for use of vitamin E as a countermeasure. Clinical trials have 

documented negative side effects of pharmacologic vitamin E supplementation alone or 

with other antioxidants; it can increase risks of cancer in humans and animals (765-768). 

The term “vitamin C” actually refers to 2 different compounds, ascorbic acid and 

dehydroascorbic acid, both of which have activity against scurvy (769). Vitamin C 

functions as an antioxidant because it acts as a reducing agent for most physiologically 

relevant reactive oxygen species, reactive nitrogen species, singlet oxygen, and 

hypochlorite. It serves as a cofactor for enzymes involved in the biosynthesis of 

collagen, carnitine, and neurotransmitters (769). Vitamin C also provides antioxidant 

protection by returning ɑ-tocopherol to its biologically active state during lipid oxidation. 

The reducing agents glutathione and either reduced nicotinamide adenine dinucleotide 

(NADH) or reduced nicotinamide adenine dinucleotide phosphate (NADPH) regenerate 

the oxidation products of ascorbate (769). 

It has been suggested that vitamin C requirements should be greater for persons 

who are under excessive physical or emotional stress, given the role of ascorbate 

in the biosynthesis of steroid hormones and neurotransmitters. However, no substantial 

data show that vitamin C metabolism is altered in healthy subjects under mental or 

emotional stress (762). 

As a cofactor in collagen synthesis, vitamin C has been investigated for potential effects 

on bone health. Although in the Framingham Study higher vitamin C intake was found 

to be associated with lower bone mass (770), the significance of this association was 

marginal when data were adjusted for potassium intake. This suggests that vitamin C 
81

may be a secondary factor in bone loss, related to fruit and vegetable reduction 

(274, 771-773), as described elsewhere in this text. Other studies have found that 

vitamin C intake or supplement use is related to improved bone health and bone mass, 

but this relationship depends on adequate calcium intake (774-777). Vitamin C has been 

related to cataract and cancer incidence (778-780), both of concern for space travelers. 

Deficiency of vitamin C leads to fatigue, depressed immune function, scurvy (fatigue, 

muscle cramps, bruised and/or bleeding gums), and eventually even death. As noted 

in the introduction, scurvy resulted in more sailor deaths during the age of sail than 

all other causes of death combined (1). Toxicity of vitamin C leads to gastrointestinal 

distress, and has been reported in subjects consuming more than 1000 mg/d (778). 

Vitamin C assessments of ISS crewmembers have been conducted, with generally 

no changes after landing relative to before launch (unpublished data). Recent long- and 

short-duration bed rest studies documented no statistically significant change in vitamin 

C, but results showed a trend for an increase, which might be related to dietary intake 

during the study relative to the subjects’ nominal intake (134, 316). 

The stability of vitamin C in food supplies has been studied, and it is generally unstable 

at a neutral or alkaline pH and in high-oxygen environments (781). Vitamin C is also 

unstable when exposed to light or heat (781), and in irradiated foods (782, 783). 

Salem (783) found that gamma irradiation of fresh onion bulbs significantly reduced 

their vitamin C content. This group also found that vitamin C content of onion bulbs 

had decreased about 50% after 6 months of storage. The destructive effects of gamma 

irradiation (10 kGy) on vitamin C were also evident in commercial spices such as 

basil, black pepper, cinnamon, nutmeg, oregano, parsley, rosemary, and sage (784). 

Exposure of these spices to gamma rays for >3 months resulted in a marked increase 

in the concentration of quinone radicals. Evaluation of vitamin C stability in space foods 

showed an average loss of 27% after 1 year of storage (785). Fortunately, the vitamin C 

content of food items and potential menus is high enough that, depending on mission 

profile, intake of vitamin C might still meet requirements (785). 

A major concern for space flight is the possibility that vitamin C could be degraded 

in foods during extended-duration missions when space foods are exposed to large 

amounts of radiation and undergo long-term storage (foods may be sent to Mars in 

advance of the crew, and left there for up to 5 y). This could be catastrophic. 

Free-radical formation is increased in space because greater amounts of radiation 

are present than on Earth. Because of this and increases in other oxidative stressors, 

antioxidants such as vitamin C are in greater demand by the body to act as buffers and 

minimize the oxidative damage. Studies have shown that supplementation with vitamin 

C and other antioxidants can modify human tissue radiosensitivity and protect DNA 

against damage (786, 787). Just as important to consider, however, is the possibility 

that vitamin C could induce DNA damage. Cai and colleagues (787) found that vitamin 

C can act as an antioxidant to prevent DNA damage caused by ionizing radiation, but 

in the presence of copper, it can also act as a reducing agent to induce DNA damage. 

Because vitamin C can reduce redox-active metals such as iron and copper, this 

“antioxidant” can increase the pro-oxidant chemistry of these metals (788). Thus, 

vitamin C can serve as both a pro-oxidant and an antioxidant, and the amount of it 

required by exploration crewmembers needs to be carefully addressed (as does the 

amount of almost all nutrients). 

Vitamin C content and stability in the space food supply need to be determined. 

Evaluation of the impact of vitamin C supplementation during exposure to oxygen or 

high-linear energy transfer radiation should be investigated before recommendations 
82

can be made for supplement use during flight. This should be evaluated in a coordinated 

effort to find an antioxidant profile for space travelers. After data have been gathered 

about vitamin C status during and after flight, and preferably after data are available 

pertaining to the influence of space flight-induced stress on vitamin C, an evaluation 

of intake requirements needs to be made. 

Expedition 16 Commander Peggy Whitson pauses for a photo while working on the S5 Truss 

during Bearing Motor Roll Ring Module removal and replacement operations. 
83

11 Pharmacology and 

Drug-Nutrient Interactions 

Supplements versus Whole Foods 

The benefits of supplements are such that individual nutrients can be obtained when 

the diet cannot meet needs. Drawbacks to supplements include the potential for side 

effects, interactions with medications, and, with respect to bone, the fact that isolated 

nutrients may not provide the same protective effect on bone as they would in the 

matrix of the whole food. For example, more than 100 phytochemicals in tomatoes likely 

contribute to the chemoprotective effect of tomato puree in addition to the lycopene 

known to protect against certain types of cancers. Tomato puree has much stronger 

dose-dependent, antimutagenic effects and lowers biomarkers of oxidative stress more 

than pure lycopene alone (789, 790). Similarly, omega-3 fatty acids have different effects 

on vasodilation, depending on whether they are supplied as a supplement or in 

a whole food (791, 792). 

Besides the fact that supplements lack the synergistic effects of nutrients in whole 

foods, there are numerous examples of negative side effects associated with 

supplement use. Symptoms can range from gastrointestinal effects, dizziness, 

or decreased white blood cell count (from ipriflavone, which is synthesized from 

the soy isoflavone daidzein) to increased cancer risk (ß-carotene in the CARET study) 

or increased risk of stroke (667, 793-795). 

Provision of nutrients through supplements also ignores the fact that in some cases, 

for example the omega 3:omega 6 ratio, the negative effects of other foods cannot 

be overcome simply by provision of supplements. Although this phenomenon is more 

difficult to document, it is likely the reason that epidemiological data continue to show 

benefits of dietary patterns over supplements. 

The issue of supplement use often arises with discussion of nutrient requirements for 

space travelers and the use of nutrients as countermeasures to the negative effects 

of space flight, especially oxidative damage and radiation-induced cancer risk. It is 

generally agreed that nutrients should be provided to astronauts in standard foods 

instead of supplements (35, 425, 426, 741, 796). The need for more detailed information 

about the “psychophysiology of hunger and eating” was noted decades ago during the 

early space programs (8), but this topic has yet to be studied in detail. It is clear from 

astronauts’ experiences on the Mir that when humans are in an isolated environment 

far from home, food becomes a psychological factor that can be a source of support 

or a source of frustration. 

NASA currently does not recommend that astronauts take general nutritional 

supplements during flight, for several reasons. Experience to date indicates that 

crewmembers do not consume the recommended amount of energy intake, and 

accordingly, intake of many individual nutrients is therefore also inadequate. 

Unfortunately, the concept of a vitamin and mineral supplement to remedy this is 

unwarranted, as the primary problem—inadequate intake of food/energy—will not be 

resolved by a supplement. This situation may even be worsened if crewmembers believe 

that taking the supplement reduces the need for adequate food consumption, and thus 

eat even less. Furthermore, when many nutrients are provided as oral supplements, 
84

they are not metabolized by the body as they are when in foods (797). Changes in 

bioavailability and metabolism of nutrients can increase the risk of malnutrition. 

Vitamin or mineral supplements should be used only when the nutrient content of the 

nominal food system does not meet the requirements for a given nutrient, or when data 

show that the efficacy of single (or multiple) nutrient supplementation is advantageous. 

To date, 1 supplement has met this standard, and that is vitamin D. Vitamin D 

supplements have been provided to all US crewmembers on ISS. Early crews received 

400 IU vitamin D3 per day (6), but recently this was raised to 800 IU per day (425); this 

level allows maintenance of serum 25-hydroxyvitamin D around 75 nmol/L (31). 

Before a supplement is recommended, a clear deficit of that nutrient in the space food 

system must be identified, as was the case with vitamin D. Stability of nutrients in the 

form of supplements would also need to be addressed; shelf lives for exploration travel 

must be particularly long. Supplements, if they are recommended, would need to be 

tested in ground models for their efficacy in maintaining nutrient status, their stability 

over a long duration (3 to 5 y), and their potential interaction with pharmaceuticals. 

Most importantly, supplements will need rigorous testing to demonstrate that the level 

used is not toxic to other body systems, and will need close monitoring during flight to 

ensure that their interactions with the space flight environment do not prevent them from 

being effective or safe. For example, ground-based studies have shown that high doses 

of antioxidants, when provided in situations where oxidant stressors are present (such 

as cigarette smoking), can actually have a detrimental effect (798). Recent studies have 

also found that supplementation with certain antioxidants such as vitamin E and vitamin 

A can increase risks of cancer and all-cause mortality (767, 768). 

An understanding of interactions between nutrients and drugs used in medical care 

is necessary to implement safe and effective medical care and clinical intervention 

operations for astronauts on long-duration missions. The most common studies of 

nutrient-drug interactions concern their effects on a nutrient’s or drug’s absorption, 

distribution, biotransformation, and excretion. 

Normally, drugs must undergo biotransformation to allow their activation or excretion. 

For the activity of a drug to be terminated by excretion, the compound must be made 

water-soluble by biotransformation. For most drugs, this process yields a watersoluble compound that is less active than the original compound. Biotransformation 

occurs in 2 phases. Phase I is an oxidation or hydrolysis reaction to expose, add, or 

cleave a functional group. Cytochrome P450 enzymes are involved in this process. 

Humans have 12 families of cytochrome P450 enzymes, but CYP1, CYP2, and CYP3 

are the forms most commonly used in drug metabolism (799). Cytochrome P450 

enzymes are unique in their ability to use a wide range of substrates (800). Phase II 

biotransformation involves the conjugation of the parent compound to a polar group 

(acetate, glucuronides, sulfates, amino acids, glutathione), which inactivates most 

drugs. Biotransformation of drugs is influenced by several factors that could be affected 

by space flight and the space food system: dietary factors, nutrient metabolism, 

monoamine oxidase inhibitors, and antacids and proton pump inhibitors. 

Dietary Factors 

Dietary factors (either excess or deficiency) can influence both phases of drug 

biotransformation. In phase I, 3 factors are required: a sufficient energy source 

(because of the high energy demands of this system), a protein source for enzyme 

synthesis, and iron for cytochrome formation (801). Phase II requires glucose, 

sulfur-containing amino acids, and glutathione (801). 
85

The effects of nutrients on drug metabolism have been well studied in animal models; 

however, relatively few dietary factors have been studied in humans (801, 802). Results 

from animal studies must be carefully weighed because of some differences between 

the cytochrome P450 enzymes of animals and humans. 

One of the most well-documented food-drug interactions is between grapefruit juice 

and a number of medications (803, 804). Flavonoid compounds such as naringin, 

naringenin, limonin, and obacunone, which are present in grapefruit juice, act as 

substrates for particular intestinal cytochrome P450 enzymes (CYP3A4 and CYP1A2). 

Within hours of ingestion, grapefruit juice decreases CYP3A4 protein expression for 

up to 24 hours (805, 806). The decrease in CYP3A4 is associated with a decreased 

capability for drug metabolism, and therefore increased drug bioavailability. 

Other foods, nutrients, or supplements known to affect phase I and II biotransformations 

and cytochrome P450 enzymes include protein, carbohydrates, lipids, certain vitamins, 

minerals, char-broiled foods, red wine, monosodium glutamate and aspartate, and herbs 

such as St. John’s wort (801, 802, 807-810). Generally, high-protein diets increase drug 

metabolism, and low-protein diets decrease drug metabolism. For instance, antipyrine 

and theophylline are metabolized more rapidly when subjects are on a high-protein diet 

(802). Other macronutrients, including carbohydrates, can affect phase I and phase II 

biotransformation reactions when intakes are very high or low. Theophylline (for asthma) 

is particularly sensitive to dietary protein:carbohydrate ratios; increasing the ratio can 

decrease effectiveness of the drug, and decreasing the ratio may lead to toxicity of the 

drug (811). Fatty acids in the diet can also affect cytochrome P450 enzymes because 

they can be metabolized by these enzymes. Specifically, CYP2E1 is responsible for 

lipid peroxidation, and activity of this enzyme is enhanced in the presence of highly 

polyunsaturated fatty acids such as fish oils. 

Metabolism of Nutrients 

Some nutrients are metabolized by cytochrome P450 enzymes; therefore, drugs or 

other nutrients that alter the activity of these enzymes can alter nutrient metabolism. 

Vitamin D and vitamin A are 2 examples of nutrients whose metabolism involves 

cytochrome P450 enzymes. 

Exposure of 7-dehydrocholesterol to sunlight converts this substrate to previtamin D3. 

Previtamin D3 undergoes an isomerization to form vitamin D3, a biologically inactive 

compound. CYP27A is a mitochondrial mixed-function oxidase that is responsible 

for hydroxylating vitamin D3 to form 25-hydroxyvitamin D3 (812). CYP3A4 has been 

found to be a 25-hydroxylase as well (813). CYP27B converts 25-hydroxyvitamin D3 to 

1,25-dihydroxyvitamin D3. CYP24 is a 24-hydroxylase that hydroxylates the vitamin D 

side chain and ultimately terminates hormonal activity of the vitamin. Inhibition of CYP24 

has recently been targeted in the development of novel anti-cancer drugs. Because 

1,25-dihydroxyvitamin D3 exerts antiproliferative and differentiating effects on many cell 

types including cancer, preventing its inactivation by inhibiting CYP24 activity may prove 

to be beneficial in treating cancer (814). Certain drugs are known to activate CYP24 

activity, including rifampin, isoniazid, and phenobarbital (815, 816). Several studies show 

a relationship between the use of these drugs and osteomalacia (817, 818), which is 

caused by a deficiency of vitamin D. The discovery of the involvement of CYP3A4 in the 

metabolism of vitamin D may explain the effects on vitamin D metabolism of numerous 

drugs, including inducers or inhibitors of this enzyme (for example, grapefruit juice, 

erythromycin, omeprazole, carbamazepine, and dexamethasone), or implicate them 

in unexplained effects on vitamin D metabolism. 
86

Vitamin A metabolism involves the actions of CYP1A2 and CYP4A4 in the conversion 

of retinol to retinoic acid (819, 820). Inducers of CYP1A2 (cigarette smoke, cruciferous 

vegetables, broiled beef, rifampin) may affect vitamin A metabolism. 

Monoamine Oxidase Inhibitors 

First-generation monoamine oxidase inhibitors include agents such as antidepressants 

(phenelzine, tranylcypromine, pargyline, and selegiline), chemotherapeutic drugs 

(procarbazine), antiprotozoal drugs (furazolidone), and analgesics (meperidine). 

Monoamine oxidase is responsible for metabolizing dietary phenylethylamines, including 

tyramine, in the gastrointestinal tract and in the liver. Inhibitors of monoamine oxidase 

prevent the breakdown of these compounds, and therefore the compounds are taken 

up in the brain. In the brain, tyramine displaces norepinephrine from storage vesicles, 

which results in release of a flood of norepinephrine at synapses. Acute hypertension 

and the potential for stroke or myocardial infarction could result from this process 

(801). Fermented foods and protein-rich foods that have begun to spoil are rich in 

phenylethylamines (801). 

Antacids and Proton Pump Inhibitors 

By altering the pH of the stomach, chronic antacid or proton pump medications 

can negatively affect the bioavailability of several nutrients, including phosphate, 

thiamin, folate, vitamin B12, vitamin C, iron, and vitamin A (801, 821, 822). Antacids 

can precipitate folic acid at a pH greater than 4.0, thus rendering it insoluble and not 

available for absorption (823). A high pH also affects thiamin bioavailability because 

the vitamin is not stable at high pH (801). Similarly, at a neutral pH, the antioxidant 

action of vitamin C on dietary nitrites is hindered. Normally, dietary nitrite is quickly 

reduced to nitric oxide by ascorbic acid in the acidic gastric juice and it is then 

absorbed by the mucosa. However, at neutral pH, the nitrite does not react with 

ascorbic acid and instead accumulates in the stomach, which can increase the 

likelihood that potentially carcinogenic N-nitroso compounds will be formed (822). 

These changes are observed mostly in subjects who are infected by Helicobacter 

pylori and are taking proton-pump inhibitors (822). 

Vitamin B12 and vitamin A are also malabsorbed at higher pH because an acidic 

environment is essential for their release from dietary proteins. Because large stores 

of vitamin B12 exist in the body, malabsorption of this vitamin is unlikely to lead to 

deficiency unless a subject has been taking proton pump inhibitors chronically for 

at least 2 years (821). This would be particularly harmful if vitamin B12 stores were 

low before initiation of therapy. 

Summary of Pharmacology and Drug-Nutrient Interactions 

Currently no data are available that pertain to specific drug-nutrient interactions 

during space flight. The main concerns for a long-duration mission involve use of 

pharmacological agents that are taken chronically. Side effects will be especially harmful 

if the status of all nutrients is not adequate at the beginning of a long-duration mission. 

Addressing these concerns of drug-nutrient interactions will be especially crucial for 

crewmembers who embark on exploration-class missions lasting several years. 
87
 

As we write this section of the book in the spring of 2014, the Space Shuttle has been 

retired for a few years, the International Space Station (ISS) has blossomed into an 

incredible science platform, commercial vehicles are delivering cargo to the ISS (and 

in the coming few years they will transport crews to and from ISS as well), preparations 

are underway for the first 1-year stay on ISS, and NASA is targeting development of 

a vehicle that will allow exploration beyond low Earth orbit, destination at this point 

TBD—to be determined. These exploration-class missions will require a food system 

with items having even longer shelf lives than those currently available for the ISS 

missions (56, 824). As NASA designs the vehicles for these missions, the challenges 

for the food system will be very similar to those met by all previous space food systems. 

Mass and volume of the food system and its associated packaging will need to be 

limited. Refrigerators and freezers will most likely not be available. Acceptability of the 

food items will become even more important on a multi-year mission. New challenges 

for this exploration food system will include the need for a 3- to 5-year shelf life and 

the possibility that the increased radiation encountered on a trip to Mars might affect 

the nutritional content and quality of the food over time. 

Long-term plans for exploration will include the establishment of habitats, which will 

no doubt include the growing of plants to aid in the recycling of air and water within 

the habitat (825). These crops will be available for use in the food system. The presence 

of partial gravity will allow crops to be processed into ingredients (for example, milling 

wheat into flour) and then used to prepare menu items for crew consumption (56). The 

research to support these endeavors, especially the growing of crops that will sustain 

a crew and not just supplement meals made on Earth, still has some distance to go. 

These long-term missions will require careful planning of nutrition. Understanding 

nutrient requirements and utilizing the food system to fulfill them will allow mitigation 

of some of the negative effects of microgravity on human physiology. Even a marginal 

nutrient deficiency over a long enough period could be devastating. After the 

requirements are defined and we have a detailed understanding of absorption, 

metabolism, and excretion of each nutrient, provision of these nutrients and an 

understanding of their stability in the space environment (for the months to years 

before they are consumed) will be critical. 

Nutrition is essential for health—on Earth and in space. Determining the nutritional 

requirements for travelers on short-, medium-, and long-duration exploration missions 

will be crucial for ensuring crew health and safety, during the mission and after their 

return. Only the surface has been scratched in our understanding of the ability of 

nutrients and nutrition to mitigate negative effects of space travel, but the initial results 

are promising already, as documented throughout this text. In many ways, nutrition 

offers a suite of countermeasures that require no more crew time than that already 

allotted for meals. While care clearly needs to be taken to avoid excess amounts of 

any nutrient, the risks of using nutritional countermeasures relative to those of using 

pharmacological countermeasures are negligible. 

12 Summary 
88

This document details the evidence collected to date that shows why inadequate 

nutrition is a risk during long-term space travel. The evidence is substantial, and drives 

a significant ongoing effort to understand and optimize nutrition for space travelers and 

to use nutrition as a tool to mitigate the health risks of microgravity exposure. Just as 

for the sailors who left Europe in sailing ships, it is not enough to have food; one must 

have the right food. 
89

13 Appendices 

References 

1.		 Bown SR. Scurvy. New York: St. Martin's Press; 2003. 

2.		 Smith SM, Zwart SR, Kloeris V, Heer M. Nutritional biochemistry of space flight. New York: 

Nova Science Publishers; 2009. 

3.		 Marshall-Bowman K, Barratt MR, Gibson CR. Ophthalmic changes and increased intracranial 

pressure associated with long duration spaceflight: an emerging understanding. Acta 

Astronaut. 2013;87:77-87. 

4.		 Smith SM, Davis-Street JE, Rice BL, Nillen JL, et al. Nutritional status assessment in 

semiclosed environments: ground-based and space flight studies in humans. J Nutr. 2001 

Jul;131(7):2053-61. 

5.		 Soller BR, Cabrera M, Smith SM, Sutton JP. Smart medical systems with application to 

nutrition and fitness in space. Nutrition. 2002;18(10):930-6. 

6.		 Smith SM, Zwart SR, Block G, Rice BL, et al. The nutritional status of astronauts is 

altered after long-term space flight aboard the International Space Station. J Nutr. 2005 

Mar;135(3):437-43. 

7.		 Zwart SR, Launius RD, Coen GK, Morgan JLL, et al. Body mass changes during long-duration 

spaceflight. Aviat Space Environ Med. 2014;85:897-904. 

8.		 Smith MC, Berry CA. Dinner on the moon. Nutr Today. 1969;4:37-42. 

9.		 Waligora JM, Horrigan DJ. Metabolism and heat dissipation during Apollo EVA periods. 

In: Johnston RS, Dietlein LF, Berry CA, editors. Biomedical results of Apollo (NASA SP-368). 

Washington, DC: National Aeronautics and Space Administration; 1975. p. 115-28. 

10. Lane HW, Gretebeck RJ, Smith SM. Nutrition, endocrinology, and body composition during 

space flight. Nutr Res. 1998 Nov;18(11):1923-34. 

11. Lane HW, Gretebeck RJ, Schoeller DA, Davis-Street J, et al. Comparison of ground-based 

and space flight energy expenditure and water turnover in middle-aged healthy male US 

astronauts. Am J Clin Nutr. 1997 Jan;65(1):4-12. 

12. Stein TP, Leskiw MJ, Schluter MD, Hoyt RW, et al. Energy expenditure and balance 

during spaceflight on the space shuttle. Am J Physiol Regul Integr Comp Physiol. 

1999;276:R1739-R48. 

13. Schoeller DA, Ravussin E, Schutz Y, Acheson KJ, et al. Energy expenditure by doubly labeled 

water: validation in humans and proposed calculation. Am J Physiol Regul Integr Comp 

Physiol. 1986 May;250(5 Pt 2):R823-30. 

14. Gretebeck RJ, Schoeller DA, Gibson EK, Lane HW. Energy expenditure during antiorthostatic 

bed rest (simulated microgravity). J Appl Physiol. 1995 Jun;78(6):2207-11. 

15. Stein TP, Schluter MD, Leskiw MJ. Cortisol, insulin and leptin during space flight and bed rest. 

J Gravit Physiol. 1999 Jul;6(1):P85-6. 

16. Lovejoy JC, Smith SR, Zachwieja JJ, Bray GA, et al. Low-dose T(3) improves the bed rest 

model of simulated weightlessness in men and women. Am J Physiol Endocrinol Metab. 1999 

Aug;277(2 Pt 1):E370-9. 

17. World Health Organization. Energy and protein requirements. Report of a joint FAO/WHO/UNU 

expert consultation. Geneva, Switzerland: World Health Organization; 1985. 
90

18. Institute of Medicine. Dietary reference intakes for energy, carbohydrate, fiber, fat, fatty 

acids, cholesterol, protein, and amino acids (macronutrients). Washington, DC: The National 

Academies Press; 2002. 

19. Leach CS, Alfrey CP, Suki WN, Leonard JI, et al. Regulation of body fluid compartments 

during short-term spaceflight. J Appl Physiol. 1996 Jul;81(1):105-16. 

20. Smith SM, Wastney ME, O'Brien KO, Morukov BV, et al. Bone markers, calcium metabolism, 

and calcium kinetics during extended-duration space flight on the Mir space station. J Bone 

Miner Res. 2005 Feb;20(2):208-18. 

21. Heer M, Boerger A, Kamps N, Mika C, et al. Nutrient supply during recent European missions. 

Pflugers Arch. 2000;441(2-3 Suppl):R8-14. 

22. Altman PL, Talbot JM. Nutrition and metabolism in spaceflight. J Nutr. 1987;117:421-7. 

23. Johnson PC, Leach CS, Rambaut PC. Estimates of fluid and energy balances of Apollo 17. 

Aerosp Med. 1973;44:1227-30. 

24. Rambaut PC, Smith MC, Jr, Wheeler HO. Nutritional studies. In: Johnston RS, Dietlein LF, 

Berry CA, editors. Biomedical results of Apollo (NASA SP-368). Washington, DC: National 

Aeronautics and Space Administration; 1975. p. 277-302. 

25. Rambaut PC, Leach CS, Johnson PC. Calcium and phosphorus change of the Apollo 17 

crew members. Nutr Metab. 1975;18(2):62-9. 

26. Stein TP, Schluter MD. Excretion of amino acids by humans during space flight. Acta 

Astronaut. 1998 Jan-Apr;42(1-8):205-14. 

27. Heer M. Nutritional interventions related to bone turnover in European space missions and 

simulation models. Nutrition. 2002 Oct;18(10):853-6. 

28. Rambaut P, Leach C, Leonard J. Observations in energy balance in man during spaceflight. 

Am J Physiol. 1977;233:R208-12. 

29. Leach CS, Rambaut PC. Biochemical responses of the Skylab crewmen: an overview. 

In: Johnston RS, Dietlein LF, editors. Biomedical results from Skylab (NASA SP-377). 

Washington, DC: National Aeronautics and Space Administration; 1977. p. 204-16. 

30. Drummer C, Hesse C, Baisch F, Norsk P, et al. Water and sodium balances and their relation 

to body mass changes in microgravity. Eur J Clin Invest. 2000 Dec;30(12):1066-75. 

31. Smith SM, Heer MA, Shackelford L, Sibonga JD, et al. Benefits for bone from resistance 

exercise and nutrition in long-duration spaceflight: evidence from biochemistry and 

densitometry. J Bone Miner Res. 2012 May 1;27:1896-906. 

32. Bourland C, Kloeris V, Rice B, Vodovotz Y. Food systems for space and planetary flights. 

In: Lane HW, Schoeller DA, editors. Nutrition in spaceflight and weightlessness models. 

Boca Raton, FL: CRC Press; 2000. p. 19-40. 

33. Smith SM, Lane HW. Spaceflight metabolism and nutritional support. In: Barratt MR, Pool SL, 

editors. Principles of clinical medicine for space flight. New York: Springer; 2008. p. 559-76. 

