b o r d e n i n s t i t u t e m o n o g r a p h s e r i e s

Water Requirements and Soldier

 Hydration

Scott J. Montain, PhD, and Matthew Ely, MS

e d i t e d b y

William R. Santee, PhD 

Karl E. Friedl, PhD, Colonel, US Army

b o r d e n i n s t i t u t e m o n o g r a p h s e r i e s

borden institute

Water 

Requirements 

and Soldier 

Hydration

Scott J. Montain, PhD, and Matthew Ely, MS

e d i t e d b y

William R. Santee, PhD

Karl E. Friedl, PhD, Colonel, US Army
It has long been known that individuals can survive much longer without food than 

they can without water. Only in recent decades has science been able to quantify the 

factors that influence the body’s ability to maintain optimal water levels and the physiological consequences of water imbalance. This monograph presents information 

to assist the caregiver—whether physician, medic, unit leader, or fellow soldier—in 

understanding the influence of environment, physical activity, body size and gender, 

and load carriage in maintaining water balance. 

This monograph was prepared for military medical educational use. The focus of the information is to foster discussion that may form the basis of doctrine and policy. The opinions or assertions contained herein are the private views of the authors and are not to be construed as official or as reflecting the views of the Department of the Army or the Department of Defense.

Dosage Selection: The authors and publisher have made every effort to ensure the accuracy of 

dosages cited herein. However, it is the responsibility of every practitioner to consult appropriate information sources to ascertain correct dosages for each clinical situation, especially for 

new or unfamiliar drugs and procedures. The authors, editors, publisher, and the Department 

of Defense cannot be held responsible for any errors found in this book.

Use of Trade or Brand Names: Use of trade or brand names in this publication is for illustrative 

purposes only and does not imply endorsement by the Department of Defense.

Neutral Language: Unless this publication states otherwise, masculine nouns and pronouns do 

not refer exclusively to men.

Certain parts of this publication pertain to copyright restrictions. 

ALL RIGHTS RESERVED.

No copyrighted parts of this publication may be reproduced or transmitted in any form 

or by any means, electronic or mechanical (including photocopy, recording, or any information storage and retrieval system), without permission in writing from the publisher or 

copyright owner.

Borden Institute

Walter Reed Army Medical Center

US Army Medical Department Center & School

This monograph was originally submitted to the Borden Institute as a chapter intended for 

publication in Military Quantitative Physiology, a book in the Textbooks of Military Medicine 

series. Because changes may be made before publication, this monograph is made available 

with the understanding that it will not be cited or reproduced without the permission of the 

publisher.
CONTENTS

Foreword v

Introduction 1

Water-Electrolyte Balance 5

Thirst and Satiation 9

Physiological Consequences of Body Water Imbalance 11

Dehydration 11

Hyperhydration 14

Water Intoxication or Hyponatremia 16

Factors Influencing Daily Water Requirements 19

Environment 19

Physical Activity 20

Body Size and Gender 20

Load Carriage 20

Impact of Hot Environments on Fluid Requirements 23

Effect of Hot Environments on Thirst and Voluntary 

Dehydration 24

Effect of Hot Environments on Water Delivery 24

Hot Weather Water Requirements 25
Effect of Cold Weather Operations on Water Requirements 27

Effect of Cold Weather on Thirst and Voluntary Dehydration 29

Effect of Cold Weather on Water Delivery 29

Daily Water Requirements 30

Effect of High-Altitude Operations on Water Requirements 31

Effect of High-Altitude Operations on Water Balance 31

Effect of Hypoxia on Thirst and Voluntary Water Intake 32

Other Factors Associated With High-Altitude Operations That 

Affect Water Requirements 32

Effect of High-Altitude Operations on Water Delivery 33

Water Requirements 33

Effect of Water Immersion on Water Requirements 35

Barriers to Rehydration 37

Water Availability 37

Palatability of Water 39

Gastric Emptying 39

Lost Solutes 40

Strategies to Sustain Hydration in Harsh Environments 43

Do Not Skip Meals 43

Make Time to Drink 43

Maximize Palatability 44

Minimize Body Water Losses 47

Provide Education 47

Summary 51

References 53

Cover photo: Courtesy of US Army

Scott J. Montain is a Research Physiologist, Military Nutrition Division, US Army Research 

Institute of Environmental Medicine, Building 42, Kansas Street, Natick, Massachusetts 

01760-5007.

Matthew Ely is a Researcher, US Army Research Institute of Environmental Medicine, Building 42, Kansas Street, Natick, Massachusetts 01760-5007.
Foreword

Water is the largest chemical component of the human body, typically 

accounting for about 60% of body weight. Animals were able to leave the 

primordial ocean by developing what Claude Bernard (the grandfather 

of physiology) called the milieu intérieur, an internal saline environment 

to bathe our cells. This special capability allows us to moderate our internal environment through homeostatic mechanisms and survive in the 

face of a wide variety of external stressors. Enhancing this capability to 

endure extreme environments is of particular importance to the Army, 

wherein any advantage to better exploit hostile conditions may be critical to mission success. Thus, water logistical planning and optimization 

of soldier osmoregulation are, in essence, tactical weapons. Relatively 

small derangements in hydration status can significantly impair mission 

performance. More severe dehydration can produce heat injury casualties. 

Therefore, it should not be surprising that the Army has been the leader 

in physiology research regarding water balance. Given the importance 

of ensuring adequate hydration during military operations, it is actually 

astounding that there remain large gaps in our knowledge of this critical 

area. This monograph summarizes the state of our knowledge. 