34. Smith SM, Zwart SR. Nutritional biochemistry of spaceflight. Adv Clin Chem. 2008;46:87-130. 

35. Lane HW, Smith SM. Nutrition in space. In: Shils ME, Olson JA, Shike M, Ross AC, editors. 

Modern nutrition in health and disease. 9th ed. Baltimore, MD: Lippincott Williams & Wilkins; 

1999. p. 783-8. 

36. Agureev AN, Kalandarov S, Segal DE. [Optimization of cosmonauts' nutrition during the 

period of acute adaptation and at the closing stage of the mission]. Aviakosm Ekolog Med. 

1997;31(6):47-51. 

37. Olabi AA, Lawless HT, Hunter JB, Levitsky DA, et al. The effect of microgravity and space 

flight on the chemical senses. J Food Sci. 2002 Mar;67(2):468-78. 

38. Baranski S, Kubiczkowa J, Piorko A, Skibniewski F, et al. Electrogustometric investigations 

during manned space flight. Aviat Space Environ Med. 1983 Jan;54(1):1-5. 
91

39. Seddon MR, Fettman MJ, Phillips RW. Practical and clinical nutritional concerns during 

spaceflight. Am J Clin Nutr. 1994 Nov;60(5):825S-30S. 

40. Watt DG, Money KE, Bondar RL, Thirsk RB, et al. Canadian medical experiments on Shuttle 

flight 41-G. Can Aeronaut Space J. 1985 Sep;31(3):215-26. 

41. Budylina SM, Khvatova VA, Volozhin AI. Effect of orthostatic and antiorthostatic hypokinesia 

on taste sensitivity in men. Kosm Biol Aviakosm Med. 1976;10:27-30. 

42. Kurliandskii V, Khvatova VA, Budylina SM. [Functional mobility of taste receptors of the 

tongue under conditions of prolonged hypodynamia]. Stomatologiia (Mosk). 1974 NovDec;53(6):13-5. 

43. Rice BL, Vickers ZM, Rose MS, Lane HW, editors. Fluid shifts during head-down bed rest 

do not influence flavor sensitivity. 67th Annual Scientific Meeting of the Aerospace Medical 

Association; 1996 May 5-9; Atlanta, GA. 

44. Lane HW, LeBlanc AD, Putcha L, Whitson PA. Nutrition and human physiological adaptations 

to space flight. Am J Clin Nutr. 1993;58:583-8. 

45. Smirnov KV, Ugolev AM. Digestion and absorption. In: Leach Huntoon CL, Antipov VV, 

Grigoriev AI, editors. Space biology and medicine. Vol. III, Book 1, Humans in spaceflight. 

Reston, VA: American Institute for Aeronautics and Astronautics; 1996. p. 211-30. 

46. Heer M, Paloski WH. Space motion sickness: incidence, etiology, and countermeasures. 

Auton Neurosci. 2006 Oct 30;129(1-2):77-9. 

47. Reschke MF, Bloomberg JJ, Harm DL, Paloski WH. Space flight and neurovestibular 

adaptation. J Clin Pharmacol. 1994 Jun;34(6):609-17. 

48. Lackner JR, Dizio P. Space motion sickness. Exp Brain Res. 2006 Nov;175(3):377-99. 

49. Nicogossian A. Medicine and space exploration. Lancet. 2003 Dec;362 Suppl:s8-9. 

50. Smith SM, Zwart SR, Heer MA, Hudson EK, Shackelford LC, Morgan JLL. Men and women 

in space: bone loss and kidney stone risk after long-duration space flight. J Bone Miner Res 

29:1639-45, 2014. 

51. Lane HW, Schulz LO. Nutritional questions relevant to space flight. Annu Rev Nutr. 

1992;12:257-78. 

52. Leonard JI, Leach CS, Rambaut PC. Quantitation of tissue loss during prolonged space flight. 

Am J Clin Nutr. 1983;38:667-79. 

53. LaChance PA, Berry CA. Luncheon in space. Nutr Today 1967 June:2-11. 

54. Leach CS. Medical results from STS 1-4: analysis of body fluids. Aviat Space Environ Med. 

1983;54(12 Suppl):S50-4. 

55. Smith SM, Wastney ME, Morukov BV, Larina IM, et al. Calcium metabolism before, during, 

and after a 3-mo spaceflight: kinetic and biochemical changes. Am J Physiol. 1999 Jul;277(1 

Pt 2):R1-10. 

56. Lane HW, Kloeris V, Perchonok M, Zwart S, et al. Food and nutrition for the moon base: 

what have we learned in 45 years of spaceflight. Nutr Today. 2007;42(3):102-10. 

57. Brozek J, Grande F, Taylor HL, Anderson JT, et al. Changes in body weight and body 

dimensions in men performing work on a low calorie carbohydrate diet. J Appl Physiol. 

1957;10(3):412-20. 

58. Faintuch J, Soriano FG, Ladeira JP, Janiszewski M, et al. Changes in body fluid and 

energy compartments during prolonged hunger strike. Rev Hosp Clin Fac Med Sao Paulo. 

2000;55(2):47-54. 

59. Johnson PC, Driscoll TB, Alexander WC, Lambertsen CJ. Body fluid volume changes during 

a 14-day continuous exposure to 5.2% O2 in N2 at pressure equivalent to 100 FSW (4 ata). 

Aerosp Med. 1973;44:860-3. 
92

60. Stein TP, Leskiw MJ, Schluter MD, Donaldson MR, et al. Protein kinetics during and after 

long-duration spaceflight on MIR. Am J Physiol Endocrinol Metab. 1999;276:E1014-21. 

61. Biolo G, Ciocchi B, Stulle M, Bosutti A, et al. Calorie restriction accelerates the catabolism 

of lean body mass during 2 wk of bed rest. Am J Clin Nutr. 2007 Aug;86(2):366-72. 

62. Wade CE, Miller MM, Baer LA, Moran MM, et al. Body mass, energy intake, and water 

consumption of rats and humans during space flight. Nutrition. 2002 Oct;18(10):829-36. 

63. Florian J, Curren M, Baisch F, Pawelczyk J. Caloric restriction decreases orthostatic tolerance. 

FASEB J. 2004;18:478.6. 

64. Blanc S, Somody L, Gharib C. Are energy metabolism alterations involved in cardiovascular 

deconditioning after weightlessness? An hypothesis. Pflugers Arch. 2000;441(2-3 

Suppl):R39-47. 

65. Ferrando AA, Paddon-Jones D, Wolfe RR. Alterations in protein metabolism during space 

flight and inactivity. Nutrition. 2002 Oct;18(10):837-41. 

66. Ihle R, Loucks AB. Dose-response relationships between energy availability and bone 

turnover in young exercising women. J Bone Miner Res. 2004 Aug;19(8):1231-40. 

67. Baek K, Barlow AA, Allen MR, Bloomfield SA. Food restriction and simulated microgravity: 

effects on bone and serum leptin. J Appl Physiol. 2008 Apr;104(4):1086-93. 

68. Oritsland NA. Starvation survival and body composition in mammals with particular reference 

to Homo sapiens. Bull Math Biol. 1990;52(5):643-55. 

69. Korcok M. Hunger strikers may have died of fat, not protein, loss. JAMA. 1981 Oct 23­

30;246(17):1878-9. 

70. Leiter LA, Marliss EB. Survival during fasting may depend on fat as well as protein stores. 

JAMA. 1982 Nov 12;248(18):2306-7. 

71. Phillips WJ. Starvation and survival: some military considerations. Mil Med. 1994 

Jul;159(7):513-6. 

72. Biolo G, Agostini F, Simunic B, Sturma M, et al. Positive energy balance is associated with 

accelerated muscle atrophy and increased erythrocyte glutathione turnover during 5 wk of 

bed rest. Am J Clin Nutr. 2008 Oct;88(4):950-8. 

73. Blanc S, Normand S, Pachiaudi C, Fortrat JO, et al. Fuel homeostasis during physical 

inactivity induced by bed rest. J Clin Endocrinol Metab. 2000 Jun;85(6):2223-33. 

74. Stein TP, Schluter MD, Galante AT, Soteropoulos P, et al. Effect of hind limb muscle unloading 

on liver metabolism of rats. J Nutr Biochem. 2005 Jan;16(1):9-16. 

75. Stein TP, Wade CE. Metabolic consequences of muscle disuse atrophy. J Nutr. 2005 

Jul;135(7):1824S-8S. 

76. Zahariev A, Bergouignan A, Caloin M, Normand S, et al. Skinfold thickness versus isotope 

dilution for body fat assessment during simulated microgravity: results from three bed-rest 

campaigns in men and women with and without countermeasures. Eur J Appl Physiol. 2005 

Oct;95(4):344-50. 

77. Levine DS, Greenleaf JE. Immunosuppression during spaceflight deconditioning. Aviat Space 

Environ Med. 1998;69:172-7. 

78. Stubbs RJ, Harbron CG, Murgatroyd PR, Prentice AM. Covert manipulation of dietary fat and 

energy density: effect on substrate flux and food intake in men eating ad libitum. Am J Clin 

Nutr. 1995 Aug;62(2):316-29. 

79. Maaß H, Raabe W, Wegmann HM. Effects of microgravity on glucose tolerance. In: Sahm 

PR, Keller MH, Schiewe B, editors. Proceedings of the Norderney Symposium on Scientific 

Results of the German Spacelab Mission D-2. Norderney, Germany: Wissenschaftliche 

Projectführung D-2; 1995. p. 732-5. 
93

80. Smirnov KV, Rubinova LG, Afonin BV, Noskov VB, et al. [Functional carbohydrate test during 

237-day space flight]. Kosm Biol Aviakosm Med. 1991 May-Jun;25(3):61-2. 

81. Alexandrov A, Gharib C, Grigoriev AI, Güell A, et al. [Oral glucose tolerance tests in 

man during a space flight of 150 days (Salyut 7-Soyuz T9)]. C R Seances Soc Biol Fil. 

1985;179:192-5. 

82. Biolo G, Ciocchi B, Stulle M, Piccoli A, et al. Metabolic consequences of physical inactivity. 

J Ren Nutr. 2005 Jan;15(1):49-53. 

83. Vernikos-Danellis J, Leach CS, Winget CM, Goodwin AL, et al. Changes in glucose, insulin, 

and growth hormone levels associated with bedrest. Aviat Space Environ Med. 1976 

Jun;47(6):583-7. 

84. Stuart CA, Shangraw RE, Prince MJ, Peters EJ, et al. Bed-rest-induced insulin resistance 

occurs primarily in muscle. Metabolism. 1988 Aug;37(8):802-6. 

85. Stein TP, Schulter MD, Boden G. Development of insulin resistance by astronauts during 

spaceflight. Aviat Space Environ Med. 1994 Dec;65(12):1091-6. 

86. Lane HW, Rambaut PC. Nutrition. In: Nicogossian AE, Huntoon CL, Pool SL, editors. Space 

physiology and medicine. 3rd ed. Philadelphia: Lea & Febiger; 1994. p. 305-16. 

87. Zerwekh JE, Odvina CV, Wuermser LA, Pak CY. Reduction of renal stone risk by potassiummagnesium citrate during 5 weeks of bed rest. J Urol. 2007 Jun;177(6):2179-84. 

88. Pietrzyk RA, Jones JA, Sams CF, Whitson PA. Renal stone formation among astronauts. 

Aviat Space Environ Med. 2007 Apr;78(4 Suppl):A9-13. 

89. Pietrzyk RA, Feiveson AH, Whitson PA. Mathematical model to estimate risk of calciumcontaining renal stones. Miner Electrolyte Metab. 1999 May-Jun;25(3):199-203. 

90. Dolkas CB, Greenleaf JE. Insulin and glucose responses during bed rest with isotonic and 

isometric exercise. J Appl Physiol. 1977 Dec;43(6):1033-8. 

91. Lipman RL, Ulvedal F, Schnure JJ, Bradley EM, et al. Gluco-regulatory hormone response 

to 2-deoxy-d-glucose infusion in normal subjects at bedrest. Metabolism. 1970 

Nov;19(11):980-7. 

92. Paulsrud JR, Pensler L, Whitten CF, Stewart S, et al. Essential fatty acid deficiency in infants 

induced by fat-free intravenous feeding. Am J Clin Nutr. 1972 Sep;25(9):897-904. 

93. Holman RT. Polyunsaturated fatty acid profiles in human disease. In: Bazan NG, Paoletti R, 

Iacono JM, editors. New trends in nutrition, lipid research and cardiovascular diseases. 

New York: Alan R. Liss; 1981. p. 25-42. 

94. Ritz P, Acheson KJ, Gachon P, Vico L, et al. Energy and substrate metabolism during a 42­

day bed-rest in a head-down tilt position in humans. Eur J Appl Physiol Occup Physiol. 1998 

Sep;78(4):308-14. 

95. Mazzucco S, Agostini F, Biolo G. Inactivity-mediated insulin resistance is associated 

with upregulated pro-inflammatory fatty acids in human cell membranes. Clin Nutr. 2010 

Jun;29(3):386-90. 

96. Macho L, Koska J, Ksinantova L, Pacak K, et al. The response of endocrine system to stress 

loads during space flight in human subject. Adv Space Res. 2003;31(6):1605-10. 

97. Stein TP, Schluter MD, Moldawer LL. Endocrine relationships during human spaceflight. 

Am J Physiol Endocrinol Metab. 1999 Jan;276(1 Pt 1):E155-62. 

98. Davidson LA, Nguyen DV, Hokanson RM, Callaway ES, et al. Chemopreventive n-3 

polyunsaturated fatty acids reprogram genetic signatures during colon cancer initiation 

and progression in the rat. Cancer Res. 2004 Sep 15;64(18):6797-804. 

99. Turner ND, Braby LA, Ford J, Lupton JR. Opportunities for nutritional amelioration 

of radiation-induced cellular damage. Nutrition. 2002 Oct;18(10):904-12. 
94

100. Chapkin RS, Davidson LA, Ly L, Weeks BR, et al. Immunomodulatory effects of (n-3) fatty 

acids: putative link to inflammation and colon cancer. J Nutr. 2007 Jan;137(1):200S-4S. 

101. Hong MY, Bancroft LK, Turner ND, Davidson LA, et al. Fish oil decreases oxidative DNA 

damage by enhancing apoptosis in rat colon. Nutr Cancer. 2005;52(2):166-75. 

102. Sanders LM, Henderson CE, Hong MY, Barhoumi R, et al. An increase in reactive oxygen 

species by dietary fish oil coupled with the attenuation of antioxidant defenses by dietary 

pectin enhances rat colonocyte apoptosis. J Nutr. 2004 Dec;134(12):3233-8. 

103. Tisdale MJ. Molecular pathways leading to cancer cachexia. Physiology (Bethesda). 

2005 Oct;20:340-8. 

104. Whitehouse AS, Tisdale MJ. Downregulation of ubiquitin-dependent proteolysis by 

eicosapentaenoic acid in acute starvation. Biochem Biophys Res Commun. 2001 Jul 

20;285(3):598-602. 

105. Whitehouse AS, Smith HJ, Drake JL, Tisdale MJ. Mechanism of attenuation of skeletal 

muscle protein catabolism in cancer cachexia by eicosapentaenoic acid. Cancer Res. 

2001 May 1;61(9):3604-9. 

106. Tisdale MJ. Loss of skeletal muscle in cancer: biochemical mechanisms. Front Biosci. 

2001 Feb 1;6:D164-74. 

107. Tisdale MJ. Cancer anorexia and cachexia. Nutrition. 2001 May;17(5):438-42. 

108. Wigmore SJ, Barber MD, Ross JA, Tisdale MJ, et al. Effect of oral eicosapentaenoic acid 

on weight loss in patients with pancreatic cancer. Nutr Cancer. 2000;36(2):177-84. 

109. Smith HJ, Lorite MJ, Tisdale MJ. Effect of a cancer cachectic factor on protein synthesis/ 

degradation in murine C2C12 myoblasts: modulation by eicosapentaenoic acid. Cancer Res. 

1999 Nov 1;59(21):5507-13. 

110. Wigmore SJ, Ross JA, Falconer JS, Plester CE, et al. The effect of polyunsaturated 

fatty acids on the progress of cachexia in patients with pancreatic cancer. Nutrition. 

1996 Jan;12(1 Suppl):S27-30. 

111. Bayram I, Erbey F, Celik N, Nelson JL, et al. The use of a protein and energy dense 

eicosapentaenoic acid containing supplement for malignancy-related weight loss in children. 

Pediatr Blood Cancer. 2008 Dec 17. 

112. Tisdale MJ. Mechanisms of cancer cachexia. Physiol Rev. 2009 Apr;89(2):381-410. 

113. Griel AE, Kris-Etherton PM, Hilpert KF, Zhao G, et al. An increase in dietary n-3 fatty acids 

decreases a marker of bone resorption in humans. Nutr J. 2007;6:2-10. 

114. Orchard TS, Ing SW, Lu B, Belury MA, et al. The association of red blood cell n-3 and n-6 

fatty acids with bone mineral density and hip fracture risk in the women's health initiative. 

J Bone Miner Res. 2013 Mar;28(3):505-15. 

115. Mangano K, Kerstetter J, Kenny A, Insogna K, et al. An investigation of the association 

between omega 3 FA and bone mineral density among older adults: results from the 

National Health and Nutrition Examination Survey years 2005-2008. Osteoporos Int. 

2014 Mar;25(3):1033-41. 

116. Zwart SR, Pierson D, Mehta S, Gonda S, et al. Capacity of omega-3 fatty acids or 

eicosapentaenoic acid to counteract weightlessness-induced bone loss by inhibiting 

NF-kappaB activation: from cells to bed rest to astronauts. J Bone Miner Res. 

2010;25(5):1049-57. 

117. Orchard TS, Cauley JA, Frank GC, Neuhouser ML, et al. Fatty acid consumption and risk 

of fracture in the Women's Health Initiative. Am J Clin Nutr. 2010 Dec;92(6):1452-60. 

118. Terano T. Effect of omega 3 polyunsaturated fatty acid ingestion on bone metabolism 

and osteoporosis. World Rev Nutr Diet. 2001;88:141-7. 
95

119. Rousseau JH, Kleppinger A, Kenny AM. Self-reported dietary intake of omega-3 fatty 

acids and association with bone and lower extremity function. J Am Geriatric Soc. 2009 

Oct;57(10):1781-8. 

120. Rizos EC, Ntzani EE, Bika E, Kostapanos MS, et al. Association between omega-3 fatty acid 

supplementation and risk of major cardiovascular disease events: a systematic review and 

meta-analysis. JAMA. 2012 Sep 12;308(10):1024-33. 

121. Andreeva VA, Touvier M, Kesse-Guyot E, Julia C, et al. B vitamin and/or omega-3 fatty 

acid supplementation and cancer: ancillary findings from the supplementation with folate, 

vitamins B6 and B12, and/or omega-3 fatty acids (SU.FOL.OM3) randomized trial. Arch Intern 

Med. 2012 Apr 9;172(7):540-7. 

122. Bassey EJ, Littlewood JJ, Rothwell MC, Pye DW. Lack of effect of supplementation with 

essential fatty acids on bone mineral density in healthy pre- and postmenopausal women: 

two randomized controlled trials of Efacal v. calcium alone. Br J Nutr. 2000 Jun;83(6):629-35. 

123. Silber T. Anorexia nervosa: morbidity and mortality. Pediatr Ann. 1984 Nov;13(11):851, 5-9. 

124. Garrow TS, Fletcher K, Halliday O. Body composition in severe infantile malnutrition. 

J Clin Invest. 1965;44:417-25. 

125. Kerstetter JE, Caseria DM, Mitnick ME, Ellison AF, et al. Increased circulating concentrations 

of parathyroid hormone in healthy, young women consuming a protein-restricted diet. 

Am J Clin Nutr. 1997 Nov;66(5):1188-96. 

126. Kerstetter JE, O'Brien KO, Insogna KL. Dietary protein affects intestinal calcium absorption. 

Am J Clin Nutr. 1998 Oct;68(4):859-65. 

127. Freudenheim JL, Johnson NE, Smith EL. Relationships between usual nutrient intake 

and bone-mineral content of women 35-65 years of age: longitudinal and cross-sectional 

analysis. Am J Clin Nutr. 1986 Dec;44(6):863-76. 

128. Promislow JH, Goodman-Gruen D, Slymen DJ, Barrett-Connor E. Protein consumption 

and bone mineral density in the elderly: the Rancho Bernardo Study. Am J Epidemiol. 

2002 Apr 1;155(7):636-44. 

129. Stein TP, Schluter MD. Plasma protein synthesis after spaceflight. Aviat Space Environ Med. 

2006 Jul;77(7):745-8. 

130. Rodriguez NR, DiMarco NM, Langley S. Position of the American Dietetic Association, 

Dietitians of Canada, and the American College of Sports Medicine: nutrition and athletic 

performance. J Am Diet Assoc. 2009 Mar;109(3):509-27. 

131. McCormick DB. Vitamin B6. In: Bowman BA, Russell RM, editors. Present knowledge in 

nutrition. 9th ed. Washington, DC: ILSI Press; 2006. p. 269-77. 

132. Kraemer WJ, Staron RS, Gordon SE, Volek JS, et al. The effects of 10 days of spaceflight on 

the shuttle Endeavor on predominantly fast-twitch muscles in the rat. Histochem Cell Biol. 

2000 Nov;114(5):349-55. 

133. Coburn SP, Lewis DL, Fink WJ, Mahuren JD, et al. Human vitamin B-6 pools estimated 

through muscle biopsies. Am J Clin Nutr. 1988 Aug;48(2):291-4. 

134. Zwart SR, Crawford GE, Gillman PL, Kala G, et al. Effects of 21 days of bed rest, 

with or without artificial gravity, on nutritional status of humans. J Appl Physiol. 

2009 Jul;107(1):54-62. 

135. Coburn SP, Thampy KG, Lane HW, Conn PS, et al. Pyridoxic acid excretion during low 

vitamin B-6 intake, total fasting, and bed rest. Am J Clin Nutr. 1995;62(5):979-83. 

136. Hvas AM, Juul S, Bech P, Nexo E. Vitamin B6 level is associated with symptoms of 

depression. Psychother Psychosom. 2004 Nov-Dec;73(6):340-3. 

137. Bassler KH. Use and abuse of high dosages of vitamin B6. Int J Vitam Nutr Res Suppl. 

1989;30:120-6. 
96

138. Gdynia HJ, Muller T, Sperfeld AD, Kuhnlein P, et al. Severe sensorimotor neuropathy after 

intake of highest dosages of vitamin B6. Neuromuscul Disord. 2008 Feb;18(2):156-8. 

139. Katan MB. [How much vitamin B6 is toxic?]. Ned Tijdschr Geneeskd. 2005 Nov 

12;149(46):2545-6. 

140. Zwart SR, Gibson CR, Mader TH, Ericson K, et al. Vision changes after spaceflight are 

related to alterations in folate- and vitamin B-12-dependent one-carbon metabolism. 

J Nutr. 2012 Mar;142(3):427-31. 

141. Fitts RH, Riley DR, Widrick JJ. Invited review: microgravity and skeletal muscle. 

J Appl Physiol. 2000 Aug;89(2):823-39. 

142. Adams GR, Caiozzo VJ, Baldwin KM. Skeletal muscle unweighting: spaceflight and 

ground-based models. J Appl Physiol. 2003 Dec;95(6):2185-201. 

143. LeBlanc A, Rowe R, Schneider V, Evans H, et al. Regional muscle loss after short duration 

spaceflight. Aviat Space Environ Med. 1995;66:1151-4. 

144. Desplanches D. Structural and functional adaptations of skeletal muscle to weightlessness. 

Int J Sports Med. 1997 Oct;18 Suppl 4:S259-64. 

145. Grogor'eva LS, Kozlovskaia IB. [Effect of weightlessness and hypokinesia on the 

velocity-strength properties of human muscles]. Kosm Biol Aviakosm Med. 

1987 Jan-Feb;21(1):27-30. 

146. Tesch PA, Berg HE, Bring D, Evans HJ, et al. Effects of 17-day spaceflight on knee extensor 

muscle function and size. Eur J Appl Physiol. 2005 Jan;93(4):463-8. 

147. Gopalakrishnan R, Genc KO, Rice AJ, Lee SM, et al. Muscle volume, strength, endurance, 

and exercise loads during 6-month missions in space. Aviat Space Environ Med. 2010 

Feb;81(2):91-102. 

148. Michel EL, Rummel JA, Sawin CF, Buderer MC, et al. Results of Skylab medical experiment 

M-171 - Metabolic Activity. In: Johnston RS, Dietlein LF, editors. Biomedical results from 

Skylab (NASA SP-377). Washington, DC: National Aeronautics and Space Administration; 

1977. p. 372-87. 

149. Antonutto G, Capelli C, Girardis M, Zamparo P, et al. Effects of microgravity on maximal 

power of lower limbs during very short efforts in humans. J Appl Physiol. 1999 Jan;86(1): 

85-92. 

150. Zange J, Muller K, Schuber M, Wackerhage H, et al. Changes in calf muscle performance, 

energy metabolism, and muscle volume caused by long-term stay on space station MIR. 

Int J Sports Med. 1997 Oct;18 Suppl 4:S308-9. 

151. Bajotto G, Shimomura Y. Determinants of disuse-induced skeletal muscle atrophy: 

exercise and nutrition countermeasures to prevent protein loss. J Nutr Sci Vitaminol (Tokyo). 

2006 Aug;52(4):233-47. 

152. Narici MV, de Boer MD. Disuse of the musculo-skeletal system in space and on earth. 

Eur J Appl Physiol. 2011 Mar;111(3):403-20. 

153. Convertino VA. Physiological adaptations to weightlessness: effects on exercise and work 

performance. Exerc Sport Sci Rev. 1990;18:119-66. 

154. Baldwin KM. Effect of spaceflight on the functional, biochemical, and metabolic properties 

of skeletal muscle. Med Sci Sports Exerc. 1996 Aug;28(8):983-7. 

155. Bodine SC. Disuse-induced muscle wasting. Int J Biochem Cell Biol. 2013 Oct;45(10): 

2200-8. 

156. Stein TP. Nutrition and muscle loss in humans during spaceflight. Adv Space Biol Med. 

1999;7:49-97. 

157. Cirillo M, De Santo NG, Heer M, Norsk P, et al. Urinary albumin in space missions. 

J Gravit Physiol. 2002 Jul;9(1):P193-4. 
97

158. Cirillo M, De Santo NG, Heer M, Norsk P, et al. Low urinary albumin excretion in astronauts 

during space missions. Nephron Physiol. 2003;93(4):p102-5. 

159. Cirillo M, Stellato D, Heer M, Drummer C, et al. Urinary albumin in head-down bed rest. 

J Gravit Physiol. 2002 Jul;9(1):P195-6. 

160. Whedon GD, Lutwak L, Rambaut PC, Whittle MW, et al. Mineral and nitrogen metabolic 

studies, experiment M071. In: Johnston RS, Dietlein LF, editors. Biomedical results from 

Skylab (NASA SP-377). Washington, DC: National Aeronautics and Space Administration; 

1977. p. 164-74. 

161. Thornton WE, Rummel JA. Muscle deconditioning and its prevention in space flight. 

In: Johnston RS, Dietlein LF, editors. Biomedical results from Skylab (NASA SP-377). 

Washington, DC: National Aeronautics and Space Administration; 1977. p. 191-7. 

162. Stein TP, Leskiw MJ, Schluter MD. Diet and nitrogen metabolism during spaceflight on 

the shuttle. J Appl Physiol (1985). 1996 Jul;81(1):82-97. 

163. Ushakov AS, Vlasova TF. Free amino acids in human blood plasma during space flights. 

Aviat Space Environ Med. 1976 Oct;47(10):1061-4. 

164. Ushakov AS, Vlasova TF. Amino acid spectrum of human blood plasma during space flight 

and in antiorthostatic hypokinesia. Life Sci Space Res. 1976;14:257-62. 