Scott Montain and Matthew Ely have focused on the very pragmatic 

aspects of various standards and predictions that currently drive military planning factors for water, key operational factors that affect these 

predictions, and approaches to maintaining appropriate levels of water 

intake. This monograh reduces a great deal of physiological research to 
practical guidance that is of great importance to the military. However, 

hidden behind clear, concise guidance is more specific scientific literature 

that is the basis for a deeper understanding of the mechanisms governing 

thirst, sweating, and other (nonsweating) water losses through the skin 

and respiratory tract, renal regulation, and fluid shifts between compartments affected by exercise, nutrition, and other key factors. Some of 

these topics are covered in other chapters that will be part of the Borden 

Institute’s volume on Military Quantitative Physiology. Other aspects 

of this area may be found in clinical textbooks on water and electrolyte 

management of patients.

So, what more is there to be learned about this subject? Consider 

camels and other mammals that have adapted to extreme desert conditions. Although we cannot mimic animals that have adapted to the desert 

over millennia, we can learn from them. Camels can lose water totaling 

30% of their body weight and still maintain plasma volume. They can 

also completely restore this water deficit in a single bout of drinking. 

Humans can tolerate less than half of this magnitude of water loss and, 

more significantly, reflect the deficit disproportionately in plasma volume 

losses. This compromise is enhanced with dehydration, especially during 

work in hot environments when peripheral vasodilation is maximally 

activated for cooling. We do not replace our losses by the amount of the 

deficit and must rely on meals to encourage additional rehydration (thus, 

part of the reason for a need to ensure food consumption in the field in 

addition to water consumption). 

The differences in tolerance to dehydration between humans and 

camels are remarkable, and there is still much to be learned from other 

desert adaptations, as well as from the behavioral adaptations of human 

desert dwellers that might allow elite forces to enhance operations in dry 

environments. T. E. Lawrence (also known as “Lawrence of Arabia”) 

thought that he and his Bedouin fighters could superhydrate in advance 

of a challenging event in the desert by forcibly overconsuming water; 

more recent experiments tried to do this with glycerol-enhanced water 

loading, and neither of these methods was effective. 

US Army doctrine earlier in the past century held that soldiers could 

adapt to dehydration by training with restricted water consumption. A 

study with Austrian Special Forces tested this premise and determined 

that soldiers were seriously and consistently incapacitated with 5 days 

of restricted water consumption of 1 liter per day during commando 

training exercises, thus reinforcing earlier research findings. 

It may be significant that water doctrine was heavily enforced during 

vi | Foreword
the first Gulf War and that there were relatively few heat casualties, or 

maybe we were simply lucky. However, we did everything that we knew 

to do to sustain hydration, even to the point of public discussion about 

the quality of bottled water being provided to troops in the field. In fact, 

the US Army was so effective in delivering this message of hydration 

to prevent heat injury that a series of cases of hyponatremia occurred 

in training units, and new upper limits to the hydration guidance had 

to be developed. Despite the great efforts by the US Army to reduce 

environmental injuries, heatstroke cases have not been eliminated from 

military training and seem to fluctuate with cycles of training cadre and 

heightened awareness caused by new tragedies. 

Technologies exist today to create a minimally invasive hydration 

monitor that queries the interstitial environment and provides a realtime assessment of hydration status and could alert soldiers, medics, 

or commanders to a timely intervention. The technologies also exist to 

create a “Dune suit” (from the Frank Herbert science fiction series) that 

would collect, purify, and recycle all water leaving the body via cutaneous, respiratory, and urinary routes. Neither of these materiel solutions 

has been produced. With other modern methods (and a commitment 

of resources to this effort), many yet unanswered questions about fluid 

distribution between body compartments, especially in the environments 

and conditions to which soldiers may be exposed, and the relationship to 

physical and mental performance, could be addressed. Many other questions, such as apparent sex differences in water turnover rates, need to 

be further elucidated. Strategies to produce the “human camel” have yet 

to be proposed, but should not be far behind with the current advances 

in genomics, proteomics, and genetic engineering.

It is generally understood that there are enduring military needs for 

research and development in certain fundamental lanes wherein new 

military options should be continuously evolving with new emerging 

technologies and understanding. Osmoregulation and water balance are 

enduring requirements. Much of today’s knowledge is founded on the 

extensive and practical research studies of physiologists mobilized from 

the Harvard Fatigue Laboratory in World War II, conducted at the Fort 

Knox Armored Medical Research Laboratory and other Army facilities, by E. F. Adolf and D. B. Dill in their famous desert studies, and by 

others who followed them, as described in this monograph. Today, the 

research capability in hydration research resides almost exclusively at 

the US Army Research Institute of Environmental Medicine (USARIEM) 

with Dr. Montain and his colleagues. The computational models of water 

Foreword | vii
requirements produced in the 1970s and 1980s by USARIEM are cited as 

national standards by many other federal agencies and standards-creating 

bodies. The work is essential to the US Army, but the research also benefits workers in industry, athletes, emergency responders, government 

agencies (eg, NASA [National Aeronautics and Space Administration], 

OSHA [Occupational Safety and Health Administration], USDA [US 

Department of Agriculture]), and many others. The National Institutes 

of Health relies on the US Army for medical research on performance 

optimization in healthy men and women. 