165. Stein TP, Schluter MD. Plasma amino acids during human spaceflight. Aviat Space Environ 

Med. 1999 Mar;70(3 Pt 1):250-5. 

166. Leach CS, Rambaut PC, Di Ferrante N. Amino aciduria in weightlessness. Acta Astronaut. 

1979;6:1323-33. 

167. Smith GI, Patterson BW, Mittendorfer B. Human muscle protein turnover--why is it so 

variable? J Appl Physiol. 2011 Feb;110(2):480-91. 

168. Stein TP, Leskiw MJ, Schluter MD. Effect of spaceflight on human protein metabolism. 

Am J Physiol Endocrinol Metab. 1993;264:E824-8. 

169. Stein TP. Protein and muscle homeostasis: the role of nutrition. In: Lane HW, Schoeller DA, 

editors. Nutrition in spaceflight and weightlessness models. Boca Raton, FL: CRC Press; 

2000. p. 141-77. 

170. Navasiolava NM, Custaud MA, Tomilovskaya ES, Larina IM, et al. Long-term dry immersion: 

review and prospects. Eur J Appl Physiol. 2011 Jul;111(7):1235-60. 

171. Biolo G, Ciocchi B, Lebenstedt M, Barazzoni R, et al. Short-term bed rest impairs amino 

acid-induced protein anabolism in humans. J Physiol. 2004 Jul 15;558(Pt 2):381-8. 

172. Biolo G, Ciocchi B, Lebenstedt M, Heer M, et al. Sensitivity of whole body protein 

synthesis to amino acid administration during short-term bed rest. J Gravit Physiol. 

2002 Jul;9(1):P197-8. 

173. Drummond MJ, Dickinson JM, Fry CS, Walker DK, et al. Bed rest impairs skeletal 

muscle amino acid transporter expression, mTORC1 signaling, and protein synthesis 

in response to essential amino acids in older adults. Am J Physiol Endocrinol Metab. 

2012 May;302(9):E1113-22. 

174. Ferrando AA, Lane HW, Stuart CA, Davis-Street J, et al. Prolonged bed rest decreases 

skeletal muscle and whole body protein synthesis. Am J Physiol Endocrinol Metab. 

1996 Apr;270(4 Pt 1):E627-E33. 

175. Ferrando AA, Tipton KD, Bamman MM, Wolfe RR. Resistance exercise maintains skeletal 

muscle protein synthesis during bed rest. J Appl Physiol (1985). 1997 Mar;82(3):807-10. 

176. Stuart CA, Shangraw RE, Peters EJ, Wolfe RR. Effect of dietary protein on bed-rest-related 

changes in whole-body-protein synthesis. Am J Clin Nutr. 1990 Sep;52(3):509-14. 

177. Symons TB, Sheffield-Moore M, Chinkes DL, Ferrando AA, et al. Artificial gravity maintains 

skeletal muscle protein synthesis during 21 days of simulated microgravity. J Appl Physiol. 

2009 Jul;107(1):34-8. 
98

178. Glover EI, Phillips SM, Oates BR, Tang JE, et al. Immobilization induces anabolic resistance 

in human myofibrillar protein synthesis with low and high dose amino acid infusion. 

J Physiol. 2008 Dec 15;586(Pt 24):6049-61. 

179. Blanc S, Normand S, Ritz P, Pachiaudi C, et al. Energy and water metabolism, body 

composition, and hormonal changes induced by 42 days of enforced inactivity and 

simulated weightlessness. J Clin Endocrinol Metab. 1998 Dec;83(12):4289-97. 

180. Rambaut PC, Leach CS, Whedon GD. A study of metabolic balance in crewmembers 

of Skylab IV. Acta Astronaut. 1979 Oct;6(10):1313-22. 

181. Heer M, De Santo NG, Cirillo M, Drummer C. Body mass changes, energy, and protein 

metabolism in space. Am J Kidney Dis. 2001 Sep;38(3):691-5. 

182. Drummond MJ, Timmerman KL, Markofski MM, Walker DK, et al. Short-term bed rest 

increases TLR4 and IL-6 expression in skeletal muscle of older adults. Am J Physiol Regul 

Integr Comp Physiol. 2013 Aug;305(3):R216-23. 

183. Bosutti A, Malaponte G, Zanetti M, Castellino P, et al. Calorie restriction modulates 

inactivity-induced changes in the inflammatory markers C-reactive protein and pentraxin-3. 

J Clin Endocrinol Metab. 2008 Aug;93(8):3226-9. 

184. Millet C, Custaud MA, Maillet A, Allevard AM, et al. Endocrine responses to 7 days of headdown bed rest and orthostatic tests in men and women. Clin Physiol. 2001 Mar;21(2):172-83. 

185. Paddon-Jones D, Sheffield-Moore M, Cree MG, Hewlings SJ, et al. Atrophy and impaired 

muscle protein synthesis during prolonged inactivity and stress. J Clin Endocrinol Metab. 

2006 Dec;91(12):4836-41. 

186. Paddon-Jones D, Sheffield-Moore M, Urban RJ, Aarsland A, et al. The catabolic effects 

of prolonged inactivity and acute hypercortisolemia are offset by dietary supplementation. 

J Clin Endocrinol Metab. 2005 Mar;90(3):1453-9. 

187. Fitts RH, Romatowski JG, Peters JR, Paddon-Jones D, et al. The deleterious effects of bed 

rest on human skeletal muscle fibers are exacerbated by hypercortisolemia and ameliorated 

by dietary supplementation. Am J Physiol Cell Physiol. 2007 Jul;293(1):C313-20. 

188. Baldwin KM. Future research directions in seeking countermeasures to weightlessness. 

J Gravit Physiol. 1995;2(1):P51-3. 

189. di Prampero PE, Narici MV. Muscles in microgravity: from fibres to human motion. 

J Biomech. 2003 Mar;36(3):403-12. 

190. Trappe S, Costill D, Gallagher P, Creer A, et al. Exercise in space: human skeletal muscle 

after 6 months aboard the International Space Station. J Appl Physiol. 2009 Apr;106(4): 

1159-68. 

191. LeBlanc A, Lin C, Rowe R, Belichenko O, et al. Muscle loss after long duration spaceflight 

on Mir 18/STS-71 [abstract]. AIAA Life Sciences and Space Medicine Conference;1996. 

p. 53-4, Abstract 96-LS-71. 

192. Shackelford LC, LeBlanc AD, Driscoll TB, Evans HJ, et al. Resistance exercise as a 

countermeasure to disuse-induced bone loss. J Appl Physiol. 2004 July;97(1):119-29. 

193. Tesch PA, Ekberg A, Lindquist DM, Trieschmann JT. Muscle hypertrophy following 5-week 

resistance training using a non-gravity-dependent exercise system. Acta Physiol Scand. 

2004 Jan;180(1):89-98. 

194. Alkner BA, Berg HE, Kozlovskaya I, Sayenko D, et al. Effects of strength training, using a 

gravity-independent exercise system, performed during 110 days of simulated space station 

confinement. Eur J Appl Physiol. 2003 Sep;90(1-2):44-9. 

195. Alkner BA, Tesch PA. Knee extensor and plantar flexor muscle size and function 

following 90 days of bed rest with or without resistance exercise. Eur J Appl Physiol. 

2004 Dec;93(3):294-305. 
99

196. Reeves ND, Maganaris CN, Ferretti G, Narici MV. Influence of 90-day simulated microgravity 

on human tendon mechanical properties and the effect of resistive countermeasures. 

J Appl Physiol. 2005 Jun;98(6):2278-86. 

197. Rittweger J, Felsenberg D, Maganaris C, Ferretti JL. Vertical jump performance after 90 days 

bed rest with and without flywheel resistive exercise, including a 180 days follow-up. 

Eur J Appl Physiol. 2007 Jul;100(4):427-36. 

198. Milesi S, Capelli C, Denoth J, Hutchinson T, et al. Effects of 17 days bed rest on the 

maximal isometric torque of the flexors and extensors of the ankle. J Gravit Physiol. 

1997 Jul;4(2):P125-6. 

199. Schwandt DF, Whalen RT, Watenpaugh DE, Parazynski SE, et al. Development of exercise 

devices to minimize musculoskeletal and cardiovascular deconditioning in microgravity. 

Physiologist. 1991;34 Suppl 1:S189-90. 

200. Greenleaf JE, Bulbulian R, Bernauer EM, Haskell WL, et al. Exercise-training protocols 

for astronauts in microgravity. J Appl Physiol. 1989;67(6):2191-204. 

201. Macias BR, Groppo ER, Eastlack RK, Watenpaugh DE, et al. Space exercise and Earth 

benefits. Curr Pharm Biotechnol. 2005 Aug;6(4):305-17. 

202. Hawkey A. The importance of exercising in space. Interdiscip Sci Rev. 2003 Jun;28(2):130-8. 

203. Krainski F, Hastings JL, Heinicke K, Romain N, et al. The Effect of Rowing Ergometry and 

Resistive Exercise on Skeletal Muscle Structure and Function during Bed Rest. J Appl 

Physiol. 2014 May 1. 

204. Lee SM, Schneider SM, Feiveson AH, Macias BR, et al. WISE-2005: Countermeasures to 

prevent muscle deconditioning during bed rest in women. J Appl Physiol. 2014 Jan 23. 

205. Ploutz-Snyder LL, Downs M, Ryder J, Hackney K, et al. Integrated resistance and aerobic 

exercise protects fitness during bed rest. Med Sci Sports Exerc. 2014 Feb;46(2):358-68. 

206. Belavy DL, Hides JA, Wilson SJ, Stanton W, et al. Resistive simulated weightbearing exercise 

with whole body vibration reduces lumbar spine deconditioning in bed-rest. Spine. 2008 Mar 

1;33(5):E121-31. 

207. Rubin C, Recker R, Cullen D, Ryaby J, et al. Prevention of postmenopausal bone loss by 

a low-magnitude, high-frequency mechanical stimuli: a clinical trial assessing compliance, 

efficacy, and safety. J Bone Miner Res. 2004 Mar;19(3):343-51. 

208. Bleeker MW, De Groot PC, Rongen GA, Rittweger J, et al. Vascular adaptation to 

deconditioning and the effect of an exercise countermeasure: results of the Berlin 

Bed Rest study. J Appl Physiol. 2005 Oct;99(4):1293-300. 

209. Zange J, Mester J, Heer M, Kluge G, et al. 20-Hz whole body vibration training fails to 

counteract the decrease in leg muscle volume caused by 14 days of 6 degrees head 

down tilt bed rest. Eur J Appl Physiol. 2009 Jan;105(2):271-7. 

210. Belavy DL, Armbrecht G, Gast U, Richardson CA, et al. Countermeasures against lumbar 

spine deconditioning in prolonged bed rest: resistive exercise with and without whole body 

vibration. J Appl Physiol. 2010 Dec;109(6):1801-11. 

211. Mikhael M, Orr R, Fiatarone Singh MA. The effect of whole body vibration exposure on 

muscle or bone morphology and function in older adults: a systematic review of the 

literature. Maturitas. 2010 Jun;66(2):150-7. 

212. Prisby RD, Lafage-Proust MH, Malaval L, Belli A, et al. Effects of whole body vibration on 

the skeleton and other organ systems in man and animal models: what we know and what 

we need to know. Ageing Res Rev. 2008 Dec;7(4):319-29. 

213. Rittweger J, Belavy D, Hunek P, Gast U, et al. Highly demanding resistive vibration exercise 

program is tolerated during 56 days of strict bed-rest. Int J Sports Med. 2006 Jul;27(7): 

553-9. 

214. Frost HM. Bone "mass" and the "mechanostat": a proposal. Anat Rec. 1987 Sep;219(1):1-9. 
100

215. Frost HM. Bone's mechanostat: a 2003 update. Anat Rec A Discov Mol Cell Evol Biol. 

2003 Dec;275(2):1081-101. 

216. Baecker N, Frings-Meuthen P, Heer M, Mester J, et al. Effects of vibration training on 

bone metabolism: results from a short-term bed rest study. Eur J Appl Physiol. 2012 

May;112(5):1741-50. 

217. Armbrecht G, Belavy DL, Gast U, Bongrazio M, et al. Resistive vibration exercise attenuates 

bone and muscle atrophy in 56 days of bed rest: biochemical markers of bone metabolism. 

Osteoporos Int. 2010 Apr;21(4):597-607. 

218. Belavy DL, Beller G, Ritter Z, Felsenberg D. Bone structure and density via HR-pQCT in 60d 

bed-rest, 2-years recovery with and without countermeasures. J Musculoskelet Neuronal 

Interact. 2011 Sep;11(3):215-26. 

219. Belavy DL, Miokovic T, Armbrecht G, Rittweger J, et al. Resistive vibration exercise reduces 

lower limb muscle atrophy during 56-day bed-rest. J Musculoskelet Neuronal Interact. 2009 

Oct-Dec;9(4):225-35. 

220. Trudel G, Coletta E, Cameron I, Belavy DL, et al. Resistive exercises, with or without whole 

body vibration, prevent vertebral marrow fat accumulation during 60 days of head-down tilt 

bed rest in men. J Appl Physiol. 2012 Jun;112(11):1824-31. 

221. Wimalawansa SM, Chapa MT, Wei JN, Westlund KN, et al. Reversal of weightlessnessinduced musculoskeletal losses with androgens: quantification by MRI. J Appl Physiol. 

1999 Jun;86(6):1841-6. 

222. Wimalawansa SM, Wimalawansa SJ. Simulated weightlessness-induced attenuation of 

testosterone production may be responsible for bone loss. Endocrine. 1999 Jun;10(3): 

253-60. 

223. Strollo F, Boitani C, Basciani S, Pecorelli L, et al. The pituitary-testicular axis in microgravity: 

analogies with the aging male syndrome. J Endocrinol Invest. 2005;28(11 Suppl 

Proceedings):78-83. 

224. Strollo F. Hormonal changes in humans during spaceflight. Adv Space Biol Med. 1999;7: 

99-129. 

225. Strollo F, Barger L, Fuller C. Testosterone urinary excretion rate increases during hypergravity 

in male monkeys. J Gravit Physiol. 2000 Jul;7(2):P181-2. 

226. Strollo F, Masini MA, Pastorino M, Ricci F, et al. Microgravity-induced alterations in cultured 

testicular cells. J Gravit Physiol. 2004 Jul;11(2):P187-8. 

227. Strollo F, Riondino G, Harris B, Strollo G, et al. The effect of microgravity on testicular 

androgen secretion. Aviat Space Environ Med. 1998 Feb;69(2):133-6. 

228. Strollo F, Strollo G, More M, Ferretti C, et al. Changes in human adrenal and gonadal function 

onboard Spacelab. J Gravit Physiol. 1997 Jul;4(2):P103-4. 

229. Strollo F, Strollo G, Morè M, Mangrossa N, et al. Space flight induces endocrine changes 

at both the pituitary and peripheral level in the absence of any major chronobiologic 

disturbances. In: Sahm PR, Keller MH, Schiewe B, editors. Proceedings of the Norderney 

Symposium on Scientific Results of the German Spacelab Mission D-2. Norderney, 

Germany: Wissenschaftliche Projectführung D-2; 1995. p. 743-7. 

230. Strollo F, Strollo G, More M, Bollanti L, et al. Hormonal adaptation to real and simulated 

microgravity. J Gravitational Physiol. 1998 Jul;5(1):P89-P92. 

231. Wade CE, Stanford KI, Stein TP, Greenleaf JE. Intensive exercise training suppresses 

testosterone during bed rest. J Appl Physiol. 2005 Jul;99(1):59-63. 

232. Space Science Board. Endocrinology. Life beyond the Earth's environment: the biology of 

living organisms in space. Washington, D.C.: National Academy of Sciences; 1979. p. 65. 

233. Strollo F, Norsk P, Roecker L, Strollo G, et al. Indirect evidence of CNS adrenergic pathways 

activation during spaceflight. Aviat Space Environ Med. 1998 Aug;69(8):777-80. 
101

234. Alemany JA, Nindl BC, Kellogg MD, Tharion WJ, et al. Effects of dietary protein content 

on IGF-I, testosterone, and body composition during 8 days of severe energy deficit and 

arduous physical activity. J Appl Physiol. 2008 Jul;105(1):58-64. 

235. Kyrolainen H, Karinkanta J, Santtila M, Koski H, et al. Hormonal responses during a 

prolonged military field exercise with variable exercise intensity. Eur J Appl Physiol. 

2008 Mar;102(5):539-46. 

236. Cangemi R, Friedmann AJ, Holloszy JO, Fontana L. Long-term effects of calorie restriction 

on serum sex-hormone concentrations in men. Aging Cell. 2010 Apr;9(2):236-42. 

237. Smith SM, Heer M, Wang Z, Huntoon CL, et al. Long-duration space flight and bed rest 

effects on testosterone and other steroids. J Clin Endocrinol Metab. 2012 Jan;97(1):270-8. 

238. Murdaca G, Setti M, Brenci S, Fenoglio D, et al. Modifications of immunological and neuroendocrine parameters induced by antiorthostatic bed-rest in human healthy volunteers. 

Minerva Med. 2003 Dec;94(6):363-78. 

239. Zachwieja JJ, Smith SR, Lovejoy JC, Rood JC, et al. Testosterone administration preserves 

protein balance but not muscle strength during 28 days of bed rest. J Clin Endocrinol Metab. 

1999 Jan;84(1):207-12. 

240. Greenleaf JE, Bernauer EM, Ertl AC, Trowbridge TS, et al. Work capacity during 30 days 

of bed rest with isotonic and isokinetic exercise training. J Appl Physiol. 1989 

Nov;67(5):1820-6. 

241. Zorbas YG, Naexu KA, Federenko YF. Blood serum biochemical changes in physically 

conditioned and unconditioned subjects during bed rest and chronic hyperhydration. 

Clin Exp Pharmacol Physiol. 1992 Feb;19(2):137-45. 

242. Smorawinski J, Nazar K, Kaciuba-Uscilko H, Kaminska E, et al. Effects of 3-day bed rest on 

physiological responses to graded exercise in athletes and sedentary men. J Appl Physiol. 

2001 Jul;91(1):249-57. 

243. Tou J, Ronca A, Grindeland R, Wade C. Models to study gravitational biology of Mammalian 

reproduction. Biol Reprod. 2002 Dec;67(6):1681-7. 

244. Vasques M, Lang C, Grindeland RE, Roy RR, et al. Comparison of hyper- and microgravity 

on rat muscle, organ weights and selected plasma constituents. Aviat Space Environ Med. 

1998 Jun;69(6 Suppl):A2-8. 

245. Ortiz RM, Wade CE, Morey-Holton E. Urinary excretion of LH and testosterone from male 

rats during exposure to increased gravity: post-spaceflight and centrifugation. Proc Soc Exp 

Biol Med. 2000 Oct;225(1):98-102. 

246. Amann RP, Deaver DR, Zirkin BR, Grills GS, et al. Effects of microgravity or simulated launch 

on testicular function in rats. J Appl Physiol. 1992 Aug;73(2 Suppl):174S-85S. 

247. Ortiz RM, Wang TJ, Wade CE. Influence of centrifugation and hindlimb suspension 

on testosterone and corticosterone excretion in rats. Aviat Space Environ Med. 1999 

May;70(5):499-504. 

248. Tash JS, Johnson DC, Enders GC. Long-term (6-wk) hindlimb suspension inhibits 

spermatogenesis in adult male rats. J Appl Physiol. 2002 Mar;92(3):1191-8. 

249. Macho L, Kvetnansky R, Fickova M, Popova IA, et al. Effects of exposure to space flight 

on endocrine regulations in experimental animals. Endocr Regul. 2001 Jun;35(2):101-14. 

250. Royland JE, Weber LJ, Fitzpatrick M. Testes size and testosterone levels in a model for 

weightlessness. Life Sci. 1994;54(8):545-54. 

251. Ricci G, Catizone A, Esposito R, Galdieri M. Microgravity effect on testicular functions. 

J Gravit Physiol. 2004 Jul;11(2):P61-2. 

252. Ricci G, Esposito R, Catizone A, Galdieri M. Direct effects of microgravity on testicular 

function: analysis of hystological, molecular and physiologic parameters. J Endocrinol Invest. 

2008 Mar;31(3):229-37. 
102

253. Smith BJ, King JB, Lucas EA, Akhter MP, et al. Skeletal unloading and dietary copper 

depletion are detrimental to bone quality of mature rats. J Nutr. 2002 Feb;132(2):190-6. 

254. Clarke BL, Khosla S. Androgens and bone. Steroids. 2009 Mar;74(3):296-305. 

255. Sheffield-Moore M, Dillon EL, Casperson SL, Gilkison CR, et al. A randomized pilot study 

of monthly cycled testosterone replacement or continuous testosterone replacement versus 

placebo in older men. J Clin Endocrinol Metab. 2011 Nov;96(11):E1831-7. 

256. Paddon-Jones D, Sheffield-Moore M, Urban RJ, Sanford AP, et al. Essential amino acid and 

carbohydrate supplementation ameliorates muscle protein loss in humans during 28 days 

bedrest. J Clin Endocrinol Metab. 2004 Sep;89(9):4351-8. 

257. Brooks N, Cloutier GJ, Cadena SM, Layne JE, et al. Resistance training and timed essential 

amino acids protect against the loss of muscle mass and strength during 28 days of bed rest 

and energy deficit. J Appl Physiol. 2008 Jul;105(1):241-8. 

258. Trappe TA, Burd NA, Louis ES, Lee GA, et al. Influence of concurrent exercise or nutrition 

countermeasures on thigh and calf muscle size and function during 60 days of bed rest in 

women. Acta Physiol (Oxf). 2007 Oct;191(2):147-59. 

259. Antonione R, Caliandro E, Zorat F, Guarnieri G, et al. Whey protein ingestion enhances 

postprandial anabolism during short-term bed rest in young men. J Nutr. 2008 

Nov;138(11):2212-6. 

260. Brooks NE, Cadena SM, Vannier E, Cloutier G, et al. Effects of resistance exercise combined 

with essential amino acid supplementation and energy deficit on markers of skeletal 

muscle atrophy and regeneration during bed rest and active recovery. Muscle Nerve. 2010 

Dec;42(6):927-35. 

261. Ferrando AA, Williams BD, Stuart CA, Lane HW, et al. Oral branched-chain amino acids 

decrease whole-body proteolysis. JPEN J Parenter Enteral Nutr. 1995 Jan-Feb;19(1):47-54. 

262. Stein TP, Blanc S. Does protein supplementation prevent muscle disuse atrophy and loss of 

strength? Crit Rev Food Sci Nutr. 2011 Oct-Nov;51(9):828-34. 

263. Dawson-Hughes B. Interaction of dietary calcium and protein in bone health in humans. 

J Nutr. 2003 Mar;133(3):852S-4S. 

264. Rafferty K, Heaney RP. Nutrient effects on the calcium economy: emphasizing the potassium 

controversy. J Nutr. 2008 Jan;138(1):166S-71S. 

265. Dawson-Hughes B. Calcium and protein in bone health. Proc Nutr Soc. 2003 May;62(2): 

505-9. 

266. Hegsted M, Linkswiler HM. Long-term effects of level of protein intake on calcium 

metabolism in young adult women. J Nutr. 1981 Feb;111(2):244-51. 

267. Borghi L, Schianchi T, Meschi T, Guerra A, et al. Comparison of two diets for the prevention 

of recurrent stones in idiopathic hypercalciuria. N Engl J Med. 2002;346(2):77-84. 

268. Dwyer J, Foulkes E, Evans M, Ausman L. Acid/alkaline ash diets: time for assessment and 

change. J Am Diet Assoc. 1985 Jul;85(7):841-5. 

269. Buclin T, Cosma M, Appenzeller M, Jacquet AF, et al. Diet acids and alkalis influence calcium 

retention in bone. Osteoporos Int. 2001;12(6):493-9. 

270. Zerwekh JE. Nutrition and renal stone disease in space. Nutrition. 2002 Oct;18(10):857-63. 

271. Palacios C. The role of nutrients in bone health, from A to Z. Crit Rev Food Sci Nutr. 

2006;46(8):621-8. 

272. Kerstetter JE, O'Brien KO, Caseria DM, Wall DE, et al. The impact of dietary protein on 

calcium absorption and kinetic measures of bone turnover in women. J Clin Endocrinol 

Metab. 2005 Jan;90(1):26-31. 

273. Lanham-New SA. Fruit and vegetables: the unexpected natural answer to the question 

of osteoporosis prevention? Am J Clin Nutr. 2006 Jun;83(6):1254-5. 
103

274. Lanham-New SA. The balance of bone health: tipping the scales in favor of potassium-rich, 

bicarbonate-rich foods. J Nutr. 2008 Jan;138(1):172S-7S. 

275. Breslau NA, Brinkley L, Hill KD, Pak CY. Relationship of animal protein-rich diet to kidney 

stone formation and calcium metabolism. J Clin Endocrinol Metab. 1988 Jan;66(1):140-6. 

276. Kaneko K, Masaki U, Aikyo M, Yabuki K, et al. Urinary calcium and calcium balance in young 

women affected by high protein diet of soy protein isolate and adding sulfur-containing 

amino acids and/or potassium. J Nutr Sci Vitaminol (Tokyo). 1990 Apr;36(2):105-16. 

277. Zwart SR, Hargens AR, Smith SM. The ratio of animal protein intake to potassium intake 

is a predictor of bone resorption in space flight analogues and in ambulatory subjects. 

Am J Clin Nutr. 2004 Oct;80(4):1058-65. 

278. Zwart SR, Davis-Street JE, Paddon-Jones D, Ferrando AA, et al. Amino acid 

supplementation alters bone metabolism during simulated weightlessness. 

J Appl Physiol. 2005 Jul;99(1):134-40. 

279. Zwart SR, Hargens AR, Lee SM, Macias BR, et al. Lower body negative pressure treadmill 

exercise as a countermeasure for bed rest-induced bone loss in female identical twins. 

Bone. 2007 Feb;40(2):529-37. 

280. Zwart SR, Smith SM. The impact of space flight on the human skeletal system and potential 

nutritional countermeasures. Intl SportMed J. 2005 Dec;6(4):199-214. 

281. Smith SM, Davis-Street JE, Fesperman JV, Calkins DS, et al. Evaluation of treadmill exercise 

in a lower body negative pressure chamber as a countermeasure for weightlessness-induced 

bone loss: a bed rest study with identical twins. J Bone Miner Res. 2003;18:2223-30. 

282. Vormann J, Remer T. Dietary, metabolic, physiologic, and disease-related aspects of 

acid-base balance: foreword to the contributions of the second International Acid-Base 

Symposium. J Nutr. 2008 Feb;138(2):413S-4S. 

283. Zwart SR, Oliver SM, Fesperman JV, Kala G, et al. Nutritional status assessment before, 

during, and after long-duration head-down bed rest. Aviat Space Environ Med. 2009;80(5 

(Suppl)):A15-22. 

284. Fenton TR, Eliasziw M, Lyon AW, Tough SC, et al. Meta-analysis of the quantity of calcium 

excretion associated with the net acid excretion of the modern diet under the acid-ash diet 

hypothesis. Am J Clin Nutr. 2008 Oct;88(4):1159-66. 

285. Kerstetter JE, Looker AC, Insogna KL. Low dietary protein and low bone density. Calcif 

Tissue Int. 2000 Apr;66(4):313. 

286. Kerstetter JE, O'Brien KO, Insogna KL. Low protein intake: the impact on calcium and bone 

homeostasis in humans. J Nutr. 2003 Mar;133(3):855S-61S. 

287. Kerstetter JE, Svastisalee CM, Caseria DM, Mitnick ME, et al. A threshold for low-proteindiet-induced elevations in parathyroid hormone. Am J Clin Nutr. 2000 Jul;72(1):168-73. 

288. Smith MC, Jr, Rambaut PC, Vogel JM, Whittle MW. Bone mineral measurement—experiment 

M078. In: Johnston RS, Dietlein LF, editors. Biomedical results from Skylab (NASA SP-377). 