Water and hydration standards are just as relevant today as they 

were in World War II. We still have deaths in training from inadequate 

fluid consumption, and, in rare cases, even from excessive hydration and 

hyponatremia. Today, military operations and humanitarian aid missions 

frequently take place where the local water resources are lacking. Water 

is a major logistical burden in Iraq and Afghanistan, putting soldier lives 

at risk in every water supply convoy and calling for the greatest possible 

precision in water planning tables. The soldier load on patrol in remote 

regions is also critical; and water is a significant portion of this load. 

The availability of potable water throughout the world is becoming an 

increasing problem that may even drive some future international conflicts. Understanding the impact of water turnover rates and the effects of 

dehydration will become even more important. Dr. Montain’s monograph 

is a must read for every preventive medicine officer and military medical 

consultants, advisors, and surgeons assigned to operational units.

Karl E. Friedl, PhD, Colonel, US Army

 Director, Telemedicine and Advanced Technology 

Research Center 

 US Army Medical Research and Materiel Command

William R. Santee, PhD

Research Physical Scientist

 US Army Research Institute of Environmental Medicine

viii | Foreword
Introduction

The provision of sufficient potable water to sustain soldier hydration 

is a prerequisite for successful deployment. Adolph et al1 and Eichna et 

al2 documented that dehydration can degrade soldier morale, as well as 

the desire to work (Figure 1). Body water deficits as little as 2% body 

weight can impair physical performance.3 Water deficits of 5% to 7% of 

initial body weight are associated with dyspnea, headache, dizziness, and 

apathy.1 An extreme example of the threat dehydration poses to troops 

is shown by the deaths of many Egyptian soldiers who were cut off from 

needed supplies and who suffered water deficits and heat illness during 

the 1967 Six-Day War. 

During military operations, an individual soldier’s daily water requirements to sustain hydration can range from 2 L/d to an excess of 

12 L/d, depending on weather conditions, workload, and physical size. 

Although 2 L/d is relatively easily achieved both in garrison and during 

deployment, 12 L/d will require concerted effort and coordination by 

military logistic support personnel and field commanders to ensure that 

adequate fluid is available (Figure 2). It is the responsibility of medical 

personnel to educate unit leaders about the importance of hydration 

and the necessary measures to optimize fluid intake and retention. This 

monograph discusses the effect of harsh environments on fluid requirements and presents methods to sustain water balance during deployment 

to environmental extremes. 
2 | Water Requirements and Soldier Hydration

Figure 1. E. F. Adolph stands alongside a military jeep in the desert in 1943. Adolph and colleagues were in California conducting experiments for the military on the adaptability of man 

to a desert environment.

Photograph: Courtesy of the University of Rochester archives.
Figure 2. World War II soldier using steel helmet to hold water and to bathe. Earlier beliefs 

that water restriction would improve human tolerance to dehydration were not borne out by 

the careful studies of Adolph and others in the 1940s.

Photograph: Courtesy of the National Archives, photo no. 208-AA-4NN-24.

Introduction | 3

Water-Electrolyte Balance

The human body is composed mainly of water. It accounts for 50% to 

70% of body weight for an adult man and 40% to 60% of body weight 

for an adult woman.4 Water is primarily distributed in nonfat tissue 

and accounts for 72% to 73% of lean body mass. The large, betweenindividual variation in total body water relative to body weight is primarily a function of body fat (ie, adipose tissue is a major contributor 

to an increase in body weight without a corresponding increase in body 

water). The typical male soldier has 47 L of body water, whereas the 

typical female soldier has 33 L of body water. 

On a daily basis, body water is relatively stable, provided adequate 

fluid is available to offset water lost by sweating, respiration, and urination. Water balance is regulated daily to within ~0.66% of body weight 

in healthy, active populations.5 This tight balance is achieved through 

fluid ingestion associated with eating and drinking, coupled with metabolic water production and physiological systems that regulate fluid loss 

(eg, renal, cardiovascular, and hormonal systems). Table 1 illustrates the 

sources of water losses and production,3 as well as the lower estimates 

of the turnover from these sources. It should be recognized, however, 

that the values presented can vary markedly. Respiratory water losses, 

for example, can range from 0.2 L/d in humid air to more than 1.5 L/d 

during exercise at extremely high altitudes. Cutaneous sweating can 

increase from 0.5 L/d to more than 10 L/d with prolonged exercise in hot 

environments. Similarly, diarrhea can greatly increase fecal water losses. 
For more information, the Institute of Medicine has published dietary 

reference intakes for water (Table 2) and electrolytes.3

Body water levels can vary markedly throughout a day. It is not uncommon for athletes or soldiers to lose 2% to 5% of body weight during 

physical activity, 6–9 and make up the short-term water deficit by drinking 

during rest periods and at meals.1,10,11 This pattern of voluntary dehydration during physical activity—followed by drinking the majority of daily 

water intake at mealtime—results in marked fluctuations that are not apparent during day-to-day examinations of total body water measurements. 

Body water is distributed in three principal fluid compartments: (1) 

intracellular, (2) interstitial, and (3) intravascular. Movement within the 

intracellular and extracellular compartments is determined largely by 

osmotic forces;12 therefore, maintenance of total body water is dependent 

on total solute and distribution of that solute. Dehydration mediated 

by sweating influences each fluid space as a consequence of free fluid 

exchange. Nose and colleagues13 determined the distribution of body 

water loss among the fluid spaces, as well as among different body 

organs in rats when they were dehydrated by 10% initial body weight 

with exercise-heat stress. The water deficit was apportioned between 

the intracellular (41%) and extracellular (59%) spaces, and also among 

the organs as follows: 

•	 40% from muscle,

•	 30% from skin,

•	 14% from viscera, and

•	 14% from bone.