Washington, DC: National Aeronautics and Space Administration; 1977. p. 183-90. 

289. Vogel JM, Whittle MW. Proceedings: Bone mineral content changes in the Skylab astronauts. 

Am J Roentgenol Radium Ther Nucl Med. 1976 Jun;126(6):1296-7. 

290. Oganov VS, Grigor'ev AI, Voronin LI, Rakhmanov AS, et al. [Bone mineral density in 

cosmonauts after flights lasting 4.5-6 months on the Mir orbital station]. Aviakosm Ekolog 

Med. 1992 Sep-Dec;26(5-6):20-4. 

291. Vico L, Collet P, Guignandon A, Lafage-Proust MH, et al. Effects of long-term microgravity 

exposure on cancellous and cortical weight-bearing bones of cosmonauts. Lancet. 2000 

May 6;355(9215):1607-11. 

292. LeBlanc A, Schneider V, Shackelford L, West S, et al. Bone mineral and lean tissue loss after 

long duration space flight. J Musculoskelet Neuronal Interact. 2000 Dec;1(2):157-60. 
104

293. Collet P, Uebelhart D, Vico L, Moro L, et al. Effects of 1- and 6-month spaceflight on bone 

mass and biochemistry in two humans. Bone. 1997 Jun;20(6):547-51. 

294. Lang T, LeBlanc A, Evans H, Lu Y, et al. Cortical and trabecular bone mineral loss from the 

spine and hip in long-duration spaceflight. J Bone Miner Res. 2004 June;19(6):1006-12. 

295. Weaver CM, LeBlanc A, Smith SM. Calcium and related nutrients in bone metabolism. In: 

Lane HW, Schoeller DA, editors. Nutrition in spaceflight and weightlessness models. Boca 

Raton, FL: CRC Press; 2000. p. 179-201. 

296. Sibonga JD. Spaceflight-induced bone loss: is there an osteoporosis risk? Curr Osteoporos 

Rep. 2013 Jun;11(2):92-8. 

297. Sibonga JD, Cavanagh PR, Lang TF, LeBlanc AD, et al. Adaptation of the skeletal system 

during long-duration spaceflight. Clin Rev Bone Miner Metab. 2008;5(4):249-61. 

298. Whitson P, Pietrzyk R, Pak C, Cintron N. Alterations in renal stone risk factors after space 

flight. J Urol. 1993;150:803-7. 

299. Whitson PA, Pietrzyk RA, Pak CY. Renal stone risk assessment during Space Shuttle flights. 

J Urol. 1997 Dec;158(6):2305-10. 

300. LeBlanc AD, Spector ER, Evans HJ, Sibonga JD. Skeletal responses to space flight and the 

bed rest analog: a review. J Musculoskelet Neuronal Interact. 2007 Jan-Mar;7(1):33-47. 

301. Orwoll ES, Adler RA, Amin S, Binkley N, et al. Skeletal health in long-duration astronauts: 

nature, assessment and management recommendations from the NASA bone summit. 

J Bone Miner Res. 2013 Apr 2;28(6):1243-55. 

302. Sibonga JD, Evans HJ, Sung HG, Spector ER, et al. Recovery of spaceflight-induced bone 

loss: bone mineral density after long-duration missions as fitted with an exponential function. 

Bone. 2007 Dec;41(6):973-8. 

303. Tilton FE, Degioanni JJC, Schneider VS. Long-term follow-up of Skylab bone 

demineralization. Aviat Space Environ Med. 1980;51:1209-13. 

304. LeBlanc AD, Schneider VS, Evans HJ, Engelbretson DA, et al. Bone mineral loss and 

recovery after 17 weeks of bed rest. J Bone Miner Res. 1990;5:843-50. 

305. Smith SM, Nillen JL, LeBlanc A, Lipton A, et al. Collagen cross-link excretion during space 

flight and bed rest. J Clin Endocrinol Metab. 1998 Oct;83(10):3584-91. 

306. Caillot-Augusseau A, Lafage-Proust MH, Soler C, Pernod J, et al. Bone formation and 

resorption biological markers in cosmonauts during and after a 180-day space flight 

(Euromir 95). Clin Chem. 1998 Mar;44(3):578-85. 

307. Seo H, Itoh T, Murata Y, Ohmori S, et al. Changes in urinary excretion of pyridinium 

cross-links during Spacelab-J. Biol Sci Space. 1997 Dec;11(4):321-6. 

308. Zittermann A, Heer M, Caillot-Augusso A, Rettberg P, et al. Microgravity inhibits intestinal 

calcium absorption as shown by a stable strontium test. Eur J Clin Invest. 2000;30:1036-43. 

309. Sorva A, Valimaki M, Risteli J, Risteli L, et al. Serum ionized calcium, intact PTH and 

novel markers of bone turnover in bedridden elderly patients. Eur J Clin Invest. 1994 

Dec;24(12):806-12. 

310. Baecker N, Frings-Meuthen P, Smith SM, Heer M. Short-term high dietary calcium intake 

during bedrest has no effect on markers of bone turnover in healthy men. Nutrition. 2010 

Sep 16;26(5):522-7. 

311. Pavy-Le Traon A, Heer M, Narici MV, Rittweger J, et al. From space to Earth: advances in 

human physiology from 20 years of bed rest studies (1986-2006). Eur J Appl Physiol. 2007 

Sep;101(2):143-94. 

312. Zerwekh JE, Ruml LA, Gottschalk F, Pak CY. The effects of twelve weeks of bed rest on bone 

histology, biochemical markers of bone turnover, and calcium homeostasis in eleven normal 

subjects. J Bone Miner Res. 1998 Oct;13(10):1594-601. 
105

313. Vico L, Chappard D, Alexandre C, Palle S, et al. Effects of a 120 day period of bed-rest 

on bone mass and bone cell activities in man: attempts at countermeasure. Bone Miner. 

1987;2:383-94. 

314. Spector ER, Smith SM, Sibonga JD. Skeletal effects of long-duration head-down bed rest. 

Aviat Space Environ Med. 2009;80(5 Suppl):A23-8. 

315. LeBlanc A, Schneider V, Spector E, Evans H, et al. Calcium absorption, endogenous 

excretion, and endocrine changes during and after long-term bed rest. Bone. 1995 Apr;16 

(4 Suppl):301S-4S. 

316. Morgan JL, Zwart SR, Heer M, Ploutz-Snyder R, et al. Bone metabolism and nutritional 

status during 30-day head-down-tilt bed rest. J Appl Physiol. 2012 Nov;113(10):1519-29. 

317. Smith SM, Zwart SR, Heer M, Lee SMC, et al. WISE-2005: Supine treadmill exercise 

within lower body negative pressure and flywheel resistive exercise as a countermeasure 

to bed rest-induced bone loss in women during 60-day simulated microgravity. Bone. 

2008 Mar;42(3):572-81. 

318. Heer M, Baecker N, Mika C, Boese A, et al. Immobilization induces a very rapid increase 

in osteoclast activity. Acta Astronaut. 2005 Jul;57(1):31-6. 

319. Arnaud SB, Sherrard DJ, Maloney N, Whalen RT, et al. Effects of 1-week head-down tilt 

bed rest on bone formation and the calcium endocrine system. Aviat Space Environ Med. 

1992 Jan;63(1):14-20. 

320. LeBlanc A, Schneider V, Krebs J, Evans H, et al. Spinal bone mineral after 5 weeks of bed 

rest. Calcif Tissue Int. 1987;41:259-61. 

321. Whedon GD. Disuse osteoporosis: physiological aspects. Calcif Tissue Int. 1984;36:S146-50. 

322. Hwang TIS, Hill K, Schneider V, Pak CYC. Effect of prolonged bedrest on the propensity for 

renal stone formation. J Clin Endocrinol Metab. 1988;66:109-12. 

323. Donaldson CL, Hulley SB, Vogel JM, Hattner RS, et al. Effect of prolonged bed rest on bone 

mineral. Metabolism. 1970 Dec;19(12):1071-84. 

324. Whedon G, Lutwak L, Rambaut P, Whittle M, et al. Effect of weightlessness on mineral 

metabolism; metabolic studies on Skylab orbital flights. Calcif Tissue Res. 1976;21 

Suppl:423-30. 

325. Hantman DA, Vogel JM, Donaldson CL, Friedman R, et al. Attempts to prevent disuse 

osteoporosis by treatment with calcitonin, longitudinal compression and supplementary 

calcium and phosphate. J Clin Endocrinol Metab. 1973 May;36(5):845-58. 

326. Deitrick JE, Whedon GD, Shorr E. Effects of immobilization upon various metabolic and 

physiologic functions of normal men. Am J Med. 1948;4:3-36. 

327. Smith SM, Zwart SR, Heer MA, Baecker N, et al. Effects of artificial gravity during bed rest 

on bone metabolism in humans. J Appl Physiol. 2009 Jul;107(1):47-53. 

328. Arnaud SB, Fung P, Popova IA, Morey-Holton ER, et al. Circulating parathyroid hormone 

and calcitonin in rats after spaceflight. J Appl Physiol. 1992 Aug;73(2 Suppl):169S-73S. 

329. Jowsey J, Riggs BL, Goldsmith RS, Kelly PJ, et al. Effects of prolonged administration 

of porcine calcitonin in postmenopausal osteoporosis. J Clin Endocrinol Metab. 1971 

Nov;33(5):752-8. 

330. Eyre DR, Dickson IR, Van Ness K. Collagen cross-linking in human bone and articular 

cartilage. Biochem J. 1988;252:405-500. 

331. Uebelhart D, Gineyts E, Chapuy MC, Delmas PD. Urinary excretion of pyridinium crosslinks: 

a new marker of bone resorption in metabolic bone disease. Bone Miner. 1990;8(1):87-96. 

332. Robins SP, Woitge H, Hesley R, Ju J, et al. Direct, enzyme-linked immunoassay for urinary 

deoxypyridinoline as a specific marker for measuring bone resorption. J Bone Miner Res. 

1994 Oct;9(10):1643-9. 
106

333. Seyedin SM, Kung VT, Daniloff YN, Hesley RP, et al. Immunoassay for urinary pyridinoline: 

the new marker of bone resorption. J Bone Miner Res. 1993 May;8(5):635-41. 

334. Convertino VA. Planning strategies for development of effective exercise and nutrition 

countermeasures for long-duration space flight. Nutrition. 2002 Oct;18(10):880-8. 

335. Schneider VS, McDonald J. Skeletal calcium homeostasis and countermeasures to prevent 

disuse osteoporosis. Calcif Tissue Int. 1984;36 Suppl 1:S151-44. 

336. Oganov VS, Bogomolov VV. [Human bone system in microgravity: review of research data, 

hypotheses and predictability of musculoskeletal system state in extended (exploration) 

missions]. Aviakosm Ekolog Med. 2009 Jan-Feb;43(1):3-12. 

337. Cavanagh PR, Licata AA, Rice AJ. Exercise and pharmacological countermeasures for bone 

loss during long-duration space flight. Gravit Space Biol Bull. 2005 Jun;18(2):39-58. 

338. Novotny SC, Perusek GP, Rice AJ, Comstock BA, et al. A harness for enhanced comfort and 

loading during treadmill exercise in space. Acta Astronaut. 2013;89:205-14. 

339. Rittweger J, Frost HM, Schiessl H, Ohshima H, et al. Muscle atrophy and bone loss after 

90 days' bed rest and the effects of flywheel resistive exercise and pamidronate: results 

from the LTBR study. Bone. 2005 Jun;36(6):1019-29. 

340. Watanabe Y, Ohshima H, Mizuno K, Sekiguchi C, et al. Intravenous pamidronate prevents 

femoral bone loss and renal stone formation during 90-day bed rest. J Bone Miner Res. 

2004 Nov;19(11):1771-8. 

341. Grigoriev AI, Morukov BV, Oganov VS, Rakhmanov AS, et al. Effect of exercise and 

bisphosphonate on mineral balance and bone density during 360 day antiorthostatic 

hypokinesia. J Bone Miner Res. 1992 Dec;7 Suppl 2:S449-55. 

342. Thomsen JS, Morukov BV, Vico L, Alexandre C, et al. Cancellous bone structure of iliac 

crest biopsies following 370 days of head-down bed rest. Aviat Space Environ Med. 

2005 Oct;76(10):915-22. 

343. Berg HE, Eiken O, Miklavcic L, Mekjavic IB. Hip, thigh and calf muscle atrophy and bone 

loss after 5-week bedrest inactivity. Eur J Appl Physiol. 2007 Feb;99(3):283-9. 

344. Loehr JA, Lee SM, English KL, Sibonga J, et al. Musculoskeletal adaptations to training with 

the advanced resistive exercise device. Med Sci Sports Exerc. 2011 Jan;43(1):146-56. 

345. Smith SM, Abrams SA, Davis-Street JE, Heer M, et al. 50 Years of human space travel: 

Implications for bone and calcium research. Annu Rev Nutr. 2014:(34:377-400). 

346. Young LR, Paloski WH. Short radius intermittent centrifugation as a countermeasure to bedrest and 0-G deconditioning: IMAG pilot study summary and recommendations for research. 

J Gravit Physiol. 2007 Jul;14(1):P31-3. 

347. Stenger MB, Evans JM, Knapp CF, Lee SM, et al. Artificial gravity training reduces bed restinduced cardiovascular deconditioning. Eur J Appl Physiol. 2012 Feb;112(2):605-16. 

348. Smith SM, Castaneda-Sceppa C, O'Brien KO, Abrams SA, et al. Calcium kinetics during bed 

rest with artificial gravity and exercise countermeasures. Osteoporos Int 2014 25:2237–44. 

349. Yang Y, Kaplan A, Pierre M, Adams G, et al. Space cycle: a human-powered centrifuge 

that can be used for hypergravity resistance training. Aviat Space Environ Med. 2007 

Jan;78(1):2-9. 

350. Vernikos J. Artificial gravity intermittent centrifugation as a space flight countermeasure. 

J Gravit Physiol. 1997 Jul;4(2):P13-6. 

351. Clément G, Bukley AP, editors. Artificial gravity. New York: Springer; 2007. 

352. Vernikos J, Ludwig DA, Ertl AC, Wade CE, et al. Effect of standing or walking on 

physiological changes induced by head down bed rest: implications for spaceflight. 

Aviat Space Environ Med. 1996 Nov;67(11):1069-79. 
107

353. Greenleaf JE, Chou JL, Stad NJ, Leftheriotis GP, et al. Short-arm (1.9 m) +2.2 Gz 

acceleration: isotonic exercise load-O2 uptake relationship. Aviat Space Environ Med. 

1999 Dec;70(12):1173-82. 

354. Yang Y, Baker M, Graf S, Larson J, et al. Hypergravity resistance exercise: the use 

of artificial gravity as potential countermeasure to microgravity. J Appl Physiol. 

2007 Nov;103(5):1879-87. 

355. Naumann FL, Bennell KL, Wark JD. The effects of +Gz force on the bone mineral density 

of fighter pilots. Aviat Space Environ Med. 2001 Mar;72(3):177-81. 

356. Naumann FL, Grant MC, Dhaliwal SS. Changes in cervical spine bone mineral density 

in response to flight training. Aviat Space Environ Med. 2004 Mar;75(3):255-9. 

357. Iwase S, Takada H, Watanabe Y, Ishida K, et al. Effect of centrifuge-induced artificial gravity 

and ergometric exercise on cardiovascular deconditioning, myatrophy, and osteoporosis 

induced by a -6 degrees head-down bedrest. J Gravit Physiol. 2004 Jul;11(2):P243-4. 

358. Clement G, Pavy-Le Traon A. Centrifugation as a countermeasure during actual and 

simulated microgravity: a review. Eur J Appl Physiol. 2004 Jul;92(3):235-48. 

359. Heer M, Baecker N, Zwart SR, Smith SM. Interactions between artificial gravity, affected 

physiological systems, and nutrition. In: Clement G, Bukley A, editors. Artificial gravity. 

New York: Springer; 2007. p. 249-70. 

360. Holguin N, Uzer G, Chiang FP, Rubin C, et al. Brief daily exposure to low-intensity vibration 

mitigates the degradation of the intervertebral disc in a frequency-specific manner. 

J Appl Physiol. 2011 Dec;111(6):1846-53. 

361. Kiel DP, Hannan MT, Barton BA, Bouxsein ML, et al. Insights from the conduct of a device 

trial in older persons: low magnitude mechanical stimulation for musculoskeletal health. 

Clin Trials. 2010 Aug;7(4):354-67. 

362. Xie L, Rubin C, Judex S. Enhancement of the adolescent murine musculoskeletal system 

using low-level mechanical vibrations. J Appl Physiol. 2008 Apr;104(4):1056-62. 

363. Holguin N, Muir J, Rubin C, Judex S. Short applications of very low-magnitude vibrations 

attenuate expansion of the intervertebral disc during extended bed rest. Spine J. 

2009 Jun;9(6):470-7. 

364. Belavy DL, Beller G, Armbrecht G, Perschel FH, et al. Evidence for an additional effect 

of whole-body vibration above resistive exercise alone in preventing bone loss during 

prolonged bed rest. Osteoporos Int. 2011 May;22(5):1581-91. 

365. Rittweger J, Beller G, Armbrecht G, Mulder E, et al. Prevention of bone loss during 

56 days of strict bed rest by side-alternating resistive vibration exercise. Bone. 2010 

Jan;46(1):137-47. 

366. Wang H, Wan Y, Tam KF, Ling S, et al. Resistive vibration exercise retards bone loss in 

weight-bearing skeletons during 60 days bed rest. Osteoporos Int. 2012 Aug;23(8):2169-78. 

367. Lockwood DR, Vogel JM, Schneider VS, Hulley SB. Effect of the diphosphonate EHDP 

on bone mineral metabolism during prolonged bed rest. J Clin Endocrinol Metab. 

1975;41:533-41. 

368. Iwamoto J, Takeda T, Sato Y. Interventions to prevent bone loss in astronauts during space 

flight. Keio J Med. 2005 Jun;54(2):55-9. 

369. LeBlanc AD, Driscol TB, Shackelford LC, Evans HJ, et al. Alendronate as an effective 

countermeasure to disuse induced bone loss. J Musculoskelet Neuronal Interact. 

2002;2(4):335-43. 

370. Shapiro J, Smith B, Beck T, Ballard P, et al. Treatment with zoledronic acid ameliorates 

negative geometric changes in the proximal femur following acute spinal cord injury. Calcif 

Tissue Int. 2007 May;80(5):316-22. 
108

371. Minaire P, Berard E, Meunier PJ, Edouard C, et al. Effects of disodium dichloromethylene 

diphosphonate on bone loss in paraplegic patients. J Clin Invest. 1981 Oct;68(4):1086-92. 

372. Maheshwari UR, Leybin L, McDonald JT, Schneider VS, et al. Effect of dichloromethylene 

diphosphonate on fluoride balance in healthy men. J Dent Res. 1983 May;62(5):559-61. 

373. Okada A, Ohshima H, Itoh Y, Yasui T, et al. Risk of renal stone formation induced by longterm bed rest could be decreased by premedication with bisphosphonate and increased 

by resistive exercise. Int J Urol. 2008 Jul;15(7):630-5. 

374. Chappard D, Alexandre C, Palle S, Vico L, et al. Effects of a bisphosphonate (1-hydroxy 

ethylidene-1,1 bisphosphonic acid) on osteoclast number during prolonged bed rest in 

healthy humans. Metabolism. 1989 Sep;38(9):822-5. 

375. Li CY, Majeska RJ, Laudier DM, Mann R, et al. High-dose risedronate treatment partially 

preserves cancellous bone mass and microarchitecture during long-term disuse. Bone. 

2005 Sep;37(3):287-95. 

376. Li CY, Price C, Delisser K, Nasser P, et al. Long-term disuse osteoporosis seems less 

sensitive to bisphosphonate treatment than other osteoporosis. J Bone Miner Res. 

2005 Jan;20(1):117-24. 

377. Zorbas YG, Kakuris KK, Deogenov VA, Yerullis KB. Inadequacy of calcium supplements 

to normalize muscle calcium deficiency in healthy subjects during prolonged hypokinesia. 

Nutrition. 2008 Mar;24(3):217-23. 

378. Heer M, Baecker N, Wnendt S, Fischer A, Biolo G, Frings-Meuthen P. How fast is recovery 

of impaired glucose tolerance after 21-day bed rest (NUC Study) in healthy adults? 

ScientificWorldJournal. 2014 Mar 11;2014:803083. doi:10.1155/2014/803083. 

379. Hulley SB, Vogel JM, Donaldson CL, Bayers JH, et al. The effect of supplemental oral 

phosphate on the bone mineral changes during prolonged bed rest. J Clin Invest. 

1971;50:2506-18. 

380. Sun D, Krishnan A, Zaman K, Lawrence R, et al. Dietary n-3 fatty acids decrease 

osteoclastogenesis and loss of bone mass in ovariectomized mice. J Bone Miner Res. 

2003 Jul;18(7):1206-16. 

381. Schneider VS, LeBlanc A, Huntoon CL. Prevention of space flight induced soft tissue 

calcification and disuse osteoporosis. Acta Astronaut. 1993;29(2):139-40. 

382. Whedon GD, Lutwak L, Reid J, Rambaut PC, et al. Mineral and nitrogen metabolic studies 

on Skylab orbital space flights. Trans Assoc Am Physicians. 1974;87:95-110. 

383. Whedon G, Lutwak L, Rambaut P, Whittle M, et al. Mineral and nitrogen balance study 

observations: the second manned Skylab mission. Aviat Space Environ Med. 1976;47:391-6. 

384. Parfitt AM. Bone effects of space flight: analysis by quantum concept of bone remodelling. 

Acta Astronaut. 1981;8(9-10):1083-90. 

385. Rambaut PC, Johnston RS. Prolonged weightlessness and calcium loss in man. 

Acta Astronaut. 1979;6:1113-22. 

386. Grigoriev AI, Oganov VS, Bakulin AV, Poliakov VV, et al. [Clinical and physiological evaluation 

of bone changes among astronauts after long-term space flights]. Aviakosm Ekolog Med. 

1998;32:21-5. 

387. Minaire P, Meunier P, Edouard C, Bernard J, et al. Quantitative histological data on disuse 

osteoporosis: comparison with biological data. Calcif Tissue Int. 1974;17:57-73. 

388. Giangregorio L, Blimkie CJ. Skeletal adaptations to alterations in weight-bearing activity: 

a comparison of models of disuse osteoporosis. Sports Med. 2002;32(7):459-76. 

389. Scott JM, Warburton DE, Williams D, Whelan S, et al. Challenges, concerns and common 

problems: physiological consequences of spinal cord injury and microgravity. Spinal Cord. 

2011 Jan;49(1):4-16. 
109

390. Institute of Medicine. Dietary reference intakes for calcium and vitamin D. Washington, DC: 

National Academies Press; 2011. 

391. Davis CD, Hartmuller V, Freedman DM, Hartge P, et al. Vitamin D and cancer: current 

dilemmas and future needs. Nutr Rev. 2007 Aug;65(8 Pt 2):S71-4. 

392. Holick MF. Vitamin D deficiency in 2010: health benefits of vitamin D and sunlight: a D-bate. 

Nat Rev Endocrinol. 2011 Feb;7(2):73-5. 

393. Bikle DD. Vitamin D regulation of immune function. Vitam Horm. 2011;86:1-21. 

394. Bikle D. Nonclassic actions of vitamin D. J Clin Endocrinol Metab. 2009 Jan;94(1):26-34. 

395. Davis CD, Milner JA. Vitamin D and colon cancer. Expert Rev Gastroenterol Hepatol. 

2011 Feb;5(1):67-81. 

396. Scragg R. Vitamin D and public health: an overview of recent research on common diseases 

and mortality in adulthood. Public Health Nutr. 2011 Sep;14(9):1515-32. 

397. Grant WB. An ecologic study of dietary and solar ultraviolet-B links to breast carcinoma 

mortality rates. Cancer. 2002 Jan 1;94(1):272-81. 

398. Ahonen MH, Tenkanen L, Teppo L, Hakama M, et al. Prostate cancer risk and prediagnostic 

serum 25-hydroxyvitamin D levels (Finland). Cancer Causes Control. 2000 Oct;11(9):847-52. 

399. Grant WB. An estimate of premature cancer mortality in the U.S. due to inadequate doses 

of solar ultraviolet-B radiation. Cancer. 2002 Mar 15;94(6):1867-75. 

400. Hewison M, Zehnder D, Chakraverty R, Adams JS. Vitamin D and barrier function: a novel 

role for extra-renal 1 alpha-hydroxylase. Mol Cell Endocrinol. 2004 Feb 27;215(1-2):31-8. 

401. Norman AW. Vitamin D. In: Bowman BA, Russell RM, editors. Present knowledge in nutrition. 

8th ed. Washington, DC: International Life Sciences Institute Press; 2001. 

402. Holick MF, Chen TC, Lu Z, Sauter E. Vitamin D and skin physiology: a D-lightful story. 

J Bone Miner Res. 2007 Dec;22 Suppl 2:V28-33. 

403. Gilchrest BA. Sun exposure and vitamin D sufficiency. Am J Clin Nutr. 2008 Aug;88(2): 

570S-7S. 

404. Mohr SB, Garland CF, Gorham ED, Grant WB, et al. Relationship between low 

ultraviolet B irradiance and higher breast cancer risk in 107 countries. Breast J. 

2008 May-Jun;14(3):255-60. 

405. Gilchrest BA. Sun protection and Vitamin D: three dimensions of obfuscation. 

J Steroid Biochem Mol Biol. 2007 Mar;103(3-5):655-63. 

406. Smith SM, Gardner KK, Locke J, Zwart SR. Vitamin D supplementation during Antarctic 

winter. Am J Clin Nutr. 2009 Apr;89(4):1092-8. 

407. Zwart SR, Mehta SK, Ploutz-Snyder R, Bourbeau Y, et al. Response to vitamin D 

supplementation during Antarctic winter is related to BMI, and supplementation can 

mitigate Epstein-Barr virus reactivation. J Nutr. 2011 Apr 1;141(4):692-7. 

408. Zwart SR, Parsons H, Kimlin M, Innis SM, et al. A 250 μg/week dose of vitamin D 

was as effective as a 50 μg/d dose in healthy adults, but a regimen of four weekly followed 

by monthly doses of 1250 μg raised the risk of hypercalciuria. Br J Nutr. 2013 Apr 18; 

110:1866-72. 

409. Smith SM, Heer M, Zwart SR. Nutrition and bone health in space. In: Holick M, Nieves J, 

editors. Nutrition and bone health, 2nd ed. New York: Springer; in press Nov 2014. 

410. Bjorkman M, Sorva A, Risteli J, Tilvis R. Vitamin D supplementation has minor effects on 

parathyroid hormone and bone turnover markers in vitamin D-deficient bedridden older 

patients. Age Ageing. 2008 Jan;37(1):25-31. 

411. Thys-Jacobs S, Chan FKW, Koberle LMC, et a. Hypercalcemia due to vitamin D toxicity. In: 

Feldman D, Glorieux FH, Pike JW, editors. Vitamin D. San Diego, CA: Academic Press; 1997. 

p. 883-901. 
110

412. Hathcock JN, Shao A, Vieth R, Heaney R. Risk assessment for vitamin D. Am J Clin Nutr. 

2007 Jan;85(1):6-18. 

413. Vieth R. Vitamin D supplementation, 25-hydroxyvitamin D concentrations, and safety. 

Am J Clin Nutr. 1999 May;69(5):842-56. 

414. Vieth R. Vitamin D toxicity, policy, and science. J Bone Miner Res. 2007 Dec;22 Suppl 2: 

V64-8. 

415. Ferland G. Vitamin K. In: Bowman BA, Russell RM, editors. Present knowledge in nutrition. 

9th ed. Washington, DC: International Life Sciences Institute Press; 2006. p. 220-30. 

416. Wolf J, Vermeer C. Potential effect of vitamin K on microgravity-induced bone loss. 

J Gravit Physiol. 1996;3(2):29-32. 

417. Caillot-Augusseau A, Vico L, Heer M, Voroviev D, et al. Space flight is associated with rapid 

decreases of undercarboxylated osteocalcin and increases of markers of bone resorption 

without changes in their circadian variation: observations in two cosmonauts. Clin Chem. 

2000;46:1136-43. 

418. Vermeer C, Wolf J, Craciun AM, Knapen MH. Bone markers during a 6-month space flight: 

effects of vitamin K supplementation. J Gravit Physiol. 1998 Oct;5(2):65-9. 

419. Zwart SR, Booth SL, Peterson JW, Wang Z, et al. Vitamin K status in spaceflight and 

ground-based models of spaceflight. J Bone Miner Res. 2011 May;26(5):948-54. 

420. Heaney RP, McCarron DA, Dawson-Hughes B, Oparil S, et al. Dietary changes favorably 

affect bone remodeling in older adults. J Am Diet Assoc. 1999 Oct;99(10):1228-33. 

421. Anderson JJB, Klemmer PJ, Watts MLS, Garner SC, et al. Phosphorus. In: Bowman BA, 

Russell RM, editors. Present knowledge in nutrition. 9th ed. Washington, DC: International 

Life Sciences Institute Press; 2006. p. 383-99. 

422. Calvo MS, Moshfegh AJ, Tucker KL. Assessing the health impact of phosphorus in the 

food supply: issues and considerations. Adv Nutr. 2014;5(1):104-13. 

423. Chang AR, Lazo M, Appel LJ, Gutierrez OM, et al. High dietary phosphorus intake is 

associated with all-cause mortality: results from NHANES III. Am J Clin Nutr. 2014 

Feb;99(2):320-7. 

424. Takeda E, Yamamoto H, Yamanaka-Okumura H, Taketani Y. Increasing dietary phosphorus 

intake from food additives: potential for negative impact on bone health. Adv Nutr. 

2014;5(1):92-7. 

425. National Aeronautics and Space Administration Johnson Space Center. Nutrition 

requirements, standards, and operating bands for exploration missions. Houston, TX: 

Lyndon B. Johnson Space Center; 2005. Report No.: JSC-63555. 

426. National Aeronautics and Space Administration Johnson Space Center. Nutritional 

requirements for International Space Station (ISS) missions up to 360 days. Houston, TX: 

National Aeronautics and Space Administration Lyndon B. Johnson Space Center; 1996. 

Report No.: JSC-28038. 

427. Shils ME. Magnesium. In: Shils ME, Olson JA, Shike M, Ross AC, editors. Modern nutrition 

in health and disease. 9th ed. Baltimore, MD: Lippincott Williams & Wilkins; 1999. p. 169-92. 

428. Leach CS, Rambaut PC. Biochemical observations of long duration manned orbital 

spaceflight. J Am Med Womens Assoc. 1975 Apr;30(4):153-72. 

429. Leach-Huntoon CS, Schneider H, Cintron NM, Landry R. Combined blood investigations. In: 

Bungo MW, Bagian TM, Bowman MA, Levitan BM, editors. Results of the life sciences DSOs 

conducted aboard the Space Shuttle 1981-1986. Houston: Space Biomedical Research 

Institute, Johnson Space Center; 1987. p. 7-11. 

430. Leach CS. Biochemical and hematologic changes after short-term space flight. 

Microgravity Q. 1992;2:69-75. 
111

431. Prokhonchukov AA, Zaitsev VP, Shakhunov BA, Zhizhina NA, et al. [Effect of space flight 

on the concentration of sodium, copper, manganese and magnesium in the bones of the 

skeleton]. Patol Fiziol Eksp Ter. 1978 Nov-Dec(6):65-70. 

432. Schwille PO, Schmiedl A, Herrmann U, Fan J, et al. Magnesium, citrate, magnesium citrate 

and magnesium-alkali citrate as modulators of calcium oxalate crystallization in urine: 

observations in patients with recurrent idiopathic calcium urolithiasis. Urol Res. 1999 

Apr;27(2):117-26. 

433. Whitson PA, Pietrzyk RA, Jones JA, Nelman-Gonzalez M, et al. Effect of potassium citrate 

therapy on the risk of renal stone formation during spaceflight. J Urol. 2009 Nov;182(5): 

2490-6. 

434. Cousins RJ. Zinc. In: Bowman BA, Russell RM, editors. Present knowledge in nutrition. 

9th ed. Washington, DC: International Life Sciences Institute Press; 2006. p. 445-57. 

435. Krebs JM, Schneider VS, LeBlanc AD, Kuo MC, et al. Zinc and copper balances in healthy 

adult males during and after 17 wk of bed rest. Am J Clin Nutr. 1993 Dec;58(6):897-901. 

436. Krebs JM, Schneider VS, LeBlanc AD. Zinc, copper, and nitrogen balances during bed rest 

and fluoride supplementation in healthy adult males. Am J Clin Nutr. 1988 Mar;47(3):509-14. 

437. Cann CE, Adachi RR. Bone resorption and mineral excretion in rats during spaceflight. 

Am J Physiol. 1983;13:R327-31. 

438. Kondrashov V, Rothenberg SJ, Chettle D, Zerwekh J. Evaluation of potentially significant 

increase of lead in the blood during long-term bed rest and space flight. Physiol Meas. 

2005 Feb;26(1):1-12. 

439. Kondrashov VS. Cosmonauts and lead: resorption and increased blood lead levels during 

long term space flight. J Med Toxicol. 2006 Dec;2(4):172-3. 

440. Garcia HD, Hays SM, Tsuji JS. Modeling of blood lead levels in astronauts exposed to lead 

from microgravity-accelerated bone loss. Aviat Space Environ Med. 2013 Dec;84(12): 

1229-34. 

441. Smith SM, McCoy T, Gazda D, Morgan JL, et al. Space flight calcium: implications for 

astronaut health, spacecraft operations, and Earth. Nutrients. 2012 Dec;4(12):2047-68. 

442. Morgan JL, Skulan JL, Gordon GW, Romaniello SJ, et al. Rapidly assessing changes 

in bone mineral balance using natural stable calcium isotopes. Proc Natl Acad Sci U S A. 

2012 Jun 19;109(25):9989-94. 

443. Russell WA, Papanastassiou DA, Tombrello TA. Ca isotope fractionation on earth and 

other solar-system materials. Geochim Cosmochim Acta. 1978;42(8):1075-90. 

444. Morgan JL, Gordon GW, Arrua RC, Skulan JL, et al. High-precision measurement of 

variations in calcium isotope ratios in urine by multiple collector inductively coupled 

plasma mass spectrometry. Anal Chem. 2011 Sep 15;83(18):6956-62. 

445. Skulan J, Bullen T, Anbar AD, Puzas JE, et al. Natural calcium isotopic composition 

of urine as a marker of bone mineral balance. Clin Chem. 2007 Jun;53(6):1155-8. 

446. Skulan JL, DePaolo DJ. Calcium isotope fractionation between soft and mineralized tissue 

as a monitor of calcium use in vertebrates. Proc Natl Acad Sci U S A. 1999;96(24):13709-13. 

447. Beard JL. Iron. In: Bowman BA, Russell RM, editors. Present knowledge in nutrition. 9th ed. 

Washington, DC: International Life Sciences Institute; 2006. p. 430-44. 

448. Fairbanks VF. Iron in medicine and nutrition. In: Shils ME, Olson JA, Shike M, Ross AC, 

editors. Modern nutrition in health and disease. 9th ed. Baltimore, MD: Lippincott 

Williams & Wilkins; 1999. p. 193-221. 

449. Milne DB, Gallagher SK, Nielsen FH. Response of various indices of iron status to acute iron 

depletion produced in menstruating women by low iron intake and phlebotomy. Clin Chem. 

1990 Mar;36(3):487-91. 
112

450. Prieto J, Barry M, Sherlock S. Serum ferritin in patients with iron overload and with acute 

and chronic liver disease. Gastroenterology. 1975;68:525-33. 

451. Viteri FE, Toran B. Anemia and physical work capacity. Clinical Hematology. 1974;3:609-26. 

452. Lozoff B. Behavioral alterations in iron deficiency. Adv Pediatr. 1988;35:331-59. 

453. Dallman PR. Iron deficiency and the immune response. Am J Clin Nutr. 1987;46:329-34. 

454. Yip R, Dallman PR. Iron. In: Ziegler EE, Filer LJ, Jr., editors. Present knowledge in nutrition. 

7th ed. Washington, DC: International Life Sciences Institute; 1996. p. 277-92. 

455. Cook JD. Adaptation in iron metabolism. Am J Clin Nutr. 1990 Feb;51(2):301-8. 

456. Finch CA, Huebers HA. Perspectives in iron metabolism. N Engl J Med. 1982;306:1520-8. 

457. Dao MC, Meydani SN. Iron biology, immunology, aging, and obesity: four fields connected 

by the small peptide hormone hepcidin. Adv Nutr. 2013;4(6):602-17. 

458. Sazawal S, Black RE, Ramsan M, Chwaya HM, et al. Effects of routine prophylactic 

supplementation with iron and folic acid on admission to hospital and mortality in preschool 

children in a high malaria transmission setting: community-based, randomised, placebocontrolled trial. Lancet. 2006 Jan 14;367(9505):133-43. 

459. Doherty CP. Host-pathogen interactions: the role of iron. J Nutr. 2007 May;137(5):1341-4. 

460. Alfrey CP, Rice L, Udden MM, Driscoll TB. Neocytolysis: physiological down-regulator 

of red-cell mass. Lancet. 1997 May 10;349(9062):1389-90. 

461. Alfrey CP, Udden MM, Leach-Huntoon C, Driscoll T, et al. Control of red blood cell mass 

in spaceflight. J Appl Physiol. 1996;81:98-104. 

462. Johnson PC. The erythropoietic effects of weightlessness. In: Dunn CDR, editor. Current 

concepts in erythropoiesis. New York: John Wiley & Sons Ltd.; 1983. p. 279-300. 

463. Udden MM, Driscoll TB, Pickett MH, Leach-Huntoon CS, et al. Decreased production of red 

blood cells in human subjects exposed to microgravity. J Lab Clin Med. 1995;125:442-9. 

464. Smith SM. Red blood cell and iron metabolism during space flight. Nutrition. 

2002 Oct;18(10):864-6. 

465. Institute of Medicine. Dietary reference intakes for vitamin A, vitamin K, arsenic, boron, 

chromium, copper, iodine, iron, manganese, molybdenum, nickel, silicon, vanadium, 

and zinc. Washington, DC: National Academy Press; 2001. 

466. Smith SM, Davis-Street JE, Fontenot TB, Lane HW. Assessment of a portable clinical blood 

analyzer during space flight. Clin Chem. 1997 Jun;43(6 Pt 1)(6 Pt 1):1056-65. 

467. Convertino VA. Clinical aspects of the control of plasma volume at microgravity and 

during return to one gravity. Med Sci Sports Exerc. 1996 Oct;28(10 Suppl):S45-52. 

468. Lane HW, Alfrey CP, Driscoll TB, Smith SM, et al. Control of red blood cell mass during 

spaceflight. J Gravitational Physiol. 1996 Sep;3(2):87-8. 

469. Zwart SR, Morgan JL, Smith SM. Iron status and its relations with oxidative damage 

and bone loss during long-duration space flight on the International Space Station. 

Am J Clin Nutr. 2013 May 29;98:217-23. 

470. Beilby J, Olynyk J, Ching S, Prins A, et al. Transferrin index: an alternative method for 

calculating the iron saturation of transferrin. Clin Chem. 1992 Oct;38(10):2078-81. 

471. Mendes JF, Arruda SF, Siqueira EM, Ito MK, et al. Iron status and oxidative stress 

biomarkers in adults: a preliminary study. Nutrition. 2009 Apr;25(4):379-84. 

472. Tuomainen TP, Loft S, Nyyssonen K, Punnonen K, et al. Body iron is a contributor 

to oxidative damage of DNA. Free Radic Res. 2007 Mar;41(3):324-8. 

473. Syrovatka P, Kraml P, Potockova J, Fialova L, et al. Relationship between increased 

body iron stores, oxidative stress and insulin resistance in healthy men. Ann Nutr Metab. 

2009 Jul 27;54(4):268-74. 
113

474. Dunn CDR, Lange RD, Kimzey SL, Johnson PC, et al. Serum erythropoietin titers during 

prolonged bedrest; relevance to the "anaemia" of space flight. Eur J Appl Physiol. 

1984;52:178-82. 

475. Rice L, Ruiz W, Driscoll T, Whitley CE, et al. Neocytolysis on descent from altitude: 

a newly recognized mechanism for the control of red cell mass. Ann Intern Med. 

2001 Apr 17;134(8):652-6. 

476. Smith SM, Davis-Street JE, Fesperman JV, Smith MD, et al. Nutritional assessment during 

a 14-d saturation dive: the NASA Extreme Environment Mission Operations V Project. 

J Nutr. 2004;134:1765-71. 

477. Bader N, Bosy-Westphal A, Koch A, Mueller MJ. Influence of vitamin C and E 

supplementation on oxidative stress induced by hyperbaric oxygen in healthy men. 

Ann Nutr Metab. 2006;50(3):173-6. 

478. Rocco M, Antonelli M, Letizia V, Alampi D, et al. Lipid peroxidation, circulating cytokine 

and endothelin 1 levels in healthy volunteers undergoing hyperbaric oxygenation. 

Minerva Anestesiol. 2001 May;67(5):393-400. 

479. Alleva R, Nasole E, Di Donato F, Borghi B, et al. alpha-Lipoic acid supplementation inhibits 

oxidative damage, accelerating chronic wound healing in patients undergoing hyperbaric 

oxygen therapy. Biochem Biophys Res Commun. 2005 Jul 29;333(2):404-10. 

480. Djurhuus R, Segadal K, Svardal AM. Glutathione in blood cells decreases without DNA 

breaks after a simulated saturation dive to 250 msw. Aviat Space Environ Med. 2006 

Jun;77(6):597-604. 

481. Zwart SR, Jessup JM, Ji J, Smith SM. Saturation diving alters folate status and biomarkers 

of DNA damage and repair. PLoS One. 2012;7(2):e31058. 

482. Kim BJ, Ahn SH, Bae SJ, Kim EH, et al. Iron overload accelerates bone loss in healthy 

postmenopausal women and middle-aged men: a 3-year retrospective longitudinal study. 

J Bone Miner Res. 2012 Nov;27(11):2279-90. 

483. He X, Hahn P, Iacovelli J, Wong R, et al. Iron homeostasis and toxicity in retinal degeneration. 

Prog Retin Eye Res. 2007 Nov;26(6):649-73. 

484. Morgan JL, Ritchie LE, Crucian BE, Theriot C, et al. Increased dietary iron and radiation in 

rats promote oxidative stress, induce localized and systemic immune system responses, 

and alter colon mucosal environment. FASEB J. 2014 Mar;28(3):1486-98. 

485. Sempos CT, Looker AC, Gillum RF, Makuc DM. Body iron stores and the risk of coronary 

heart disease. N Engl J Med. 1994;330(16):1119-24. 

486. Ascherio A, Willett WC. Are body iron stores related to the risk of coronary heart disease? 

(Editorial). N Engl J Med. 1994;330:1152-4. 

487. Sullivan JL. Stored iron and ischemic heart disease: empirical support for a new paradigm 

(Editorial Comment). Circulation. 1992;86:1036-7. 

488. Lauffer RB. Iron stores and the international variation in mortality from coronary artery 

disease. Lancet. 1991;2:1288-9. 

489. Salonen JT, Nyyssonen K, Korpela H, Tuomilehto J, et al. High stored iron levels are 

associated with excess risk of myocardial infarction in eastern Finnish men. Circulation. 

1992;86(3):803-11. 

490. Sullivan JL. The iron paradigm of ischemic heart disease. Am Heart J. 1989;117:1177-88. 

491. Knekt P, Reunanen A, Takkunen H, Aromaa A, et al. Body iron stores and risk of cancer. 

Int J Cancer. 1994;56(3):379-82. 

492. Mainous AG, 3rd, Wells BJ, Koopman RJ, Everett CJ, et al. Iron, lipids, and risk of cancer 

in the Framingham Offspring cohort. Am J Epidemiol. 2005 Jun 15;161(12):1115-22. 
114

493. Schreiber WE. Iron, porphyrin, and bilirubin metabolism. In: Kaplan LA, Pesce AJ, editors. 

Clinical chemistry: theory, analysis, and correlation. St. Louis, MO: Mosby-Year Books, Inc.; 

1996. p. 696-715. 

494. Wilson JW, Ott CM, Honer zu Bentrup K, Ramamurthy R, et al. Space flight alters bacterial 

gene expression and virulence and reveals a role for global regulator Hfq. Proc Natl Acad 

Sci U S A. 2007 Oct 9;104(41):16299-304. 

495. Payne SM. Iron and virulence in the family Enterobacteriaceae. Crit Rev Microbiol. 

1988;16(2):81-111. 

496. Prohaska JR. Copper. In: Bowman BA, Russell RM, editors. Present knowledge in nutrition. 

9th ed. Washington, DC: International Life Sciences Institute Press; 2006. p. 458-70. 

497. Davis AT, Franz FP, Courtnay DA, Ullrey DE, et al. Plasma vitamin and mineral status in home 

parenteral nutrition patients. JPEN J Parenter Enteral Nutr. 1987 Sep-Oct;11(5):480-5. 

498. Food and Nutrition Board, Institute of Medicine. Dietary Reference Intakes: a risk assessment 

model for establishing upper intake levels for nutrients. Washington, DC: National Academy 

Press; 1998. 

499. Higuchi S, Higashi A, Nakamura T, Matsuda I. Nutritional copper deficiency in severely 

handicapped patients on a low copper enteral diet for a prolonged period: estimation of 

the required dose of dietary copper. J Pediatr Gastroenterol Nutr. 1988 Jul-Aug;7(4):583-7. 

500. Milne DB. Copper intake and assessment of copper status. Am J Clin Nutr. 1998 May; 

67(5 Suppl):1041S-5S. 

501. Bourre JM. Effects of nutrients (in food) on the structure and function of the nervous system: 

update on dietary requirements for brain. Part 1: micronutrients. J Nutr Health Aging. 

2006 Sep-Oct;10(5):377-85. 

502. Taylor GR, Konstantinova I, Sonnenfeld G, Jennings R. Changes in the immune system 

during and after spaceflight. Adv Space Biol Med. 1997;6:1-32. 

503. Borchers AT, Keen CL, Gershwin ME. Microgravity and immune responsiveness: implications 

for space travel. Nutrition. 2002 Oct;18(10):889-98. 

504. Tipton CM, Greenleaf JE, Jackson CG. Neuroendocrine and immune system responses 

with spaceflights. Med Sci Sports Exerc. 1996 Aug;28(8):988-98. 

505. Crucian BE, Stowe RP, Pierson DL, Sams CF. Immune system dysregulation following 

short- vs long-duration spaceflight. Aviat Space Environ Med. 2008 Sep;79(9):835-43. 

506. Devine A, Hodgson JM, Dick IM, Prince RL. Tea drinking is associated with benefits on 

bone density in older women. Am J Clin Nutr. 2007 Oct;86(4):1243-7. 

507. Norsk P. Blood pressure regulation IV: adaptive responses to weightlessness. 

Eur J Appl Physiol. 2014 Mar;114(3):481-97. 

508. Norsk P, Damgaard M, Petersen L, Gybel M, et al. Vasorelaxation in space. Hypertension. 

2006;47(1):69-73. 

509. Aubert AE, Beckers F, Verheyden B. Cardiovascular function and basics of physiology 

in microgravity. Acta Cardiol. 2005 Apr;60(2):129-51. 

510. Zhang LF. Region-specific vascular remodeling and its prevention by artificial gravity 

in weightless environment. Eur J Appl Physiol. 2013 Dec;113(12):2873-95. 

511. Moore AD, Lee SMC, Stenger MB, Platts SH. Cardiovascular exercise in the U.S. space 

program: past, present, and future. Acta Astronaut. 2010;66:974-88. 

512. Lee SM, Moore AD, Everett ME, Stenger MB, et al. Aerobic exercise deconditioning and 

countermeasures during bed rest. Aviat Space Environ Med. 2010 Jan;81(1):52-63. 

513. Iwasaki K, Levine BD, Zhang R, Zuckerman JH, et al. Human cerebral autoregulation 

before, during and after spaceflight. J Physiol. 2007 Mar 15;579(Pt 3):799-810. 
115

514. Nie ZL, Wang ZM, Zhou B, Tang ZP, et al. Magnesium intake and incidence of stroke: 

meta-analysis of cohort studies. Nutr Metab Cardiovasc Dis. 2013 Mar;23(3):169-76. 

515. Rowe WJ. As with space flight, a magnesium deficit may be responsible for both peripheral 

vascular dysfunction and kidney disease. Am J Cardiol. 2010 Apr 15;105(8):1203-4. 

516. Volpe SL. Magnesium in disease prevention and overall health. Adv Nutr. 

2013;4(3):378S-83S. 

517. Bellavia A, Larsson SC, Bottai M, Wolk A, et al. Fruit and vegetable consumption and 

all-cause mortality: a dose-response analysis. Am J Clin Nutr. 2013 Aug;98(2):454-9. 

518. Boeing H, Bechthold A, Bub A, Ellinger S, et al. Critical review: vegetables and fruit in 

the prevention of chronic diseases. Eur J Nutr. 2012 Sep;51(6):637-63. 

519. Cocate PG, Natali AJ, de Oliveira A, Longo GZ, et al. Fruit and vegetable intake and related 

nutrients are associated with oxidative stress markers in middle-aged men. Nutrition. 

2014 Jun;30(6):660-5. 

520. Leenders M, Sluijs I, Ros MM, Boshuizen HC, et al. Fruit and vegetable consumption and 

mortality: European prospective investigation into cancer and nutrition. Am J Epidemiol. 

2013 Aug 15;178(4):590-602. 

521. Macready AL, George TW, Chong MF, Alimbetov DS, et al. Flavonoid-rich fruit and 

vegetables improve microvascular reactivity and inflammatory status in men at risk 

of cardiovascular disease--FLAVURS: a randomized controlled trial. Am J Clin Nutr. 

2014 Mar;99(3):479-89. 

522. Oyebode O, Gordon-Dseagu V, Walker A, Mindell JS. Fruit and vegetable consumption 

and all-cause, cancer and CVD mortality: analysis of Health Survey for England data. 

J Epidemiol Community Health. 2014 Mar 31. 

523. Oh MS, Uribarri J. Electrolytes, water, and acid-base balance. In: Shils ME, Olson JA, 

Shike M, Ross AC, editors. Modern nutrition in health and disease. 9th ed. Baltimore, MD: 

Lippincott Williams & Wilkins; 1999. p. 105-40. 

524. Tietz NW, Pruden EL, Siggaard-Andersen O. Electrolytes. In: Burtis CA, Ashwood ER, 

editors. Tietz textbook of clinical chemistry. 2nd ed. Philadelphia, PA: WB Saunders 

Company; 1994. p. 1887-973. 

525. Preuss HG. Electrolytes: sodium, chloride, and potassium. In: Bowman BA, Russell RM, 

editors. Present knowledge in nutrition. 8th ed. Washington, DC: International Life Sciences 

Institute Press; 2006. p. 409-21. 

526. Pitts RF. Ionic composition of body fluids. The physiological basis of diuretic therapy. 

Springfield, IL: Charles C Thomas Publisher; 1959. 

527. Lynn MP, Fouad F, Cook SA, Napoli CA, et al. Alterations in cardiac function and 

cardiopulmonary blood volume in chronic sodium depletion in dogs. Clin Sci (Lond). 

1980 Dec;59 Suppl 6:393s-5s. 

528. Benedict FG. A study of prolonged fasting. Washington, DC: Carnegie Institution 

of Washington; 1915. 

529. Keys AB, Brozek J, Henschel A. The biology of human starvation. Minneapolis: 

University of Minnesota Press; 1950. 

530. Gamble JL, Ross GS, Tisdall FF. The metabolism of fixed base during fasting. 

J Biol Chem. 1923;57(3):633-95. 

531. Smith SM, Krauhs JM, Leach CS. Regulation of body fluid volume and electrolyte 

concentrations in spaceflight. Adv Space Biol Med. 1997;6:123-65. 

532. Heer M, Zittermann A, Hoetzel D. Role of nutrition during long-term spaceflight. 

Acta Astronaut. 1995 Feb-Mar;35(4-5):297-311. 
116

533. Gerzer R, Heer M. Regulation of body fluid and salt homeostasis - from observations 

in space to new concepts on Earth. Curr Pharm Biotechnol. 2005 Aug;6(4):299-304. 

534. Norsk P, Drummer C, Christensen NJ, Cirillo M, et al. Revised hypothesis and future 

perspectives. Am J Kidney Dis. 2001 Sep;38(3):696-8. 

535. Lane HW, Leach C, Smith SM. Fluid and electrolyte homeostasis. In: Lane HW, Schoeller DA, 

editors. Nutrition in spaceflight and weightlessness models. Boca Raton, FL: CRC Press; 

2000. p. 119-39. 

536. Drummer C, Heer M, Dressendörfer RA, Strasburger CJ, et al. Reduced natriuresis during 

weightlessness. Clin Investig. 1993;71:678-86. 

537. Heer M, Frings-Meuthen P, Titze J, Boschmann M, et al. Increasing sodium intake from 

a previous low or high intake affects water, electrolyte and acid-base differently. Br J Nutr. 

2009 May;101(9):1286-94. 

538. Machnik A, Dahlmann A, Kopp C, Goss J, et al. Mononuclear phagocyte system depletion 

blocks interstitial tonicity-responsive enhancer binding protein/vascular endothelial growth 

factor C expression and induces salt-sensitive hypertension in rats. Hypertension. 2010 

Mar;55(3):755-61. 

539. Machnik A, Neuhofer W, Jantsch J, Dahlmann A, et al. Macrophages regulate salt-dependent 

volume and blood pressure by a vascular endothelial growth factor-C-dependent buffering 

mechanism. Nat Med. 2009 May;15(5):545-52. 

540. Rakova N, Juttner K, Dahlmann A, Schroder A, et al. Long-term space flight simulation 

reveals infradian rhythmicity in human Na(+) balance. Cell Metab. 2013 Jan 8;17(1):125-31. 

541. Titze J, Dahlmann A, Lerchl K, Kopp C, et al. Spooky sodium balance. Kidney Int. 

2014 Apr;85(4):759-67. 

542. Titze J, Krause H, Hecht H, Dietsch P, et al. Reduced osmotically inactive Na storage 

capacity and hypertension in the Dahl model. Am J Physiol Renal Physiol. 2002 

Jul;283(1):F134-41. 

543. Titze J, Shakibaei M, Schafflhuber M, Schulze-Tanzil G, et al. Glycosaminoglycan 

polymerization may enable osmotically inactive Na+ storage in the skin. Am J Physiol 

Heart Circ Physiol. 2004 Jul;287(1):H203-8. 

544. Ivanova LN, Archibasova VK, Shterental I. [Sodium-depositing function of the skin in white 

rats]. Fiziol Zh SSSR Im I M Sechenova. 1978 Mar;64(3):358-63. 

545. Goulding A. Effects of dietary NaCl supplements on parathyroid function, bone turnover 

and bone composition in rats taking restricted amounts of calcium. Miner Electrolyte Metab. 

1980;4:203-8. 

546. Goulding A, Lim PE. Effects of varying dietary salt intake on the fasting urinary excretion 

of sodium, calcium and hydroxyproline in young women. N Z Med J. 1983;96:853-4. 

547. Evans CE, Chughtai AY, Blumsohn A, Giles M, et al. The effect of dietary sodium on calcium 

metabolism in premenopausal and postmenopausal women. Eur J Clin Nutr. 1997;51(6): 

394-9. 

548. Castenmiller JJM, Mensink RP, van der Heijden L, Kouwenhoven T, et al. The effect of dietary 

sodium on urinary calcium and potassium excretion in normotensive men with different 

calcium intakes. Am J Clin Nutr. 1985;41:52-60. 