Table 1. Estimates of Minimal Daily Water Losses and Production* 

Source Loss (mL/d) Production (mL/d)

Respiratory loss –250 to –350

Urinary loss –500 to –1,000

Fecal loss –100 to –200

Insensible loss –450 to –1,900

Metabolic production — +250 to +350

Total –1,300 to –3,450 +250 to +350

Net loss –1,050 to –3,100

*Assuming conditions in which there is minimal water loss from sweating. 

Adapted from: Food and Nutrition Board, Institute of Medicine. Dietary Reference Intakes for Water, Potassium, 

Sodium, Chloride, and Sulfate. Washington, DC: National Academy Press; 2005.

6 | Water Requirements and Soldier Hydration
Table 2. Criteria and Dietary Reference Intake Values* for Total Water†

Adequate Intake‡ (L/d)

Male Female

Life-Stage From From Total From From Total

Group Criterion Foods Beverages Water Foods Beverages Water

0–6 mos Average consumption of 0.0 0.7 0.7 0.0 0.7 0.7

water from human milk

7–12 mos Average consumption of 0.2 0.6 0.8 0.2 0.6 0.8

water from human milk

and complementary 

foods

1 yr Median total water intake 0.4 0.9 1.3 0.4 0.9 1.3

from NHANES III

4–8 yrs Median total water intake 0.5 1.2 1.7 0.5 1.2 1.7

from NHANES III

9–13 yrs Median total water intake 0.6 1.8 2.4 0.5 1.6 2.1

from NHANES III

14–18 yrs Median total water intake 0.7 2.6 3.3 0.5 1.8 2.3

from NHANES III

>19 yrs Median total water intake 0.7 3.0 3.7 0.5 2.2 2.7

from NHANES III

Pregnancy

14–50 yrs Same as median intake 0.7 2.3 3.0

for nonpregnant women 

from NHANES III

Lactation

14–50 yrs Same as median intake 0.7 3.1 3.8

for nonlactating women 

from NHANES III

NHANES III: Third National Health and Nutrition Examination Survey (1988–1994)

*No Tolerable Upper Intake Level is established; however, maximal capacity to excrete excess water in individuals with 

normal kidney function is 0.7 L/h.

†Total water represents drinking water, water in other beverages, and water (moisture) from food. (See Table S-1 in 

Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate for the median percentage if total water 

intake from beverages [including drinking water] and from foods reported in the NHANES III.)

‡The observed average or experimentally determined intake by a defined population or subgroup that appears to sustain 

a defined nutritional status, such as growth rate, normal circulating nutrient values, or other functional indicators of health. 

Adequate intake is used if sufficient scientific evidence is not available to derive an Estimated Average Requirement. 

Bold numbers = adequate intake is not equivalent to a Recommended Dietary Allowance. (Adequate 

intake is provided where no Recommended Dietary Allowance has been established.)

Adapted from: Food and Nutrition Board, Institute of Medicine. Dietary Reference Intakes for Water, Potassium, Sodium, 

Chloride, and Sulfate. Washington, DC: The National Academies Press; 2005.

Water-Electrolyte Balance | 7
Neither the brain nor liver lost significant water content. Thus, dehydration results in water redistribution largely between the intracellular and 

extracellular spaces of muscle and skin. 

Sweat-induced dehydration decreases plasma volume and increases 

plasma osmotic pressure in proportion to the level of fluid loss. Plasma 

volume decreases because it provides the precursor fluid for sweat. 

Osmolality increases because sweat is ordinarily hypotonic relative to 

plasma.14,15 Sodium and chloride are primarily responsible for elevated 

plasma osmolality.15 It is the plasma hyperosmolality that mobilizes fluid 

from the intracellular space to the extracellular space and enables plasma 

volume defense when a person is dehydrated.

8 | Water Requirements and Soldier Hydration
Thirst and Satiation

Several factors stimulate thirst and play a role in satiation. The most 

studied mechanism is in osmoreceptor cells, which are activated by intracellular dehydration. These cells—located in the preoptic/hypothalamic 

region of the brain—are sensitive to cell volume reductions induced by 

either extracellular hypertonicity or isoosmotic water loss (eg, hemorrhage, diuretic administration). Activation of these cells stimulates both 

thirst and arginine vasopressin (AVP) secretion. Stimulation of AVP 

secretion increases the desire to drink and facilitates water retention by 

increasing renal water reabsorption. 

Changes in extracellular fluid volume also contribute to thirst and 

water balance. Evidence indicating that water volume is independent of 

osmotic stimuli comes from experiments that have manipulated blood 

volume or cardiac filling or both. Increased blood volume or central venous pressure (eg, head-out immersion) leads to diuresis, reduced thirst, 

and voluntary water intake.16–18 In contrast, central venous pressure 

reductions from hemorrhage, diuretics, or lower body negative pressure 

decrease urine output and increase thirst and drinking.16 Sensitivity of 

the response is much lower, however, than the osmotic thirst mechanism 

and seems to be more effective with intact kidneys,19 thus suggesting that 

the renin-angiotensin system contributes to thirst response.