549. Cohen AJ, Roe FJ. Review of risk factors for osteoporosis with particular reference to a 

possible aetiological role of dietary salt. Food Chem Toxicol. 2000 Feb-Mar;38(2-3):237-53. 

550. Heer M, Baisch F, Drummer C, Gerzer R. Long-term elevations of dietary sodium produce 

parallel increases in the renal excretion of urodilatin and sodium. In: Sahm PR, Keller MH, 

Schiewe B, editors. Proceedings of the Norderney Symposium on Scientific Results of the 

German Spacelab Mission D-2. Norderney, Germany: Wissenschaftliche Projectführung D-2; 

1995. p. 708-14. 
117

551. Ho SC, Chen YM, Woo JL, Leung SS, et al. Sodium is the leading dietary factor associated 

with urinary calcium excretion in Hong Kong Chinese adults. Osteoporos Int. 2001; 

12:723-31. 

552. Nordin B, Need A, Morris H, Horowitz M, et al. Sodium and osteoporosis. In: Lesourd B, 

Rapin C, Sachet P, editors. Osteoporose: pour une prevention nutritionelle du risque? 

Paris: Centre Recherche et Information Nutritionnelles; 1992. p. 117. 

553. Nordin BE, Need AG, Morris HA, Horowitz M. The nature and significance of the relationship 

between urinary sodium and urinary calcium in women. J Nutr. 1993;123:1615-22. 

554. Kleeman CR, Bohannan J, Bernstein D, Long S, et al. Effect of variations in sodium intake 

on calcium excretion in normal humans. Proc Soc Exp Biol Med. 1964;115:29-32. 

555. Heer M. Einfluss alimentaer erhoehter Kochsalzzufuhr auf den Wasser- und 

Elektrolythaushalt des Menschen [dissertation]. Bonn, Germany: University of Bonn; 1996. 

556. Agus ZS, Goldfarb S. Renal regulation of calcium balance. In: Seldin DW, Giebisch G, 

editors. The kidney: physiology and pathophysiology. New York, NY: Raven Press; 1985. 

p. 1323-35. 

557. Constanzo LS, Windhager EE. Effects of PTH, ADH and cyclic AMP on distal tubular Ca 

and Na reabsorption. Am J Physiol. 1980;239:F478-F85. 

558. Meyer WJ, 3rd, Transbol I, Bartter FC, Delea C. Control of calcium absorption: effect 

of sodium chloride loading and depletion. Metabolism. 1976;9:989-93. 

559. Breslau MA, McGuire J, Zerwekh J, Pak CYC. The role of dietary sodium on renal excretion 

and intestinal absorption of calcium and on vitamin D metabolism. J Clin Endocrinol Metab. 

1982;55:369-73. 

560. Ginty F, Flynn A, Cashman KD. The effect of dietary sodium intake on biochemical markers 

of bone metabolism in young women. Br J Nutr. 1998;79:343-50. 

561. Lietz G, Avenell A, Robins SP. Short-term effects of dietary sodium intake on bone 

metabolism in postmenopausal women measured using urinary deoxypyridinoline excretion. 

Br J Nutr. 1997;78:73-82. 

562. Blackwood AM, Sagnella GA, Cook DG, Cappuccio FP. Urinary calcium excretion, sodium 

intake and blood pressure in a multi-ethnic population: results of the Wandsworth Heart 

and Stroke Study. J Hum Hypertens. 2001 Apr;15(4):229-37. 

563. Harrington M, Cashman KD. High salt intake appears to increase bone resorption in 

postmenopausal women but high potassium intake ameliorates this adverse effect. 

Nutr Rev. 2003 May;61(5 Pt 1):179-83. 

564. Harrington M, Bennett T, Jakobsen J, Ovesen L, et al. The effect of a high-protein, highsodium diet on calcium and bone metabolism in postmenopausal women and its interaction 

with vitamin D receptor genotype. Br J Nutr. 2004 Jan;91(1):41-51. 

565. Harrington M, Bennett T, Jakobsen J, Ovesen L, et al. Effect of a high-protein, high-salt 

diet on calcium and bone metabolism in postmenopausal women stratified by hormone 

replacement therapy use. Eur J Clin Nutr. 2004 Oct;58(10):1436-9. 

566. Massey LK, Whiting SJ. Dietary salt, urinary calcium, and stone risk. Nutr Rev. 1995; 

53:131-9. 

567. Massey LK, Whiting SJ. Dietary salt, urinary calcium, and bone loss. J Bone Miner Res. 

1996;11:731-6. 

568. Sellmeyer DE, Schloetter M, Sebastian A. Potassium citrate prevents increased urine calcium 

excretion and bone resorption induced by a high sodium chloride diet. J Clin Endocrinol 

Metab. 2002 May;87(5):2008-12. 

569. Nordin BE, Need AG, Steurer T, Morris HA, et al. Nutrition, osteoporosis, and aging. 

Ann N Y Acad Sci. 1998 Nov 20;854:336-51. 
 

118

570. Frassetto L, Morris RC, Jr, Sellmeyer DE, Todd K, et al. Diet, evolution and aging--the 

pathophysiologic effects of the post-agricultural inversion of the potassium-to-sodium 

and base-to-chloride ratios in the human diet. Eur J Nutr. 2001 Oct;40(5):200-13. 

571. de Wardener HE, MacGregor GA. Harmful effects of dietary salt in addition to hypertension. 

J Hum Hypertens. 2002 Apr;16(4):213-23. 

572. Fellstrom B, Danielson BG, Karlstrom B, Lithell H, et al. Dietary habits in renal stone patients 

compared with healthy subjects. Br J Urol. 1989;63:575-80. 

573. Trinchieri A, Mandressi A, Luongo P, Longo G, et al. The influence of diet on urinary risk 

factors for stones in healthy subjects and idiopathic renal calcium stone formers. 

Br J Urol. 1991;67:230-6. 

574. Sakhaee K, Harvey JA, Padalino PK, Whitson P, et al. The potential role of salt abuse 

on the risk for kidney stone formation. J Urol. 1993;150(2 Pt 1):310-2. 

575. Matkovic V, Ilich JZ, Andon MB, Hsieh LC, et al. Urinary calcium, sodium, and bone mass 

of young females. Am J Clin Nutr. 1995;62:417. 

576. Devine A, Criddle RA, Dick IM, Kerr DA, et al. A longitudinal study of the effect of sodium 

and calcium intakes on regional bone density in postmenopausal women. Am J Clin Nutr. 

1995;62:740-5. 

577. Frings-Meuthen P, Baecker N, Heer M. Low-grade metabolic acidosis may be the 

cause of sodium chloride-induced exaggerated bone resorption. J Bone Miner Res. 

2008 Apr;23(4):517-24. 

578. Arnaud SB, Wolinsky I, Fung P, Vernikos J. Dietary salt and urinary calcium excretion 

in a human bed rest spaceflight model. Aviat Space Environ Med. 2000;71:1115-9. 

579. Frings P, Baecker N, Boese A, Heer M. High sodium chloride intake causes mild metabolic 

acidosis: Is this the reason for increased bone resorption? FASEB J. 2005 Mar;19(5):A1345 

(abstract #745.2). 

580. Frings P, Baecker N, Heer M. High sodium chloride intake exacerbates immobilisation 

induced bone loss. FASEB J. 2007;21:Abstract 548.7. 581. May RC, Kelly RA, Mitch 

WE. Metabolic acidosis stimulates protein degradation in rat muscle by a glucocorticoiddependent mechanism. J Clin Invest. 1986 Feb;77(2):614-21. 

582. Buehlmeier J, Frings-Meuthen P, Remer T, Maser-Gluth C, et al. Alkaline salts to counteract 

bone resorption and protein wasting induced by high salt intake: results of a randomized 

controlled trial. J Clin Endocrinol Metab. 2012 Dec;97(12):4789-97. 

583. Compston J. Management of glucocorticoid-induced osteoporosis. Nat Rev Rheumatol. 

2010 Feb;6(2):82-8. 

584. Cooper MS, Walker EA, Bland R, Fraser WD, et al. Expression and functional 

consequences of 11beta-hydroxysteroid dehydrogenase activity in human bone. 

Bone. 2000 Sep;27(3):375-81. 

585. Dovio A, Perazzolo L, Osella G, Ventura M, et al. Immediate fall of bone formation 

and transient increase of bone resorption in the course of high-dose, short-term 

glucocorticoid therapy in young patients with multiple sclerosis. J Clin Endocrinol Metab. 

2004 Oct;89(10):4923-8. 

586. Kuroki Y, Kaji H, Kawano S, Kanda F, et al. Short-term effects of glucocorticoid therapy 

on biochemical markers of bone metabolism in Japanese patients: a prospective study. 

J Bone Miner Metab. 2008;26(3):271-8. 

587. Frassetto LA, Morris RC, Jr, Sellmeyer DE, Sebastian A. Adverse effects of sodium chloride 

on bone in the aging human population resulting from habitual consumption of typical 

American diets. J Nutr. 2008 Feb;138(2):419S-22S. 

588. Burckhardt P. The effect of the alkali load of mineral water on bone metabolism: 

interventional studies. J Nutr. 2008 Feb;138(2):435S-7S. 
119

589. Arnett TR. Extracellular pH regulates bone cell function. J Nutr. 2008 Feb;138(2):415S-8S. 

590. Kleinman LI, Lorenz JM. Physiology and pathophysiology of body water and electrolytes. 

In: Kaplan LA, Pesce AJ, editors. Clinical chemistry: theory, analysis, and correlation. 

St. Louis, MO: CV Mosby Company; 1984. p. 363-86. 

591. Alexander WC, Leach CS, Fischer CL. Clinical biochemistry. In: Johnston RS, Dietlein LF, 

Berry CA, editors. Biomedical results of Apollo (NASA SP-368). Washington, DC: National 

Aeronautics and Space Administration; 1975. p. 185-96. 

592. Sawka MN, Coyle EF. Influence of body water and blood volume on thermoregulation 

and exercise performance in the heat. Exerc Sport Sci Rev. 1999;27:167-218. 

593. Sullivan RJ, Jr. Accepting death without artificial nutrition or hydration. J Gen Intern Med. 

1993 Apr;8(4):220-4. 

594. Leach Huntoon CS, Grigoriev AI, Natochin YV. Fluid and electrolyte regulation in spaceflight. 

San Diego: Univelt, Inc.; 1998. 

595. Leach CS. A review of the consequences of fluid and electrolyte shifts in weightlessness. 

Acta Astronaut. 1979;6:1123-35. 

596. Leach CS. An overview of the endocrine and metabolic changes in manned space flight. 

Acta Astronaut. 1981;8:977-86. 

597. Leach CS. Fluid control mechanisms in weightlessness. Aviat Space Environ Med. 

1987;58(9 Section II):A74-9. 

598. Leach CS, Johnson PC, Jr. Fluid and electrolyte control in simulated and actual spaceflight. 

Physiologist. 1985;28(6 Suppl):S-34-S-7. 

599. Drummer C, Gerzer R, Baisch F, Heer M. Body fluid regulation in micro-gravity differs from 

that on Earth: an overview. Pflugers Arch. 2000;441(2-3 Suppl):R66-72. 

600. De Santo NG, Christensen NJ, Drummer C, Kramer HJ, et al. Fluid balance and kidney 

function in space: introduction. Am J Kidney Dis. 2001 Sep;38(3):664-7. 

601. Norsk P, Christensen NJ, Bie P, Gabrielsen A, et al. Unexpected renal responses in space. 

Lancet. 2000 Nov 4;356(9241):1577-8. 

602. Nicogossian AE, Sawin CF, Huntoon CL. Overall physiologic response to space flight. 

In: Nicogossian AE, Huntoon CL, Pool SL, editors. Space physiology and medicine. 

3rd ed. Philadelphia, PA: Lea & Febiger; 1994. p. 213-27. 

603. Johnson P, Driscoll T, LeBlanc A. Blood volume changes. In: Johnston R, Dietlein L, editors. 

Biomedical results from Skylab (NASA SP-377). Washington, DC: National Aeronautics and 

Space Administration; 1977. p. 235-41. 

604. Bungo MW, Johnson PC, Jr. Cardiovascular examinations and observations of 

deconditioning during the space shuttle orbital flight test program. Aviat Space Environ Med. 

1983;54:1001-4. 

605. Vernikos J, Convertino VA. Advantages and disadvantages of fludrocortisone or saline load 

in preventing post-spaceflight orthostatic hypotension. Acta Astronaut. 1994;33:259-66. 

606. Hyatt KH, West DA. Reversal of bedrest-induced orthostatic intolerance by lower body 

negative pressure and saline. Aviat Space Environ Med. 1977 Feb;48(2):120-4. 

607. Thornton WE, Ord J. Physiological mass measurements in Skylab. In: Johnston RS, Dietlein 

LF, editors. Biomedical results from Skylab (NASA SP-377) Washington, DC: National 

Aeronautics and Space Administration; 1977. p. 175-82. 

608. Leach CS, Inners LD, Charles JB. Changes in total body water during spaceflight. 

J Clin Pharmacol. 1991;31:1001-6. 

609. Norsk P. Cardiovascular and fluid volume control in humans in space. 

Curr Pharm Biotechnol. 2005 Aug;6(4):325-30. 
120

610. Balakhovskiy I, Natochin Y. [Metabolism under the extreme conditions of spaceflight 

and during its simulation]. Moscow: Nauka Press; 1973. 

611. Gerzer R, Drummer C, Heer M. Antinatriuretic kidney response to weightlessness. 

Acta Astronaut. 1994 Jul;33:97-100. 

612. Gerzer R, Heer M, Drummer C. Body fluid metabolism at actual and simulated microgravity. 

Med Sci Sports Exerc. 1996;28(10 Suppl):S32-5. 

613. Whitson PA, Pietrzyk RA, Morukov BV, Sams CF. The risk of renal stone formation during 

and after long duration space flight. Nephron. 2001;89:264-70. 

614. Harm DL, Jennings RT, Meck JV, Powell MR, et al. Invited review: gender issues related 

to spaceflight: a NASA perspective. J Appl Physiol. 2001 Nov;91(5):2374-83. 

615. Greenleaf JE. Mechanisms for negative water balance during weightlessness: immersion 

or bed rest? Physiologist. 1985;28(6 Suppl):S38-9. 

616. Vernikos J. Metabolic and endocrine changes. In: Sandler H, Vernikos J, editors. 

Inactivity: physiological effects. Orlando, FL: Academic Press, Inc.; 1986. p. 99-121. 

617. Norsk P. Renal adjustments to microgravity. Pflugers Arch. 2000;441(2-3 Suppl):R62-5. 

618. Norsk P, Christensen NJ, Vorobiev D, Suzuki Y, et al. Effects of head-down bed rest 

& microgravity on renal fluid excretion. J Gravit Physiol. 1998 Jul;5(1):P81-4. 

619. Regnard J, Heer M, Drummer C, Norsk P. Validity of microgravity simulation models on earth. 

Am J Kidney Dis. 2001 Sep;38(3):668-74. 

620. Chylack LT, Jr., Feiveson AH, Peterson LE, Tung WH, et al. NASCA Report 2: longitudinal 

study of relationship of exposure to space radiation and risk of lens opacity. Radiat Res. 

2012 Jul;178(1):25-32. 

621. Chylack LT, Jr., Peterson LE, Feiveson AH, Wear ML, et al. NASA study of cataract in 

astronauts (NASCA). Report 1: cross-sectional study of the relationship of exposure to 

space radiation and risk of lens opacity. Radiat Res. 2009 Jul;172(1):10-20. 

622. Cucinotta FA, Manuel FK, Jones J, Iszard G, et al. Space radiation and cataracts 

in astronauts. Radiat Res. 2001;156(5 Pt 1):460-6. 

623. Mader TH, Gibson CR, Pass AF, Kramer LA, et al. Optic disc edema, globe flattening, 

choroidal folds, and hyperopic shifts observed in astronauts after long-duration space flight. 

Ophthalmology. 2011 Oct;118(10):2058-69. 

624. Mayasnikov VI, Stepanova SI. Features of cerebral hemodynamics in cosmonauts before 

and after flight on the Mir Orbital Station. Orbital Station Mir Institute of Biomedical 

Problems: State Scientific Center of Russian Federation; 2008. p. 300-5. 

625. Nygard O, Vollset SE, Refsum H, Stensvold I, et al. Total plasma homocysteine and 

cardiovascular risk profile. The Hordaland Homocysteine Study. JAMA. 1995 Nov 15; 

274(19):1526-33. 

626. Finkelstein JD. The metabolism of homocysteine: pathways and regulation. Eur J Pediatr. 

1998 Apr;157 Suppl 2:S40-4. 

627. Pizza V, Agresta A, Agresta A, Lamaida E, et al. Migraine and genetic polymorphisms: 

an overview. Open Neurol J. 2012;6:65-70. 

628. Homocysteine Studies Collaboration. Homocysteine and risk of ischemic heart disease 

and stroke: a meta-analysis. JAMA. 2002 Oct 23-30;288(16):2015-22. 

629. Ebrahimi KB, Handa JT. Lipids, lipoproteins, and age-related macular degeneration. J Lipids. 

2011;2011:802059. doi: 10.1155/2011/802059. Epub 2011 Jul 28. 

630. Burgansky-Eliash Z, Barash H, Nelson D, Grinvald A, et al. Retinal blood flow velocity in 

patients with age-related macular degeneration. Curr Eye Res. 2014 Mar;39(3):304-11. 

631. Evans J. Should we be taking B vitamins to prevent age-related macular degeneration? 

Not yet, but worth doing more research. Am J Clin Nutr. 2013 Jul;98(1):4-5. 
121

632. Chu C, Scanlon P. Vitamin B12 deficiency optic neuropathy detected by asymptomatic 

screening. BMJ Case Rep. 2011 Apr 26;2011. pii: bcr0220113823. doi: 10.1136/ 

bcr.02.2011:3823. 

633. Jalil A, Usmani HA, Khan MI, Blakely EL, et al. Bilateral paediatric optic neuropathy 

precipitated by vitamin B12 deficiency and a novel mitochondrial DNA mutation. 

Int Ophthalmol. 2013 Dec;33(6):687-90. 

634. Gopinath B, Flood VM, Rochtchina E, Wang JJ, et al. Homocysteine, folate, vitamin B-12, 

and 10-y incidence of age-related macular degeneration. Am J Clin Nutr. 2013 Jul; 

98(1):129-35. 

635. Sekeryapan B, Oner V, Kirbas A, Turkyilmaz K, et al. Plasma homocysteine levels in 

dry eye patients. Cornea. 2013 May;32(5):e94-6. 

636. Xu F, Zhao X, Zeng SM, Li L, et al. Homocysteine, B vitamins, methylenetetrahydrofolate 

reductase gene, and risk of primary open-angle glaucoma: a meta-analysis. Ophthalmology. 

2012 Dec;119(12):2493-9. 

637. Rochtchina E, Wang JJ, Flood VM, Mitchell P. Elevated serum homocysteine, low serum 

vitamin B12, folate, and age-related macular degeneration: the Blue Mountains Eye Study. 

Am J Ophthalmol. 2007 Feb;143(2):344-6. 

638. Christen WG, Glynn RJ, Chew EY, Albert CM, et al. Folic acid, pyridoxine, and 

cyanocobalamin combination treatment and age-related macular degeneration in women: 

the Women's Antioxidant and Folic Acid Cardiovascular Study. Arch Intern Med. 

2009 Feb 23;169(4):335-41. 

639. Bailey LB, Gregory III JF. Folate. In: Bowman BA, Russell RM, editors. Present knowledge 

in nutrition. 9th ed. Washington, DC: International Life Sciences Institute; 2006. p. 278-301. 

640. Groff J, Gropper S. Advanced nutrition and human metabolism. 3rd ed. St. Paul, MN: 

Wadsworth Publishing; 2000. 

641. Herbert V. Development of human folate deficiency. In: Picciano MF, Sotokstad ELR, 

Gregory JFI, editors. Folic acid metabolism in health and disease. New York: Wiley-Liss; 

1990. p. 195-210. 

642. Herbert V. Folic acid. In: Shils ME, Olson JA, Shike M, Ross AC, editors. Modern nutrition 

in health and disease. 9th ed. Baltimore, MD: Lippincott Williams & Wilkins; 1999. 

643. Endoh K, Murakami M, Araki R, Maruyama C, et al. Low folate status increases 

chromosomal damage by X-ray irradiation. Int J Radiat Biol. 2006 Apr;82(4):223-30. 

644. Endoh K, Murakami M, Umegaki K. Vulnerability of folate in plasma and bone marrow 

to total body irradiation in mice. Int J Radiat Biol. 2007 Jan;83(1):65-71. 

645. Beetstra S, Thomas P, Salisbury C, Turner J, et al. Folic acid deficiency increases 

chromosomal instability, chromosome 21 aneuploidy and sensitivity to radiation-induced 

micronuclei. Mutat Res. 2005 Oct 15;578(1-2):317-26. 

646. Courtemanche C, Huang AC, Elson-Schwab I, Kerry N, et al. Folate deficiency and ionizing 

radiation cause DNA breaks in primary human lymphocytes: a comparison. FASEB J. 

2004 Jan;18(1):209-11. 

647. Joshi R, Adhikari S, Patro BS, Chattopadhyay S, et al. Free radical scavenging behavior 

of folic acid: evidence for possible antioxidant activity. Free Radic Biol Med. 2001 Jun 15; 

30(12):1390-9. 

648. Shin YS, Beuhring KU, Stokstad EL. The relationships between vitamin B12 and folic acid 

and the effect of methionine on folate metabolism. Mol Cell Biochem. 1975 Nov 14; 

9(2):97-108. 

649. Institute of Medicine. Dietary reference intakes for thiamin, riboflavin, niacin, vitamin 

B6, folate, vitamin B12, pantothenic acid, biotin, and choline. Washington, DC: 

National Academies Press; 1998. 
122

650. van Asselt DZ, van den Broek WJ, Lamers CB, Corstens FH, et al. Free and protein-bound 

cobalamin absorption in healthy middle-aged and older subjects. J Am Geriatr Soc. 1996 

Aug;44(8):949-53. 

651. Stabler SP. Vitamin B12. In: Bowman BA, Russell RM, editors. Present knowledge in nutrition. 

9th ed. Washington, DC: International Life Sciences Institute Press; 2006. 

p. 302-13. 

652. Camporeale G, Zempleni J. Biotin. In: Bowman BA, Russell RM, editors. Present knowledge 

in nutrition. 9th ed. Washington, DC: International Life Sciences Institute Press; 2006. 

p. 314-26. 

653. Mock DM, Malik MI. Distribution of biotin in human plasma: most of the biotin is not bound 

to protein. Am J Clin Nutr. 1992 Aug;56(2):427-32. 

654. Mock DM, Stadler DD, Stratton SL, Mock NI. Biotin status assessed longitudinally 

in pregnant women. J Nutr. 1997 May;127(5):710-6. 

655. Krause KH, Bonjour JP, Berlit P, Kynast G, et al. Effect of long-term treatment with 

antiepileptic drugs on the vitamin status. Drug Nutr Interact. 1988;5(4):317-43. 

656. Rathman SC, Gregory JF, 3rd, McMahon RJ. Pharmacological biotin supplementation 

maintains biotin status and function in rats administered dietary carbamazepine. J Nutr. 

2003 Sep;133(9):2857-62. 

657. Maeda Y, Kawata S, Inui Y, Fukuda K, et al. Biotin deficiency decreases ornithine 

transcarbamylase activity and mRNA in rat liver. J Nutr. 1996 Jan;126(1):61-6. 

658. Borboni P, Magnaterra R, Rabini RA, Staffolani R, et al. Effect of biotin on glucokinase 

activity, mRNA expression and insulin release in cultured beta-cells. Acta Diabetol. 

1996 Jul;33(2):154-8. 

659. Dakshinamurti K, Li W. Transcriptional regulation of liver phosphoenolpyruvate carboxykinase 

by biotin in diabetic rats. Mol Cell Biochem. 1994 Mar 30;132(2):127-32. 

660. Whatham A, Bartlett H, Eperjesi F, Blumenthal C, et al. Vitamin and mineral deficiencies 

in the developed world and their effect on the eye and vision. Ophthalmic Physiol Opt. 

2008 Jan;28(1):1-12. 

661. van Poppel G, Goldbohm RA. Epidemiologic evidence for beta-carotene and cancer 

prevention. Am J Clin Nutr. 1995;62 Suppl:1393S-402S. 

662. Kohlmeier L, Hastings SB. Epidemiologic evidence of a role of carotenoids in cardiovascular 

disease prevention. Am J Clin Nutr. 1995;62 Suppl:1370S-6S. 

663. Olson JA. Vitamin A, retinoids, and carotenoids. In: Shils ME, Olson JA, Shike M, editors. 

Modern nutrition in health and disease. 8th ed. Malvern, PA: Lea & Febiger; 1994. 

p. 287-307. 

664. Michaelsson K, Lithell H, Vessby B, Melhus H. Serum retinol levels and the risk of fracture. 

N Engl J Med. 2003 Jan 23;348(4):287-94. 

665. Melhus H, Michaelsson K, Kindmark A, Bergstrom R, et al. Excessive dietary intake of 

vitamin A is associated with reduced bone mineral density and increased risk for hip fracture. 

Ann Intern Med. 1998 Nov 15;129(10):770-8. 

666. Jackson HA, Sheehan AH. Effect of vitamin A on fracture risk. Ann Pharmacother. 

2005 Dec;39(12):2086-90. 

667. Omenn GS, Goodman GE, Thornquist MD, Balmes J, et al. Risk factors for lung cancer and 

for intervention effects in CARET, the Beta-Carotene and Retinol Efficacy Trial. J Natl Cancer 

Inst. 1996 Nov 6;88(21):1550-9. 

668. ATBC Cancer Prevention Study Group. The alpha-tocopherol, beta-carotene lung 

cancer prevention study: design, methods, participant characteristics, and compliance. 

Ann Epidemiol. 1994 Jan;4(1):1-10. 
123

669. Rastegar N, Eckart P, Mertz M. Radiation-induced cataract in astronauts and cosmonauts. 

Graefes Arch Clin Exp Ophthalmol. 2002 Jul;240(7):543-7. 

670. Jones JA, McCarten M, Manuel K, Djojonegoro B, et al. Cataract formation mechanisms and 

risk in aviation and space crews. Aviat Space Environ Med. 2007 Apr;78(4 Suppl):A56-66. 

671. Agte V, Tarwadi K. The importance of nutrition in the prevention of ocular disease with 

special reference to cataract. Ophthalmic Res. 2010;44(3):166-72. 

672. Cui YH, Jing CX, Pan HW. Association of blood antioxidants and vitamins with risk 

of age-related cataract: a meta-analysis of observational studies. Am J Clin Nutr. 

2013 Sep;98(3):778-86. 

673. Beisel WR. History of nutritional immunology: introduction and overview. J Nutr. 

1992 Mar;122(3 Suppl):591-6. 

674. Mizock BA. Immunonutrition and critical illness: an update. Nutrition. 

2010 Jul-Aug;26(7-8):701-7. 

675. Cunningham-Rundles S, McNeeley DF, Moon A. Mechanisms of nutrient modulation 

of the immune response. J Allergy Clin Immunol. 2005 Jun;115(6):1119-28; quiz 29. 

676. Chandra RK, Kumari S. Effects of nutrition on the immune system. Nutrition. 

1994 May-Jun;10(3):207-10. 

677. Chandra RK. Nutrient regulation of immune functions. Forum Nutr. 2003;56:147-8. 

678. Chandra RK. Nutrition and the immune system: an introduction. Am J Clin Nutr. 

1997 Aug;66(2):460S-3S. 

679. Keith ME, Jeejeebhoy KN. Immunonutrition. Baillieres Clin Endocrinol Metab. 

1997 Dec;11(4):709-38. 

680. Wintergerst ES, Maggini S, Hornig DH. Contribution of selected vitamins and trace elements 

to immune function. Ann Nutr Metab. 2007;51(4):301-23. 