Animal experiments have demonstrated that angiotensin II can be 

a potent dipsogen.20 Receptors for angiotensin II exist in the preoptic/

hypothalamic regions of the brain. Intraventricular infusion of small 
quantities of angiotensin II stimulate thirst.20 Pharmacological blockade of 

angiotensin I-type receptors and angiotensin II receptors reduces the thirst 

response accompanying food intake, hemorrhage, and water deficit.21–23

Whether systemic angiotensin II plays a direct role in stimulating thirst 

in humans remains unclear. Current evidence suggests that blood-borne 

angiotensin II binds to receptors within the preoptic/hypothalamic regions, and the transduced signal is integrated with other inputs from 

systemic arterial blood pressure and volume receptors.24

Unpleasant oral sensations also play a role in stimulating voluntary fluid intake. Several studies have demonstrated that 24 hours of 

fluid abstinence produces a marked increase in the subjective ratings 

of thirst, mouth dryness, and unpleasant taste.25 The reversal of these 

oral sensations is significantly correlated with subsequent water intake. 

Similarly, induction of graded levels of dehydration by food restriction, 

fluid restriction, and exercise-heat stress are accompanied by graded 

increases in the number and magnitude of unpleasant oral sensations.26

Sensations showing a linear trend with dehydration level include the 

following: 

•	 dry and irritated throat;

•	 chapped lips;

•	 feelings of weariness, dizziness, and appetite loss; and

•	 thinking of drinking.

To examine if these oral sensations contributed to drinking behavior, 

Phillips and colleagues10 obtained blood samples and visual analog scale 

thirst measurements at hourly intervals and when volunteers drank water during a normal working day. They found that sensations of thirst, 

mouth dryness, unpleasant mouth taste, and the pleasure of drinking 

water increased before ad libitum intake, with no concomitant changes 

in blood tonicity or volume. These results provide evidence that, during 

free access to water, humans become thirsty and drink before body water 

deficits accrue, perhaps in response to subtle oropharyngeal cues.10

10 | Water Requirements and Soldier Hydration
Physiological Consequences of 

Body Water Imbalance

Dehydration

Body water deficits can occur in all environments, either from inadequate 

water availability, insufficient voluntary intake to offset water losses in 

sweat and respiration, or excessive water loss from diuresis or diarrhea. Regardless of the specific process, water deficit or dehydration can reduce the 

ability of soldiers to perform mission-essential tasks in a timely and effective 

manner, thus increasing their likelihood of becoming medical casualties. 

Symptoms 

One of the earliest qualitative signs of dehydration is the development 

of thirst.1 With approximately 2% body weight loss, vague discomfort 

develops, and the number of complaints increase. With additional water 

loss, soldiers can develop: 

•	 flushed skin,

•	 heat oppression,

•	 weariness,

•	 sleepiness,

•	 impatience, and

•	 anorexia.
At 4% loss of body weight, soldiers display signs of apathy, and might 

complain of muscle fatigue and nausea. At water deficits of 6% body 

weight and greater, soldiers will likely display

•	 dizziness,

•	 headache,

•	 dyspnea,

•	 tingling in limbs, and

•	 very dry mouth.

They might present with indistinct speech and the inability to walk. 

Thermoregulatory and Cardiovascular Effects

Body water deficits increase the thermal strain of exercise in temperate and hot environments. Individuals performing the same physical 

task when dehydrated have higher core temperatures than they would if 

they were normally hydrated, and the magnitude of the core temperature 

increase is proportional to the body water deficit. Studies examining 

the core temperature response to graded water deficits report that core 

temperature increases 0.15°C to 0.20°C for every 1% body weight 

lost.1,27–29 This relationship is maintained independent of exercise intensity30 and is not affected by changing the pattern of drinking.1,31 The 

magnitude of core temperature increase is, however, less in temperate 

versus warmer environments. In cold environments, dehydration does 

not seem to increase the risk of hypothermia; limited data available 

suggest that body temperature falls similarly during whole-body cooling, 

depending on if an individual is normally hydrated or dehydrated.32

Whether dehydrated persons are at increased risk of peripheral cold 

injury is controversial.33–35

Dehydration exerts its effects on thermoregulation by altering the 

ability to dissipate heat to the environment. Both skin blood flow and 

sweating responses are affected. Fortney and colleagues36 reported that 

thermally induced dehydration increased the esophageal temperature 

threshold for cutaneous vasodilatation and attenuated the increase in 

cutaneous blood flow per unit increase in body temperature. Dehydration also reduces peak cutaneous vasodilatation in both humans36 and 

nonhuman primates.37,38 Montain and colleagues39 found that thermally 

induced dehydration increased the esophageal temperature threshold 

for sweating and attenuated the increase in sweating per unit increase 

in body temperature. In addition, the magnitude of the sweating gain 

12 | Water Requirements and Soldier Hydration
and sensitivity alterations increased as a function of body water loss, 

which is in agreement with the observation that temperature increases 

in proportion to the water deficit.

Singular and combined effects of hypertonicity and hypovolemia 

contribute to modulating thermoregulation when an individual is hypohydrated. Hypertonicity, per se, will increase the esophageal temperature 

threshold for sweating36 and increase core temperature during exerciseheat stress.36,40,41 Similarly, the production of an isotonic hypovolemia 

with diuretic administration adversely affects cutaneous vasodilatation42

and sweating,43 and increases core temperature during exercise-heat stress. 

Afferent signals responsible for the thermoregulatory effects of 

dehydration come from several anatomical areas. Osmoreceptor cells 

located in the preoptic/hypothalamic area are activated when intracellular 

volume declines. These cells affect the firing of thermosensitive neurons 

in the thermoregulatory control center.44,45 Cardiac mechanoreceptors or 

pulmonary baroreceptors, or both, also modulate the thermoregulatory 

response to exercise and heat stress because application of lower body 

negative pressure during heat stress augments heat storage independent of 

changes in blood volume or tonicity.46,47 These observations are consistent 

with the neurophysiological findings that osmotic and blood pressure 

stimuli modulate the firing of thermosensitive neurons in the preoptic 

and anterior hypothalamus.44

The consequences of dehydration on the cardiovascular response to 

exercise have also been characterized. During submaximal exercise with 

little thermal strain, dehydration elicits an increase in heart rate and a 

reduction in stroke volume with little or no change in cardiac output 

relative to euhydrated levels.48 During more vigorous exercise, the increase 

in heart rate does not fully compensate for reductions in stroke volume, 

and cardiac output is reduced.49 In warm environments, the detrimental effects of dehydration are exacerbated. Experiments examining the 

effect of graded levels of water loss on the cardiovascular response to 

exercise report graded increases in heart rate1,27,28,30 and stroke volume 

reductions,28,30,48 with increasing water deficit. Cardiac output falls in a 

graded manner with water deficit28,30 because the increase in heart rate 

is not sufficient to offset stroke volume reductions. 

Effects on Physical Performance

Dehydration in excess of 2% normal body mass has negative effects on morale and willingness to work. Comments from investigators 

working with soldiers during World War II illustrate how devastating 

Physiological Consequences of Body Water Imbalance | 13
dehydration can be (Figure 3). Adolph et al1

 reported that, in dehydrated 

soldiers, “walking was mechanical, and a matter of will power” and that 

soldiers “threatened to quit unless given water.” Eichna and colleagues2

found that tasks became much more difficult, and “acclimatized subjects 

who had performed a given task easily, energetically, and cheerfully” 

were “reduced to apathetic, listless, plodding men straining to finish the 

same task.” More recently, Strydom et al50,51 reported that dehydrated 

men became lethargic and morose, and were aggressive and disobedient 

toward their superiors. 

Dehydration in excess of 2% body mass compromises physical performance. Exercise endurance time is shorter during both submaximal 

and maximal exercise, and less work is accomplished per unit of time 

when dehydrated beyond 2% body mass (~1.5 L for a 70-kg soldier) in 

temperate and warmer environments. Detrimental effects of dehydration 

afflict large52–56 and small muscle mass activities.52,53,55,57–59 In contrast, 

dehydration has little effect on muscle strength, at least up to 5% loss 

of body weight.6 Evidence also indicates that somewhat greater dehydration is necessary before endurance performance is affected in cool/

cold climates.60

Dehydration also reduces heat tolerance. In a 1992 study,61 scientists 

had subjects walk to voluntary exhaustion when either euhydrated or dehydrated (8% of total body water). The experiments were designed so that the 

combined environment (Ta = 49°C, relative humidity = 20%) and exercise 

intensity (47% maximal oxygen) would not allow thermal equilibrium; 

thus, heat exhaustion eventually occurred. Dehydration reduced tolerance time from 121 minutes to 55 minutes; but, more importantly, core 

temperatures at exhaustion were approximately 0.4°C lower when 

subjects were dehydrated. These findings suggest that dehydration not 

only impairs exercise performance, but also reduces tolerance to heat 

strain.

Hyperhydration

Because moderate levels of dehydration can be detrimental to soldier 

performance, it is important to consider whether overdrinking or taking 

substances to achieve hyperhydration or greater-than-normal body water 

would be beneficial for temperature regulation and exercise performance. 

Studies that directly expanded blood volume have usually reported decreased cardiovascular strain62–64 during exercise, but disparate results 

on heat dissipation63–65 and exercise-heat performance.64–66 Other studies 

14 | Water Requirements and Soldier Hydration
Figure 3. (Top) Man falls out of formation as other troops keep marching. He has been overcome by heat exhaustion and collapses. (Bottom) Two first-aid men helping the GI who has 

been overcome by heat exhaustion. They have loosened his clothes and are giving him a drink 

of water. [Original caption]

Photographs: Courtesy of the National Archives, photo nos. SC-391634 (top) and SC-391635 

(bottom).

Physiological Consequences of Body Water Imbalance | 15
examining the effect of overdrinking to produce a transient hyperhydration have found generally reduced heart rates during submaximal 

exercise, but disparate results regarding core temperature (reviewed in 

Sawka et al6). Studies observing lower core temperatures were often 

confounded by lower core temperatures before exercise. When this effect 

was controlled,67 hyperhydration had no appreciable effect on thermoregulation during exercise. 

Although many studies have attempted to induce hyperhydration 

by overdrinking water or water-electrolyte solutions, these approaches 

have produced only a transient expansion of body water. Greater fluid 

retention can be achieved with aqueous solutions containing small quantities of glycerol.68–71 Although an early report suggested that glycerol 

hyperhydration enhanced heat dissipation during exercise-heat stress,70

this finding has not been confirmed by subsequent investigations.67,72 In 

addition, this finding has not improved endurance exercise performance73

or exercise-heat tolerance.74

Water Intoxication or Hyponatremia

Drinking too much can make soldiers sick. A relatively rare but clinically important problem for military personnel is the development of 

hyponatremia consequent to overdrinking water during training. From 

1989 to 1998, 10 to 20 US Army soldiers were hospitalized yearly with 

symptomatic hyponatremia,75 and hyponatremia was the cause of at 

least one soldier’s death.76 The majority of cases occurred during warm 

weather training and afflicted soldiers in a basic training environment.77

Symptomatic hyponatremia occurs most often in soldiers and athletes 

who perform prolonged physical activities that produce (or are believed 

will produce) relatively high sweat rates and are sustained longer than 5 

hours.78,79 The majority of cases have occurred during physical activities 

lasting longer than 8 hours. 