681. Sonnenfeld G. The immune system in space, including Earth-based benefits of space-based 

research. Curr Pharm Biotechnol. 2005 Aug;6(4):343-9. 

682. Sonnenfeld G, Shearer WT. Immune function during space flight. Nutrition. 

2002 Oct;18(10):899-903. 

683. Stein TP. Space flight and oxidative stress. Nutrition. 2002 Oct;18(10):867-71. 

684. Stein TP, Leskiw MJ. Oxidant damage during and after spaceflight. Am J Physiol Endocrinol 

Metab. 2000 Mar;278(3):E375-82. 

685. Stein TP, Gaprindashvili T. Spaceflight and protein metabolism, with special reference 

to humans. Am J Clin Nutr. 1994 Nov;60(5):806S-19S. 

686. Chandra RK, Chandra S, Gupta S. Antibody affinity and immune complexes after 

immunization with tetanus toxoid in protein-energy malnutrition. Am J Clin Nutr. 

1984 Jul;40(1):131-4. 

687. Guadagni M, Biolo G. Effects of inflammation and/or inactivity on the need for dietary 

protein. Curr Opin Clin Nutr Metab Care. 2009 Nov;12(6):617-22. 

688. Kirk SJ, Barbul A. Role of arginine in trauma, sepsis, and immunity. JPEN J Parenter Enteral 

Nutr. 1990 Sep-Oct;14(5 Suppl):226S-9S. 

689. Kirk SJ, Hurson M, Regan MC, Holt DR, et al. Arginine stimulates wound healing and 

immune function in elderly human beings. Surgery. 1993 Aug;114(2):155-9; discussion 60. 

690. Singleton KD, Wischmeyer PE. Glutamine attenuates inflammation and NF-kappaB activation 

via Cullin-1 deneddylation. Biochem Biophys Res Commun. 2008 Aug 29;373(3):445-9. 

691. Wischmeyer PE. Glutamine: role in critical illness and ongoing clinical trials. Curr Opin 

Gastroenterol. 2008 Mar;24(2):190-7. 
124

692. Wischmeyer PE. Glutamine: mode of action in critical illness. Crit Care Med. 

2007 Sep;35(9 Suppl):S541-4. 

693. Mondello S, Italiano D, Giacobbe MS, Mondello P, et al. Glutamine-supplemented total 

parenteral nutrition improves immunological status in anorectic patients. Nutrition. 

2010 Jun;26(6):677-81. 

694. Novak F, Heyland DK, Avenell A, Drover JW, et al. Glutamine supplementation in serious 

illness: a systematic review of the evidence. Crit Care Med. 2002 Sep;30(9):2022-9. 

695. van Etten E, Mathieu C. Immunoregulation by 1,25-dihydroxyvitamin D3: basic concepts. 

J Steroid Biochem Mol Biol. 2005 Oct;97(1-2):93-101. 

696. Mathieu C, Adorini L. The coming of age of 1,25-dihydroxyvitamin D(3) analogs 

as immunomodulatory agents. Trends Mol Med. 2002 Apr;8(4):174-9. 

697. Hewison M, Freeman L, Hughes SV, Evans KN, et al. Differential regulation of vitamin D 

receptor and its ligand in human monocyte-derived dendritic cells. J Immunol. 2003 Jun 1; 

170(11):5382-90. 

698. Martineau AR, Wilkinson KA, Newton SM, Floto RA, et al. IFN-gamma- and TNF-independent 

vitamin D-inducible human suppression of mycobacteria: the role of cathelicidin LL-37. 

J Immunol. 2007 Jun 1;178(11):7190-8. 

699. Martineau AR, Wilkinson RJ, Wilkinson KA, Newton SM, et al. A single dose of vitamin D 

enhances immunity to mycobacteria. Am J Respir Crit Care Med. 2007 Jul 15;176(2):208-13. 

700. Kondo Y, Kato T, Kimura O, Iwata T, et al. 1(OH) vitamin D3 supplementation improves the 

sensitivity of the immune-response during Peg-IFN/RBV therapy in chronic hepatitis C 

patients-case controlled trial. PLoS One. 2013;8(5):e63672. 

701. Chouker A. Stress challenges and immunity in space: from mechanisms to monitoring 

and preventive strategies. Berlin: Springer; 2012. 

702. Tamura J, Kubota K, Murakami H, Sawamura M, et al. Immunomodulation by vitamin B12: 

augmentation of CD8+ T lymphocytes and natural killer (NK) cell activity in vitamin B12­

deficient patients by methyl-B12 treatment. Clin Exp Immunol. 1999 Apr;116(1):28-32. 

703. Bunout D, Barrera G, Hirsch S, Gattas V, et al. Effects of a nutritional supplement on the 

immune response and cytokine production in free-living Chilean elderly. JPEN J Parenter 

Enteral Nutr. 2004 Sep-Oct;28(5):348-54. 

704. Maggini S, Wintergerst ES, Beveridge S, Hornig DH. Selected vitamins and trace elements 

support immune function by strengthening epithelial barriers and cellular and humoral 

immune responses. Br J Nutr. 2007 Oct;98 Suppl 1:S29-35. 

705. Heer M, Baisch F, Kropp J, Gerzer R, et al. High dietary sodium chloride consumption 

may not induce body fluid retention in humans. Am J Physiol Renal Physiol. 2000 

Apr;278(4):F585-95. 

706. Titze J, Lang R, Ilies C, Schwind KH, et al. Osmotically inactive skin Na+ storage in rats. 

Am J Physiol Renal Physiol. 2003 Dec;285(6):F1108-17. 

707. Marvar PJ, Gordon FJ, Harrison DG. Blood pressure control: salt gets under your skin. 

Nat Med. 2009 May;15(5):487-8. 

708. Chen Q, Ross AC. Vitamin A and immune function: retinoic acid modulates population 

dynamics in antigen receptor and CD38-stimulated splenic B cells. 

Proc Natl Acad Sci U S A. 2005 Oct 4;102(40):14142-9. 

709. Ross AC. Vitamin A and retinoic acid in T cell-related immunity. Am J Clin Nutr. 

2012 Nov;96(5):1166S-72S. 

710. Ross AC. Vitamin A status: relationship to immunity and the antibody response. 

Proc Soc Exp Biol Med. 1992 Jul;200(3):303-20. 

711. Stephensen CB. Vitamin A, infection, and immune function. Annu Rev Nutr. 2001;21:167-92. 
125

712. Stephensen CB. Examining the effect of a nutrition intervention on immune function in 

healthy humans: what do we mean by immune function and who is really healthy anyway? 

Am J Clin Nutr. 2001 Nov;74(5):565-6. 

713. Semba RD. The role of vitamin A and related retinoids in immune function. Nutr Rev. 

1998 Jan;56(1 Pt 2):S38-48. 

714. Field CJ, Johnson IR, Schley PD. Nutrients and their role in host resistance to infection. 

J Leukoc Biol. 2002 Jan;71(1):16-32. 

715. Schwager J, Schulze J. Modulation of interleukin production by ascorbic acid. 

Vet Immunol Immunopathol. 1998 Jun 30;64(1):45-57. 

716. Jeng KC, Yang CS, Siu WY, Tsai YS, et al. Supplementation with vitamins C and E enhances 

cytokine production by peripheral blood mononuclear cells in healthy adults. Am J Clin Nutr. 

1996 Dec;64(6):960-5. 

717. Park OJ, Kim HY, Kim WK, Kim YJ, et al. Effect of vitamin E supplementation on antioxidant 

defense systems and humoral immune responses in young, middle-aged and elderly Korean 

women. J Nutr Sci Vitaminol (Tokyo). 2003 Apr;49(2):94-9. 

718. Wintergerst ES, Maggini S, Hornig DH. Immune-enhancing role of vitamin C and zinc 

and effect on clinical conditions. Ann Nutr Metab. 2006;50(2):85-94. 

719. Chung S, Yao H, Caito S, Hwang JW, et al. Regulation of SIRT1 in cellular functions: role 

of polyphenols. Arch Biochem Biophys. 2010 Sep 1;501(1):79-90. 

720. Perron NR, Brumaghim JL. A review of the antioxidant mechanisms of polyphenol 

compounds related to iron binding. Cell Biochem Biophys. 2009;53(2):75-100. 

721. Gao X, Deeb D, Media J, Divine G, et al. Immunomodulatory activity of resveratrol: 

discrepant in vitro and in vivo immunological effects. Biochem Pharmacol. 2003 Dec 15; 

66(12):2427-35. 

722. Park HJ, Lee CM, Jung ID, Lee JS, et al. Quercetin regulates Th1/Th2 balance in a murine 

model of asthma. Int Immunopharmacol. 2009 Mar;9(3):261-7. 

723. Sharma S, Chopra K, Kulkarni SK, Agrewala JN. Resveratrol and curcumin suppress immune 

response through CD28/CTLA-4 and CD80 co-stimulatory pathway. Clin Exp Immunol. 

2007 Jan;147(1):155-63. 

724. Shim JH, Choi HS, Pugliese A, Lee SY, et al. (-)-Epigallocatechin gallate regulates 

CD3-mediated T cell receptor signaling in leukemia through the inhibition of ZAP-70 kinase. 

J Biol Chem. 2008 Oct 17;283(42):28370-9. 

725. Singh NP, Hegde VL, Hofseth LJ, Nagarkatti M, et al. Resveratrol (trans-3,5,4'­

trihydroxystilbene) ameliorates experimental allergic encephalomyelitis, primarily 

via induction of apoptosis in T cells involving activation of aryl hydrocarbon receptor 

and estrogen receptor. Mol Pharmacol. 2007 Dec;72(6):1508-21. 

726. Song EK, Hur H, Han MK. Epigallocatechin gallate prevents autoimmune diabetes induced 

by multiple low doses of streptozotocin in mice. Arch Pharm Res. 2003 Jul;26(7):559-63. 

727. Ganz T. Hepcidin--a peptide hormone at the interface of innate immunity and iron 

metabolism. Curr Top Microbiol Immunol. 2006;306:183-98. 

728. Ganz T. Hepcidin and iron regulation, 10 years later. Blood. 2011 Apr 28;117(17):4425-33. 

729. Lee P, Peng H, Gelbart T, Wang L, et al. Regulation of hepcidin transcription by interleukin-1 

and interleukin-6. Proc Natl Acad Sci U S A. 2005 Feb 8;102(6):1906-10. 

730. Nemeth E, Rivera S, Gabayan V, Keller C, et al. IL-6 mediates hypoferremia of inflammation 

by inducing the synthesis of the iron regulatory hormone hepcidin. J Clin Invest. 

2004 May;113(9):1271-6. 

731. Vanamala J, Glagolenko A, Yang P, Carroll RJ, et al. Dietary fish oil and pectin enhance 

colonocyte apoptosis in part through suppression of PPARdelta/PGE2 and elevation 

of PGE3. Carcinogenesis. 2008 Apr;29(4):790-6. 
126

732. Camandola S, Leonarduzzi G, Musso T, Varesio L, et al. Nuclear factor kB is activated 

by arachidonic acid but not by eicosapentaenoic acid. Biochem Biophys Res Commun. 

1996 Dec 13;229(2):643-7. 

733. Kang JX, Weylandt KH. Modulation of inflammatory cytokines by omega-3 fatty acids. 

Subcell Biochem. 2008;49:133-43. 

734. Kim HH, Lee Y, Eun HC, Chung JH. Eicosapentaenoic acid inhibits TNF-alpha-induced 

matrix metalloproteinase-9 expression in human keratinocytes, HaCaT cells. Biochem 

Biophys Res Comm. 2008 Apr 4;368(2):343-9. 

735. Magee P, Pearson S, Allen J. The omega-3 fatty acid, eicosapentaenoic acid (EPA), prevents 

the damaging effects of tumour necrosis factor (TNF)-alpha during murine skeletal muscle 

cell differentiation. Lipids Health Dis. 2008;7:24. 

736. Powers SK, Jackson MJ. Exercise-induced oxidative stress: cellular mechanisms and 

impact on muscle force production. Physiol Rev. 2008 Oct;88(4):1243-76. 

737. Azzam EI, Jay-Gerin JP, Pain D. Ionizing radiation-induced metabolic oxidative stress 

and prolonged cell injury. Cancer Lett. 2012 Dec 31;327(1-2):48-60. 

738. Li M, Gonon G, Buonanno M, Autsavapromporn N, et al. Health risks of space exploration: 

targeted and nontargeted oxidative injury by high-charge and high-energy particles. 

Antioxid Redox Signal. 2014 Mar 20;20(9):1501-23. 

739. Gernhardt ML, Jones JA, Scheuring RA, Abercromby AF, et al. Risk of compromised EVA 

performance and crew health due to inadequate EVA suit systems. In: McPhee JS, Charles 

JB, editors. Human health and performance risks of space exploration missions: evidence 

reviewed by the NASA Human Research Program. Washington, DC: Government Printing 

Office; 2009. p. 333-58. 

740. Norcross J, Norsk P, Law J, Arias D, et al. Effects of the 8 psia / 32% O2 atmosphere 

on the human in the spaceflight environment. Hanover, MD: NASA Center for AeroSpace 

Information; 2013. 

741. National Aeronautics and Space Administration Johnson Space Center. Nutritional 

requirements for Extended Duration Orbiter missions (30-90 d) and Space Station Freedom 

(30-120 d). Houston, TX: National Aeronautics and Space Administration Lyndon B. Johnson 

Space Center;1993. Report No.: JSC-32283. 

742. Reid MB. Muscle fatigue: mechanisms and regulation. In: Sen CK, Packer L, Hänninen O, 

editors. Handbook of oxidants and antioxidants in exercise. Amsterdam: Elsevier Science 

B.V.; 2000. p. 599-630. 

743. Upton AC. The biological effects of low-level ionizing radiation. Sci Am. 1982;246(2):41-9. 

744. O'Neill CA, Stebbins CL, Bonigut S, Halliwell B, et al. Production of hydroxyl radicals 

in contracting skeletal muscle of cats. J Appl Physiol. 1996 Sep;81(3):1197-206. 

745. Reid MB, Khawli FA, Moody MR. Reactive oxygen in skeletal muscle. III. Contractility 

of unfatigued muscle. J Appl Physiol. 1993 Sep;75(3):1081-7. 

746. Essig DA, Nosek TM. Muscle fatigue and induction of stress protein genes: a dual function 

of reactive oxygen species? Can J Appl Physiol. 1997 Oct;22(5):409-28. 

747. Lawler JM, Cline CC, Hu Z, Coast JR. Effect of oxidant challenge on contractile function 

of the aging rat diaphragm. Am J Physiol. 1997 Feb;272(2 Pt 1):E201-7. 

748. Jones JA, Riggs PK, Yang TC, Pedemonte CH, et al. Ionizing radiation-induced bioeffects 

in space and strategies to reduce cellular injury and carcinogenesis. Aviat Space Environ 

Med. 2007 Apr;78(4 Suppl):A67-78. 

749. Kennedy AR, Ware JH, Guan J, Donahue JJ, et al. Selenomethionine protects against 

adverse biological effects induced by space radiation. Free Radic Biol Med. 2004 Jan 15; 

36(2):259-66. 
127

750. Little JB. Radiation carcinogenesis. Carcinogenesis. 2000 Mar;21(3):397-404. 

751. Mao XW, Pecaut MJ, Stodieck LS, Ferguson VL, et al. Spaceflight environment induces 

mitochondrial oxidative damage in ocular tissue. Radiat Res. 2013 Oct;180(4):340-50. 

752. Rizzo AM, Corsetto PA, Montorfano G, Milani S, et al. Effects of long-term space flight 

on erythrocytes and oxidative stress of rodents. PLoS One. 2012;7(3):e32361. 

753. Baqai FP, Gridley DS, Slater JM, Luo-Owen X, et al. Effects of spaceflight on innate immune 

function and antioxidant gene expression. J Appl Physiol (1985). 2009 Jun;106(6):1935-42. 

754. Gridley DS, Slater JM, Luo-Owen X, Rizvi A, et al. Spaceflight effects on T lymphocyte 

distribution, function and gene expression. J Appl Physiol (1985). 2009 Jan;106(1):194-202. 

755. Zwart SR, Kala G, Smith SM. Body iron stores and oxidative damage in humans increased 

during and after a 10- to 12-day undersea dive. J Nutr. 2009 Jan;139(1):90-5. 

756. Zezerov AE, Ivanova SM, Morukov BV, Ushakov AS. [Lipid peroxidation in the human blood 

during a 120-day period of anti-orthostatic hypokinesia]. Kosm Biol Aviakosm Med. 

1989 Mar-Apr;23(2):28-33. 

757. Pross HD, Casares A, Kiefer J. Induction and repair of DNA double-strand breaks under 

irradiation and microgravity. Radiat Res. 2000 May;153(5 Pt 1):521-5. 

758. Kiefer J, Pross HD. Space radiation effects and microgravity. Mutat Res. 1999 Dec 6; 

430(2):299-305. 

759. Hollander J, Gore M, Fiebig R, Mazzeo R, et al. Spaceflight downregulates antioxidant 

defense systems in rat liver. Free Radic Biol Med. 1998 Jan 15;24(2):385-90. 

760. Sunde RA. Selenium. In: Bowman BA, Russell RM, editors. Present knowledge in nutrition. 

9th ed. Washington, DC: International Life Sciences Institute Press; 2006. p. 480-97. 

761. Traber MG. Vitamin E. In: Shils ME, Olson JA, Shike M, Ross AC, editors. Modern nutrition 

in health and disease. 9th ed. Baltimore, MD: Lippincott Williams & Wilkins; 1999. p. 347-62. 

762. Institute of Medicine. Dietary reference intakes for vitamin C, vitamin E, selenium, and 

carotenoids. Washington, DC: The National Academies Press; 2000. 

763. Fang Y, Yang S, Wu G. Free radicals, antioxidants, and nutrition. Nutrition. 2002;18:872-9. 

764. Pence BC, Yang TC. Antioxidants: radiation and stress. In: Lane HW, Schoeller DA, editors. 

Nutrition in spaceflight and weightlessness models. Boca Raton, FL: CRC Press; 2000. 

p. 233-52. 

765. Klein EA, Thompson IM Jr, Tangen CM, Crowley JJ, et al. Vitamin E and the risk of prostate 

cancer: the Selenium and Vitamin E Cancer Prevention Trial (SELECT). JAMA. 2011 Oct 12; 

306(14):1549-56. 

766. Sayin VI, Ibrahim MX, Larsson E, Nilsson JA, et al. Antioxidants accelerate lung cancer 

progression in mice. Sci Transl Med. 2014 Jan 29;6(221):221ra15. 

767. Bjelakovic G, Nikolova D, Gluud C. Antioxidant supplements and mortality. Curr Opin Clin 

Nutr Metab Care. 2014 Jan;17(1):40-4. 

768. Bjelakovic G, Nikolova D, Gluud C. Meta-regression analyses, meta-analyses, and trial 

sequential analyses of the effects of supplementation with beta-carotene, vitamin A, and 

vitamin E singly or in different combinations on all-cause mortality: do we have evidence 

for lack of harm? PLoS One. 2013;8(9):e74558. 

769. Johnston CS. Vitamin C. In: Bowman BA, Russell RM, editors. Present knowledge 

in nutrition. 9th ed. Washington, DC: International Life Sciences Institute Press; 2006. 

p. 233-41. 

770. Sahni S, Hannan MT, Gagnon D, Blumberg J, et al. High vitamin C intake is associated 

with lower 4-year bone loss in elderly men. J Nutr. 2008 Oct;138(10):1931-8. 
128

771. New SA, Robins SP, Campbell MK, Martin JC, et al. Dietary influences on bone mass 

and bone metabolism: further evidence of a positive link between fruit and vegetable 

consumption and bone health? Am J Clin Nutr. 2000 Jan;71(1):142-51. 

772. New SA, Bolton-Smith C, Grubb DA, Reid DM. Nutritional influences on bone mineral 

density: a cross-sectional study in premenopausal women. Am J Clin Nutr. 1997 

Jun;65(6):1831-9. 

773. Macdonald HM, Black AJ, Aucott L, Duthie G, et al. Effect of potassium citrate 

supplementation or increased fruit and vegetable intake on bone metabolism in healthy 

postmenopausal women: a randomized controlled trial. Am J Clin Nutr. 2008 Aug; 

88(2):465-74. 

774. Masse PG, Jougleux JL, Tranchant CC, Dosy J, et al. Enhancement of calcium/vitamin D 

supplement efficacy by administering concomitantly three key nutrients essential to bone 

collagen matrix for the treatment of osteopenia in middle-aged women: a one-year followup. J Clin Biochem Nutr. 2010 Jan;46(1):20-9. 

775. Sahni S, Hannan MT, Gagnon D, Blumberg J, et al. Protective effect of total and 

supplemental vitamin C intake on the risk of hip fracture--a 17-year follow-up from 

the Framingham Osteoporosis Study. Osteoporos Int. 2009 Nov;20(11):1853-61. 

776. Morton DJ, Barrett-Connor EL, Schneider DL. Vitamin C supplement use and bone mineral 

density in postmenopausal women. J Bone Miner Res. 2001 Jan;16(1):135-40. 

777. Hall SL, Greendale GA. The relation of dietary vitamin C intake to bone mineral density: 

results from the PEPI study. Calcif Tissue Int. 1998 Sep;63(3):183-9. 

778. Thompson J. Vitamins, minerals and supplements 5: overview of vitamin C. Community 

Pract. 2007 Jan;80(1):35-6. 

779. Yoshida M, Takashima Y, Inoue M, Iwasaki M, et al. Prospective study showing that dietary 

vitamin C reduced the risk of age-related cataracts in a middle-aged Japanese population. 

Eur J Nutr. 2007 Mar;46(2):118-24. 

780. Watters JL, Satia JA, Kupper LL, Swenberg JA, et al. Associations of antioxidant nutrients 

and oxidative DNA damage in healthy African-American and White adults. Cancer Epidemiol 

Biomarkers Prev. 2007 Jul;16(7):1428-36. 

781. Henshall JD. The effect of processing on the nutritive value of fruit and vegetable products. 

Proc Nutr Soc. 1973 May;32(1):17-22. 

782. Fan X, Thayer DW. gamma-Radiation influences browning, antioxidant activity, and 

malondialdehyde level of apple juice. J Agric Food Chem. 2002 Feb 13;50(4):710-5. 

783. Salem SA. Effect of gamma radiation on the storage of onions used in the dehydration 

industry. J Sci Food Agric. 1974 Mar;25(3):257-62. 

784. Calucci L, Pinzino C, Zandomeneghi M, Capocchi A, et al. Effects of gamma-irradiation 

on the free radical and antioxidant contents in nine aromatic herbs and spices. 

J Agric Food Chem. 2003 Feb 12;51(4):927-34. 

785. Cooper M, Perchonok M, Koslovsky M, Smith SM, et al. International Space Station menu 

micronutrient concentration and stability. in review. 

786. Jagetia GC, Rajanikant GK, Baliga MS, Rao KV, et al. Augmentation of wound healing 

by ascorbic acid treatment in mice exposed to gamma-radiation. Int J Radiat Biol. 

2004 May;80(5):347-54. 

787. Cai L, Koropatnick J, Cherian MG. Roles of vitamin C in radiation-induced DNA damage 

in presence and absence of copper. Chem Biol Interact. 2001 Jul 31;137(1):75-88. 

788. Buettner GR, Jurkiewicz BA. Catalytic metals, ascorbate and free radicals: combinations 

to avoid. Radiat Res. 1996 May;145(5):532-41. 

789. Polivkova Z, Smerak P, Demova H, Houska M. Antimutagenic effects of lycopene and tomato 

puree. J Med Food. 2010 Dec;13(6):1443-50. 
129

790. Basu A, Imrhan V. Tomatoes versus lycopene in oxidative stress and carcinogenesis: 

conclusions from clinical trials. Eur J Clin Nutr. 2007 Mar;61(3):295-303. 

791. Hoshi T, Tian Y, Xu R, Heinemann SH, et al. Mechanism of the modulation of BK potassium 

channel complexes with different auxiliary subunit compositions by the omega-3 fatty acid 

DHA. Proc Natl Acad Sci U S A. 2013 Mar 19;110(12):4822-7. 

792. Hoshi T, Wissuwa B, Tian Y, Tajima N, et al. Omega-3 fatty acids lower blood pressure 

by directly activating large-conductance Ca2+-dependent K+ channels. Proc Natl Acad 

Sci U S A. 2013 Mar 19;110(12):4816-21. 

793. Nicastro HL, Dunn BK. Selenium and prostate cancer prevention: insights from the Selenium 

and Vitamin E Cancer Prevention Trial (SELECT). Nutrients. 2013;5(4):1122-48. 

794. Dunn BK, Richmond ES, Minasian LM, Ryan AM, et al. A nutrient approach to prostate 

cancer prevention: the Selenium and Vitamin E Cancer Prevention Trial (SELECT). 

Nutr Cancer. 2010;62(7):896-918. 

795. Sesso HD, Buring JE, Christen WG, Kurth T, et al. Vitamins E and C in the prevention of 

cardiovascular disease in men: the Physicians' Health Study II randomized controlled trial. 

JAMA. 2008 Nov 12;300(18):2123-33. 

796. Smith SM, Davis-Street J, Rice BL, Lane HW. Nutrition in space. Nutrition Today. 

1997 Jan-Feb;32(1):6-12. 

797. American Dietetic Association. Position of the American Dietetic Association: fortification 

and nutritional supplements. J Am Diet Assoc. 2005 Aug;105(8):1300-11. 

798. Goodman GE, Thornquist MD, Balmes J, Cullen MR, et al. The Beta-Carotene and Retinol 

Efficacy Trial: incidence of lung cancer and cardiovascular disease mortality during 6-year 

follow-up after stopping beta-carotene and retinol supplements. J Natl Cancer Inst. 

2004 Dec 1;96(23):1743-50. 

799. Hardman J, Limbird L. Goodman and Gilman's the pharmacological basis of therapeutics. 

New York: McGraw Hill; 1996. 

800. Guengerich FP, Miller GP, Hanna IH, Martin MV, et al. Diversity in the oxidation of substrates 

by cytochrome P450 2D6: lack of an obligatory role of aspartate 301-substrate electrostatic 

bonding. Biochemistry (Mosc). 2002 Sep 10;41(36):11025-34. 

801. Utermohlen V. Diet, nutrition, and drug interactions. In: Shils ME, Olson JA, Shike M, 

Ross AC, editors. Modern nutrition in health and disease. 9th ed. Baltimore, MD: 

Lippincott Williams & Wilkins; 1998. 

802. Guengerich FP. Influence of nutrients and other dietary materials on cytochrome P-450 

enzymes. Am J Clin Nutr. 1995 Mar;61(3 Suppl):651S-8S. 

803. Bailey DG, Spence JD, Edgar B, Bayliff CD, et al. Ethanol enhances the hemodynamic 

effects of felodipine. Clin Invest Med. 1989 Dec;12(6):357-62. 

804. Kane GC, Lipsky JJ. Drug-grapefruit juice interactions. Mayo Clin Proc. 2000 Sep; 

75(9):933-42. 

805. Lown KS, Bailey DG, Fontana RJ, Janardan SK, et al. Grapefruit juice increases felodipine 

oral availability in humans by decreasing intestinal CYP3A protein expression. J Clin Invest. 

1997 May 15;99(10):2545-53. 

806. Lundahl J, Regardh CG, Edgar B, Johnsson G. Relationship between time of intake of 

grapefruit juice and its effect on pharmacokinetics and pharmacodynamics of felodipine 

in healthy subjects. Eur J Clin Pharmacol. 1995;49(1-2):61-7. 

807. Piver B, Berthou F, Dreano Y, Lucas D. Inhibition of CYP3A, CYP1A and CYP2E1 activities 

by resveratrol and other non volatile red wine components. Toxicol Lett. 2001 Dec 15; 

125(1-3):83-91. 