Symptoms and Etiology

Symptoms of hyponatremia generally occur at serum sodium concentrations <125 mEq/L and include the following:

•	 mental confusion,

•	 disorientation,

•	 malaise,

•	 weakness,

16 | Water Requirements and Soldier Hydration
•	 nausea,

•	 transient neurological deficits,

•	 seizures, and

•	 coma.

The absence of hyperthermia excludes the diagnosis of heatstroke.80

Symptoms are most severe when sodium is diluted rapidly (ie, over hours 

rather than days). This condition is a consequence of the combined loss 

of sodium in sweat and failure to excrete the large amounts of hypotonic fluid that have been ingested. The net result is a dilution of body 

sodium. Retention of excess water is not from abnormal renal function, 

but rather extrarenal factors that suppress renal water excretion81 and/or 

inappropriate AVP levels.82 For example, Poortmans83 demonstrated that 

physical exercise reduces urine flow rate even in overly hydrated men. 

In the experiment, overdrinking before performing exhaustive exercise 

produced urine flow rates of 12 mL/min. Despite continuing to drink 200 

mL of water every 20 minutes during vigorous exercise, the urine flow 

rate fell to 2 to 3 mL/min. This occurred likely because vigorous exercise reduces the glomerular filtration rate.81 In addition, various factors 

enhancing tubular sodium reabsorption causes isoosmotic reabsorption 

of water to increase. As a consequence of decreased glomerular filtration 

rate and increased water reabsorption, urine volume will decrease. 

Prevention

Episodes of hyponatremia have, in most cases, been associated with 

excessive water intake relative to sweat losses.78,79 Soldiers—especially 

noncommissioned officers, training cadre, and junior officers—need to 

be educated that overdrinking is not beneficial (ie, it does not enhance 

thermoregulation or exercise capacity). The key to prevention is to ensure 

that volunteers drink only in amounts less than or equal to their sweating rate. Dehydration less than 2% body mass does not compromise 

performance; therefore, full fluid replacement is not essential during 

work. Because sodium is the major constituent of sweat (20–60 mEq/L), 

prolonged sweating can produce substantial sodium deficits. To prevent 

sodium depletion, soldiers should have access to regular meals. In work 

environments that produce prolonged, persistent sweating, salting food 

to taste is generally sufficient to prevent salt depletion. However, when 

conditions do not permit eating meals, adding small quantities of salt 

(~0.05% NaCl solution) to the water supply, including salt-containing 

beverages (eg, commercial sports drinks), or using commercially available 

salt tablets are viable options to replace salt losses. 

Physiological Consequences of Body Water Imbalance | 17

Factors Influencing Daily 

Water Requirements

Several factors influence a soldier’s daily water requirements. This section summarizes the effect of environment, physical activity, body size, 

gender, and load carriage on daily water loss. However, these factors 

rarely occur independently, and it is their interaction that determines a 

soldier’s daily water needs. 

Environment

The environment dictates the avenues of heat loss available. In cool 

and temperate environments, a substantial quantity of heat can be dissipated via radiation and convection. During moderate physical activity 

in temperate environments, approximately 50% of the heat produced 

can be dissipated via nonsweating means. In cold environments, almost 

75% of the heat produced can be dissipated by nonsweating means. In 

contrast, in air temperatures 35°C and higher, virtually all heat produced 

must be dissipated by sweat evaporation. The greater demand on sweating in hot environments can substantially increase a soldier’s daily fluid 

requirements.
Physical Activity

A soldier’s daily water requirements increase as a function of the total 

calories expended per day (Figure 4). This occurs because sweat losses are 

dependent on exercise intensity and duration of effort. For example, in 

temperate conditions, moderate-intensity military tasks (~425 watts; 6.1 

kcal/min) will elicit sweating rates of approximately 0.3 L/h, whereas hard 

work (~600 watts, 8.6 kcal/min) will elicit sweating rates approximately 

0.7 L/h. The duration of physical activity also has a significant effect 

on water need. Although moderate-intensity work in temperate conditions might not elicit high rates of sweating (~0.3 L/h) when extended 

over an 8-hour period, a soldier’s daily fluid requirements increase an 

additional 2.4 L/d. 

Body Size and Gender

Because men typically are heavier and sweat more profusely than women 

during physical activity, they will likely have larger daily fluid requirements. This occurs because the caloric cost of ambulatory activity increases as a function of body mass; therefore, larger persons expend more 

energy walking 1 mile than do smaller persons. Thus, men will expend 

more energy during marching, and thus produce more heat, than women. 

In addition, studies show that men have fewer sweat glands per unit area 

compared with women, but they produce more sweat per gland.84 As such, 

men will generally require larger quantities of fluid than women.