808. Chan WK, Nguyen LT, Miller VP, Harris RZ. Mechanism-based inactivation of human 

cytochrome P450 3A4 by grapefruit juice and red wine. Life Sci. 1998;62(10):PL135-42. 
 

130

809. Roby CA, Anderson GD, Kantor E, Dryer DA, et al. St John's Wort: effect on CYP3A4 activity. 

Clin Pharmacol Ther. 2000 May;67(5):451-7. 

810. Fujita K. Food-drug interactions via human cytochrome P450 3A (CYP3A). Drug Metabol 

Drug Interact. 2004;20(4):195-217. 

811. Fagan TC, Walle T, Oexmann MJ, Walle UK, et al. Increased clearance of propranolol and 

theophylline by high-protein compared with high-carbohydrate diet. Clin Pharmacol Ther. 

1987 Apr;41(4):402-6. 

812. Saarem K, Pedersen JI. Sex differences in the hydroxylation of cholecalciferol and of 5 betacholestane-3 alpha, 7 alpha, 12 alpha-triol in rat liver. Biochem J. 1987 Oct 1;247(1):73-8. 

813. Gupta RP, Hollis BW, Patel SB, Patrick KS, et al. CYP3A4 is a human microsomal vitamin D 

25-hydroxylase. J Bone Miner Res. 2004 Apr;19(4):680-8. 

814. Schuster I, Egger H, Reddy GS, Vorisek G. Combination of vitamin D metabolites with 

selective inhibitors of vitamin D metabolism. Recent Results Cancer Res. 2003;164:169-88. 

815. Pascussi JM, Robert A, Nguyen M, Walrant-Debray O, et al. Possible involvement 

of pregnane X receptor-enhanced CYP24 expression in drug-induced osteomalacia. 

J Clin Invest. 2005 Jan;115(1):177-86. 

816. Self TH, Chrisman CR, Baciewicz AM, Bronze MS. Isoniazid drug and food interactions. 

Am J Med Sci. 1999 May;317(5):304-11. 

817. Shah SC, Sharma RK, Hemangini, Chitle AR. Rifampicin induced osteomalacia. Tubercle. 

1981 Sep;62(3):207-9. 

818. Goraya JS, Gupta PN, Gupta RK, Bahadur R, et al. Anticonvulsant induced osteomalacia. 

Indian Pediatr. 2000 Mar;37(3):325-9. 

819. Chen H, Howald WN, Juchau MR. Biosynthesis of all-trans-retinoic acid from 

all-trans-retinol: catalysis of all-trans-retinol oxidation by human P-450 cytochromes. 

Drug Metab Dispos. 2000 Mar;28(3):315-22. 

820. Roberts ES, Vaz AD, Coon MJ. Role of isozymes of rabbit microsomal cytochrome P-450 in 

the metabolism of retinoic acid, retinol, and retinal. Mol Pharmacol. 1992 Feb;41(2):427-33. 

821. Force RW, Nahata MC. Effect of histamine H2-receptor antagonists on vitamin B12 

absorption. Ann Pharmacother. 1992 Oct;26(10):1283-6. 

822. Mowat C, McColl KE. Alterations in intragastric nitrite and vitamin C levels during acid 

inhibitory therapy. Best Pract Res Clin Gastroenterol. 2001 Jun;15(3):523-37. 

823. Russell RM, Golner BB, Krasinski SD, Sadowski JA, et al. Effect of antacid and H2 

receptor antagonists on the intestinal absorption of folic acid. J Lab Clin Med. 1988 Oct; 

112(4):458-63. 

824. Perchonok M, Bourland C. NASA food systems: past, present, and future. Nutrition. 

2002 Oct;18(10):913-20. 

825. Barta DJ, Henninger DL. Regenerative life support systems - why do we need them? 

Adv Space Res. 1994;14(11):40-3. 

826. Lane HW, Zwart SR, Kloeris V, Smith SM. Chapter 25. Food and Nutrition for Space Flight. 

In: Berdanier CD, Dwyer JT, Heber D, editors. Handbook of Nutrition and Food 3rd ed. 

New York: CRC Press; 2013. p. 381-402. 
131

Authors and Editor 

Scott M. Smith is Senior Nutritionist and Manager for Nutritional Biochemistry at the 

NASA Johnson Space Center in Houston, Texas. The primary goal of this group is to 

determine the nutritional requirements for extended-duration space flight. This involves 

conducting both operational and research activities, and has spanned Space Shuttle, 

Mir, and International Space Station flight platforms, and planning for lunar exploration 

missions. Research activities are conducted on space missions and in laboratories on 

the ground; ground-based research projects include studies of the effects of simulated 

weightlessness on calcium and bone metabolism, vitamin D supplementation in 

crews wintering over in Antarctica, and oxidative damage in crews living 50 feet below 

the surface of the ocean, and investigations of dietary and other countermeasures 

for ameliorating space flight-induced changes in human physiology. Dr Smith also 

participated in the definition of the current nutritional recommendations for extendedduration space flight, and is Co-Chair of the Multilateral Medical Operations Panel’s 

Nutrition Working Group for the International Space Station. 

Sara R. Zwart is a Senior Scientist and Deputy Manager of the Nutritional Biochemistry 

Laboratory at the NASA Johnson Space Center in Houston, Texas. She has been 

involved with research investigating relationships between nutrition and side effects 

of space flight, including bone loss, changes in iron metabolism, and oxidative 

damage. She has also worked with ground-based analogs of space flight, including 

cell culture models, NASA Extreme Environment Mission Operations (NEEMO) projects, 

extravehicular activity analogs at the Neutral Buoyancy Laboratory at the Johnson 

Space Center, and bed rest models. 

Martina Heer is Senior Nutritionist and Adjunct Professor at the University of Bonn, 

Institute of Food and Nutrition Sciences, Bonn, Germany, and Director of Nutrition 

Sciences at PROFIL, Neuss, Germany. She also represents the European Space 

Agency (ESA) in the Multilateral Medical Operations Panel’s Nutrition Working Group 

for the International Space Station (ISS) and is a member of the ESA Nutrition Expert 

Committee. Previously she headed the Space Physiology Division, Institute of Aerospace 

Medicine, at the German Aerospace Center (DLR) for 6 years. Her main research interest 

is to understand the interaction of nutrition and nutrients (in particular sodium and 

sodium chloride) with other physiological systems such as the musculoskeletal and 

cardiovascular systems. Her space flight studies started with Space Shuttle missions 

and missions to the Mir station, and they continue with experiments on the ISS. 

The space studies are combined with extensive research in the form of space analog 

studies on the ground. 

Jane M. Krauhs is a Senior Scientist with Wyle Science, Technology and Engineering 

Group in Houston, Texas, and an Editor in the Life Sciences (Diplomate). She has edited 

many technical and nontechnical documents produced by the Nutritional Biochemistry 

Laboratory and other space life science disciplines at the Johnson Space Center. 
132

Acknowledgments 

This book represents a review of many areas of research as seen from the 

perspective of a few space nutritionists, but many people have contributed to 

this research and we would like to recognize as many as we can. At the heart 

of all space life sciences research are the astronauts who bravely soared into 

space. Beyond their required duty of flying and maintaining the spacecraft, 

and all that that entails, they also volunteered to be operator and/or subject for 

science experiments. Without their efforts and dedication to this process, none 

of the research on space physiology and medicine would be possible, and we 

are greatly in their debt. 

In the field of space nutrition that we have reviewed, our laboratory teams have 

played a central role. Although few original data are reported herein, the primary 

publications represent the efforts of many individuals. We recognize the NASA 

Nutritional Biochemistry Laboratory (NBL) team, a continually evolving group of 

dedicated individuals who work tirelessly to ensure that all samples, from flightor ground-based studies, are collected, processed, and analyzed according 

to detailed plans and procedures. Sample and data management represent a 

somewhat tedious, but absolutely critical element of this research, and the NBL 

team members handle this with unparalleled grace and tenacity. 

Space flight research is literally unlike anything on Earth. The teams of review 

and support staff, from the engineers who develop flight hardware to trainers 

who work with the crews to accomplish all on-orbit data and sample collection, 

to experiment support personnel who watch over every aspect of a study, all 

ensure that nothing escapes completion. Again, these teams represent evolving 

and ever-changing groups of names, but without their dedication, these studies 

would simply not be possible. 

Similarly, NASA management personnel across organizations and over 

the years have supported efforts to allow these research projects to happen. 

The NASA Human Research Program (HRP), established in 2006, set the stage 

for conduct of most of the studies from which we obtained the International 

Space Station data reported herein. “The HRP” includes the Human Health 

and Countermeasures Element of the HRP, where the Nutrition Risk part of the 

human space flight research plan resides. While many considered nutrition an 

afterthought on the list of potential countermeasures, or worse, thought nutrition 

to be simply what the food system provided, others championed nutrition 

research and have nourished it for the past 8 years. 

We are indebted to many of the scientists at NASA who took time to review 

sections of the text. Specifically, Brian Crucian for the Immune Function, 

Inflammation, and Nutrition section, Robert Pietrzyk for the Renal Stone Risk 

subsection, Lori Ploutz-Snyder for the Energy and Fuel Metabolism section 

and the Muscle and Protein sections, and Jean Sibonga for the Bone section. 

Their input was extremely helpful and valuable, and the authors take blame 

for any mistakes or oversights remaining. 
133

We are grateful to Cynthia Bush, Senior Graphic Design Specialist/Illustrator 

at the Johnson Space Center in Houston. The products of her efforts to create 

the cover and several of the specialized figures herein are outstanding. She was 

subsequently also responsible for the content layout work. If what you hold in 

your hands (or see on a screen) looks good—it is thanks to Cindy. 

As evidenced by the affiliations of Dr Martina Heer, the collaboration between 

US and German investigators over the past 10 to 15 years has promoted growth 

and expansion of knowledge, as any good collaboration does. European Space 

Agency and German Aerospace Center (DLR) support of nutrition research has 

contributed greatly to this field of science as reviewed herein. Similar to those 

described above for NASA, a management structure and dedicated laboratory 

support team have enabled outstanding research. 

We are indebted to many for the opportunities we have had to conduct research, 

to publish research, and to review research herein. We hope you find this volume 

useful for your own knowledge base. 

SMS, SRZ, MH. 
134

Nutrient Intake DataUpdated nutrient intake data for several space programs are reported below. For ISS, we report the data on nutrients available from the Food Frequency Questionnaire analysis. Data on planned ISS menu content and information on a wider range of nutrients are available online at http://go.nasa.gov/QS1KW1 (2). 
135

List of Figures 

Figure 1. ........................................................................................................................... 2 

Mike Lopez-Alegria collects the first blood sample on the International Space Station 

on October 5, 2006, having inserted the needle himself. The collection tubes can be 

seen (one in hand, the others in elastic bands on his belt assembly), along with a sharps 

container and detailed procedures (both Velcroed to the wall). He had set the camera to 

automatically take pictures periodically, snapping this one in the process. ...................... 

Figure 2. ........................................................................................................................... 3 

The refrigerated centrifuge on ISS. 

Figure 3. ........................................................................................................................... 3 

Left: Urine Collection Device (UCD), shown here with a syringe on the collection port, 

and a female adapter. Right: A UCD with a male adapter shown floating on ISS. 

Figure 4. ........................................................................................................................... 3 

Sunita Williams shown here with a UCD, contained in a ziplock bag to provide another 

layer of containment. 

Figure 5. ........................................................................................................................... 4 

Urine Containment Bag, used for holding discarded UCDs until they can be disposed 

of along with other trash from ISS. 

Figure 6. ........................................................................................................................... 4 

External view of a MELFI, showing the covers for the 4 dewars. 

Figure 7. ........................................................................................................................... 4 

External view of a MELFI, showing the interior of one of the dewars, with 1 tray partially 

removed. The 2 urine syringes to be placed in the freezer can be seen Velcroed to 

the wall. 

Figure 8. ........................................................................................................................... 4 

Closer view of the MELFI tray, with compartment open for sample insertion. The mesh 

bags are designed to contain the samples from a given blood collection session. Blood 

samples are shown in this image. 

Figure 9. ........................................................................................................................... 4 

Crewmember Greg Chamitoff (left) can be seen with the MELFI tray under his right hand, 

performing sample transfers from the MELFI into the Double Cold Bag (Sandy Magnus 

has her hand inside the DCB in this image). 

Figure 10. ......................................................................................................................... 5 

Peggy Whitson with the opening screen of the Food Frequency Questionnaire. After 

a crewmember enters name or initials, the second screen opens with a list of food 

categories, under each of which is a list of food items grouped together according 

to nutrient content. 

Figure 11. ......................................................................................................................... 6 

A screen shot from the Food Frequency Questionnaire. Available foods are grouped 

according to key nutrient content. Shown here are 2 groups: “Fruit” and “Beans, Soups.” 

The foods provided by the European Space Agency, the Canadian Space Agency, the 

Japanese Aerospace Exploration Agency, and NASA are shown in black font, and foods 

provided by Russia are shown in blue. 

Figure 12. ......................................................................................................................... 6 

Karen Nyberg (left), Expedition 36 Flight Engineer, using the Space Linear Acceleration 

Mass Measurement Device (SLAMMD). Tom Marshburn (right), Expedition 34 Flight 

Engineer, using the Body Mass Measurement Device (BMMD). Photo credits: NASA. 
136

Figure 13. ......................................................................................................................... 8 

In-flight dietary intake of crewmembers in different space programs. Data are expressed 

as percentage of energy requirements predicted by the World Health Organization (17) 

and are mean ± SD. Apollo N = 33, Skylab N = 9, Shuttle N = 32, Mir N = 7, ISS E1-18 

N = 26, E19-36 N =31. E = expedition. Apollo and Skylab data are from Bourland et al 

(32). Figure is adapted from earlier publications (2, 33), with additional published data 

included (6, 31, 34). 

Figure 14. ....................................................................................................................... 10 

In-flight body mass measurement data from 55 ISS crewmembers. Pre = preflight, FDx 

= flight day x, R+x = x days after landing. Data are expressed as percent change from 

preflight values and are mean ± SD. Figure and data adapted from Zwart et al (7). 

Figure 15. ....................................................................................................................... 10 

Changes in body weight on the day of landing relative to before flight. Data are 

expressed as percent change from preflight values. Each symbol represents 

1 crewmember from a mission on the Space Shuttle (green, N=25), Skylab (red, N=9), 

Mir (blue squares, N=19), ISS Expeditions 1-18 (dark purple, N=26), or Expeditions 

19-36 (light purple, N=31). Duration data have been adjusted slightly to ensure 

anonymity. Data updated from initial publications (2, 56). 

Figure 16. ....................................................................................................................... 11 

Relationship between energy intake (kcal/kg body mass/d) and weight loss (change 

in body mass/d, kg) during Apollo missions. N = 33. Data are courtesy of William 

Carpentier. 

Figure 17. ....................................................................................................................... 15 

Relationship between fish intake during long-duration flight and whole-body bone 

mineral density loss after flight on ISS. Figure adapted from (116). 

Figure 18. ....................................................................................................................... 17 

Protein intake during space flight on ISS missions (N=56). Each point represents 

an individual crewmember, and is their reported average intake over the course of 

the mission. Dashed lines represent space flight (and ground-based) protein intake 

requirements of 0.8 g protein/kg body mass, and the range of protein intakes 

(1.2.-1.7 g/kg) recommended by the American Dietetic Association, Dietitians 

of Canada, and American College of Sports Medicine for high-intensity athletes (130). 

Figure 19. ....................................................................................................................... 22 

Total serum testosterone concentrations (mean ± SD) before (L-x), during (FDx), and 

after flight on ISS. Although circulating concentrations decreased significantly after 

flight (at R+0), no other time point differed significantly from the preflight mean. N=15. 

Data are from Smith et al (237). 

Figure 20. ....................................................................................................................... 23 

Total serum testosterone concentrations (mean ± SD) in subjects before, during, and 

after bed rest with (dashed line and open circles) or without (solid line and filled squares) 

an artificial gravity countermeasure. These data have been published (134), although the 

pre-bed rest data (BR-9, BR-1) were combined as an average in the original manuscript. 

In this figure, the 2 pre-bed rest data points are graphed separately, as reported in (237). 

Figure 21. ....................................................................................................................... 28 

Urine pH (mean ± SD) of amino acid-supplemented (red) and placebo (blue) groups 

during 4 weeks of bed rest. *Significantly different from before bed rest (Pre), P < 0.05. 

#Significant difference between groups, P < 0.05. Figure is from Zwart et al, J Appl 

Physiol 2005 (278). 
137

Figure 22. ....................................................................................................................... 32 

Bone resorption (as indicated by urinary n-telopeptide, left panel) and bone formation 

(as evaluated by serum bone-specific alkaline phosphatase, right panel) during 17 weeks 

of bed rest with (solid line) or without (dashed line) heavy resistance exercise. Data are 

expressed as percentage of pre-bed rest values, and are mean ± SD. The vertical line 

separates the bed rest and post bed rest periods. Data adapted from (192). 

Figure 23. ....................................................................................................................... 32 

Bone resorption (as indicated by urinary n-telopeptide, left panel) and bone formation 

(as indicated by serum bone-specific alkaline phosphatase, right panel) during 60 days 

of bed rest with (solid line) or without (dashed line) a combination of resistance exercise 

and supine treadmill/LBNP exercise. Data are expressed as percentage of pre-bed rest 

values, and are mean ± SD. Data adapted from (317). 

Figure 24. ....................................................................................................................... 33 

Garrett Reisman shown using the interim Resistance Exercise Device (iRED, left), while 

Sandy Magnus is shown using the Advanced Resistance Exercise Device (ARED, right) 

on ISS. The iRED was launched in 2000 with the first ISS crew, and the ARED replaced 

it in late 2008. 

Figure 25. ....................................................................................................................... 33 

Body composition changes (left panel, lean body mass; right panel, total body fat) in 

astronauts on Mir and ISS missions. ISS crews had access to either iRED or ARED 

exercise devices. Data are expressed as percent change per month of flight and are 

mean ± SD. Figure adapted from (31). 

Figure 26. ....................................................................................................................... 33 

Bone mineral density loss in astronauts on Mir and ISS missions. ISS crews had access 

to either iRED or ARED exercise devices. Data are expressed as percent change per 

month of flight. Figure adapted from (31). 

Figure 27. ....................................................................................................................... 34 

Bone mineral density (BMD) loss after flight in men (N=33, open bars) and women 

(n=9, solid bars) who used either the iRED or ARED exercise device. Data are expressed 

as percent change per month of flight and are mean ± SD. Figure adapted from (50). 

Figure 28. ....................................................................................................................... 36 

Relationship between omega-3 fatty acid consumption and bone resorption during 

bed rest (left panel), and relationship between fish consumption and percent change in 

whole-body bone mineral density at landing in astronauts (right panel). Figures adapted 

from (116). 

Figure 29. ....................................................................................................................... 39 

Pre- and postflight data (mean ± SD) from testing required for medical operations 

show that vitamin D status decreased after long-duration space flight, despite vitamin 

D supplementation with 400 IU/d (black dashed line, N=16). In-flight data (purple line) 

show that 800 IU/d is enough vitamin D3 to maintain status during long-duration space 

flight (N=26). Red lines depict Institute of Medicine-defined lower acceptable limits 

(with respect to bone health), and upper advised limit (390). The green line at 80 nmol/L 

reflects what many perceive as an optimal level with respect to parathyroid hormone 

suppression and non-bone health outcomes. Figure adapted, and data updated, from 

Smith et al (409). 
138

Figure 30. ....................................................................................................................... 40
 
 

Serum phylloquinone before launch (launch minus 180 d, or L-180, L-45, and L-10), 
 


during flight (flight days 15, 30, 60, 120, and 180), on landing day (R+0), and 30 days 


post landing (R+30). The dashed lines indicate the normal range for phylloquinone. 


Data are mean ±SD, N=26. Data and N are expanded from the original publication 
 


of these findings (419).

 

Figure 31. ....................................................................................................................... 41
 
 

Dietary phosphorus:calcium ratio in ISS crewmembers during flight (approximate 
 


flight days 15, 30, 60, 120, and 180). The dashed line indicates a ratio of 1.5. 


Data are mean ±SD, N=9.
 
 

Figure 32. ....................................................................................................................... 47
 
 

The percent change from preflight (mean ± SD) in serum ferritin (red circles, solid line) 
 


and urinary 8OHdG (blue squares, dashed line) before, during, and after long-duration 
 


space flight (N=23). “Pre” was determined from the mean of preflight data points (3 for 
 


ferritin, 4 for 8OHdG), and percent change was calculated from that average. (469)
 
 

Figure 33. ....................................................................................................................... 51
 
 

Relationship between energy intake (kcal/kg body mass/d) and plasma volume loss 
 


(mL/d) during Apollo missions. N=21. Data are courtesy of William Carpentier.
 
 

Figure 34. ....................................................................................................................... 52
 
 

Postflight heart rate under 3 conditions, predicted (from factors including plasma volume 
 


loss) versus actual measured heart rate. 
 


Figure 35. ....................................................................................................................... 52
 
 

Postflight heart rate, predicted (from factors including energy intake) versus actual 
 


measured heart rate. 
 


Figure 36. ....................................................................................................................... 53
 
 

Energy intake during space flight on ISS missions (N=56). Each point represents an 
 


individual crewmember, and is their reported average daily energy intake over the course 
 


of the mission. The dashed line represents 33 kcal/kg body mass, the energy intake 
 


needed to avoid plasma volume loss. Solid lines represent the mean ± 1SD.
 
 

Figure 37. ....................................................................................................................... 58
 
 

Proposed mechanism of the effects of high dietary sodium on bone loss.
 
 

Figure 38. ....................................................................................................................... 61
 
 

Estimated supersaturation risk from the Renal Stone Risk Profile during space flight on 
 


ISS. Each symbol represents a 24-h urine pool, and the solid black line represents the 
 


group mean. The red dashed line is the point above which the risk is greater than in 
 


the non-stone forming population.

 

Figure 39. ....................................................................................................................... 62
 
 

Relationship between urinary citrate and sodium urate supersaturation. The dashed 


line at 2 represents the average risk of sodium urate stone formation for the general 
 


population, with numbers above 2 being higher-than-average risk. Citrate, which 
 


typically protects against renal stone risk, is actually positively correlated with sodium 
 


urate supersaturation (Pearson r = 0.065; P < 0.016). The red dashed line is the point 
 


above which the risk is greater than in the non-stone-forming population. Data are from 
 


1399 twenty-four-hour urine collections.
 
 
139

Figure 40. ....................................................................................................................... 64 

Overview of 1-carbon metabolism. AA, amino acids; CBS, cystathionine ß-synthase; 

CYS, cystathionine; FA, fatty acids; HCY, homocysteine; αKBT, α-ketobutyrate; MCA, 

methylcitric acid; MM-CoA, methylmalonyl coenzyme A (CoA); MMA, methylmalonic 

acid; MS, methionine synthase; 5-MTHF, 5-methyltetrahydrofolate; 5,10-MTHF, 

5,10-methylenetetrahydrofolate; MTHFR, methylenetetrahydrofolate reductase; MTRR, 

5-methyltetrahydrofolate homocysteine methyltransferase reductase; PRP-CoA, 

propionyl CoA; SAH, S-adenosylhomocysteine; SAM,S-adenosylmethionine; 

SUC-CoA, succinyl CoA; THF, tetrahydrofolate 

Figure 41. ....................................................................................................................... 65 

Artistic depiction of vision issues related to space flight, and the potential involvement 

of genetic influences in this problem. The depiction of a different DNA base pair (blue 

above and red below) in the 2 strands reflects genetic polymorphisms, or multiple forms, 

of the DNA. Inset imagery includes ISS and a Hubble Space Telescope image of the 

Cat’s Eye Nebula, to reflect that these issues are relevant for not only ISS missions, 

but exploration-class missions as well. 

Figure 42. ....................................................................................................................... 66 

RBC (left) and serum folate (right) before, during, and after long-duration space flight 

(data are mean ± SD). Note: RBC folate data are not available during flight because 

of sample processing requirements. 

Figure 43. ....................................................................................................................... 68 

Serum retinol (left) and ß-carotene (right) before, during, and after long-duration space 

flight. Dashed lines represent normal range. Data are mean ± SD. 

Figure 44. ....................................................................................................................... 73 

An interaction between serum cortisol, vitamin D status, and the probability of EBV 

shedding. Data from all 41 participants in the Antarctic study are included in the graph, 

and the data were statistically analyzed using the continuous data set of cortisol data. 

The data are split into the 2 subgroups here for presentation purposes. The graph is 

from Zwart et al (407). 

Figure 45. ....................................................................................................................... 80 

Serum ɑ-tocopherol (top left) and ɣ-tocopherol (top right), and total vitamin E:lipid ratio 

(left) before, during, and after long-duration space flight. All data are mean ± SD. 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

140

Abbreviations 

8OHdG 

ADP 

AMP 

ARED 

BR 

BSAP 

cal 

CoA 

CO2 

d 

DLR 

DNA 

DRI 

DXA 

EBV 

EE 

Eq 

ESA 

EVA 

FDx 

FFQ 

g 

g 

GLA 

Gy 

h, hr 

HRP 

Hz 

IOM 

iRED 

ISS 

IU 

J 

k 

KCit 

KMgCit 

L-x 

L 

LBNP 

μ 

m 

M 

MDA 

MELFI 

mol 

8-hydroxy-2'-deoxyguanosine 

adenosine diphosphate 

adenosine monophosphate 

Advanced Resistance Exercise Device 

bed rest (day) 

bone-specific alkaline phosphatase 

calorie 

coenzyme A 

carbon dioxide 

day, deci 

German Aerospace Center 

deoxyribonucleic acid 

dietary reference intake 

dual-energy x-ray absorptiometry 

Epstein-Barr virus 

energy expenditure 

equivalent 

European Space Agency 

extravehicular activity (space walk) 

flight day x 

Food Frequency Questionnaire 

gram 

acceleration due to gravity (1g = Earth gravity) 

gamma-carboxyglutamic acid 

Gray 

hour 

Human Research Program 

hertz 

Institute of Medicine 

interim resistance exercise device 

International Space Station 

international unit 

joule 

kilo 

potassium citrate 

potassium magnesium citrate 

x days before launch 

liter 

lower-body negative pressure 

micro 

meter, milli 

mega 

malondialdehyde 

minus-eighty (degrees) laboratory freezer for ISS 

mole 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

141

MRI magnetic resonance imaging 

n nano 

n number of subjects in a subsample 

N number of subjects in a sample of a population 

NAD nicotinamide adenine dinucleotide 

NADH reduced form of nicotinamide adenine dinucleotide 

NADP nicotinamide adenine dinucleotide phosphate 

NADPH reduced form of nicotinamide adenine dinucleotide phosphate 

NASA National Aeronautics and Space Administration 

NBL Nutritional Biochemistry Laboratory 

NEEMO NASA Extreme Environment Mission Operations 

NF-κB NF-kappa B 

NK natural killer 

NTX n-telopeptide 

P probability, phosphate 

PGF2α 8-iso-prostaglandin F2α

PL pyridoxal 

PLP pyridoxal 5'-phosphate 

PM pyridoxamine 

PMP pyridoxamine 5'-phosphate 

PN pyridoxine 

PNP pyridoxine 5'-phosphate 

Post after bed rest 

Pre before flight or bed rest 

psia pound(s) per square inch absolute 

PTH parathyroid hormone 

PUFA polyunsaturated fatty acid 

r bivariate correlation coefficient 

R+x x days after landing (recovery) or end of bed rest 

RBC red blood cell 

RDA recommended dietary allowance 

RNA ribonucleic acid 

ROS reactive oxygen species 

SD standard deviation 

TEE total energy expenditure 

THF tetrahydrofolate 

U unit 

UCB urine containment bag 

UCD urine collection device 

ULLS unilateral limb suspension 

UPA Urine Processor Assembly 

US United States 

UV ultraviolet 

VDR vitamin D receptor 

WHO World Health Organization 

y year