For ambulatory activities, rates of sweating are nearly proportional 

to the two-thirds power of body weight. Therefore, a 91-kg man will 

sweat approximately 30% more than his 59-kg companion walking at 

the same speed.1

Load Carriage

The energy cost of marching with an All-Purpose Lightweight Individual 

Carrying Equipment (ALICE) pack and load-bearing equipment (LBE) is 

much greater than marching without it. Soldiers wearing a 31-kg ALICE

pack and LBE expend 29% more energy to march 3.5 mph. Soldiers carrying a 49-kg ALICE pack–LBE ensemble expend 63% more energy to 

20 | Water Requirements and Soldier Hydration
march at the same speeds without the packs.85 In temperate environments, 

the added energy cost of carrying the 31- or 49-kg backpack ensemble 

would increase the expected sweating rate from approximately 0.3 L/h 

without the pack to 0.6 and 1.1 L/h for the two packs, respectively. An 

additional factor affecting sweating requirements is the loss of effective 

surface area for evaporation of sweat, because the pack blocks sweating evaporation from the back. Most likely, this would further increase 

sweating beyond the sweating rate calculated from the added energy 

cost alone. Adolph et al1 reported that sweating rate increased 7 g/h/kg 

of backpack weight for packs weighing 10 to 20 kg. 

Figure 4. Estimates of daily water requirements for range of environmental conditions and 

energy expenditures. 

WBGT: wet-bulb globe temperature

Data source: Sawka MN, Wenger CB, Montain SJ, et al. Heat Stress Control and Heat Casualty Management. Washington, DC: Headquarters, US Department of the Army and Air Force; 

2003: 13. TB MED 507/AFPAM 48-152. Daily Water Requirements, qt/d

Daily Means WBGT, °F

Daily Means WBGT, °C

50 60 70 80 90 100 110

5 1 10 5 20 25 30 35 40

5,500 kcal/day

4,500 kcal/day

3,500 kcal/day

2,500 kcal/day

18

16

14

12

10

8

6

4

2

0

Factors Influencing Daily Water Requirements | 21

Impact of Hot Environments 

on Fluid Requirements

In temperate environments, the daily fluid requirements of soldiers typically range from 2 to 5 L/d, depending on the intensity and duration of 

physical activities. However, as air temperature rises, daily fluid requirements can increase substantially. In extremely hot conditions, water 

requirements for soldiers performing primarily sedentary activities can 

increase from approximately 2 to 3 L/d to 5 L/d. Water requirements 

for soldiers performing heavy work or long hours of moderate work 

(4,200–5,300 kcal/d) can increase from 4 to 6 L/d in temperate environments and from 8 to 10 L/d in extremely hot environments.

Relative humidity can increase water requirements independent of 

ambient temperature. Although relative humidity has little impact on 

sweating and water requirements in temperate environments, high humidity conditions increase water requirements as much as 2-fold in warmer 

environments.2 When the humidity is high, the dramatic increase in sweating requirements underscores the tremendous water supply requirements 

posed when troops are deployed in tropical and desert climates.

Individuals exposed to hot environments generally drink insufficient 

fluids during physical activity to offset water lost from sweating.2,86 Heat 

acclimatization shortens the time delay between when sweating begins 

and when drinking is initiated.86 In addition, heat-acclimatized individuals 

drink more frequently and more closely match fluid intake with sweating 
rate; this results in less voluntary dehydration.2,86 During light exercise, 

heat-acclimatized soldiers can and often will drink sufficient quantities to 

sustain hydration.1 However, during moderate-to-heavy intensity exercise, 

drinking ad libitum generally replaces 50% to 70% of sweat losses,1,2,87–89

and deficits need to be addressed during rest breaks.

Clothing can have a significant effect on daily water requirements. 

Typically, clothing adds insulation and hinders vapor permeability, thus 

increasing the sweating rate (compared with wearing shorts and a T-shirt). 

The addition of backpacks and body armor increases the energy cost 

of locomotion and reduces the surface area available for heat transfer, 

thereby further increasing dependence on sweating. However, there are 

conditions in which clothing can actually aid in reducing water requirements. For example, in hot, arid environments, fully clothed soldiers 

wearing light, breathable clothing actually sweat less than minimally 

clothed men.1 During desert walks in the full sun, light, breathable fabric 

that reflects solar radiation can reduce sweating as much as 20%.1,90

Effect of Hot Environments on Thirst and Voluntary Dehydration

Thermal stresses associated with hot environments are a positive stimulus for thirst and voluntary intake.91 It is not uncommon, however, for 

persons working in the desert to accrue water deficits. As discussed 

previously, the objective of drinking is to prevent water deficits from 

exceeding approximately 2% of normal body mass (eg, ~1.5 L for a 70-

kg soldier). This level of water deficit is associated with the onset of reduced performance. Any imbalance between sweating rate and voluntary 

intake will likely be more prevalent and of greater magnitude in persons 

unacclimatized to working in the heat. Greenleaf et al92 revealed that 

volunteers matched water intake to sweating rate during repeated bouts 

of exercise-heat exposure. They did this by drinking earlier, and the volunteers drank more frequently during the 2-hour exercise-heat exposure 

period. The average volume consumed per drink did not change. 

Effect of Hot Environments on Water Delivery

In tropical regions, water sources are often abundant (eg, rivers, streams, 

lakes, ponds, wells, and local water systems). However, because of dense 

vegetation, poor ground transportation (via truck) can impair water 

24 | Water Requirements and Soldier Hydration
distribution and increase reliance on aerial resupply. In desert regions, 

water sources are limited and widely dispersed. Although water needs 

are greatest in these hot environments, surface fresh water is almost nonexistent, and available subsurface water can vary from region to region. 

Lack of available water in desert environments increases water storage 

and distribution requirements. 

Hot Weather Water Requirements

In hot, climatic extremes, daily water requirements for soldiers can range 

from 5 to 12 L/d. The Water Consumption Planning Factors study report 

(also known as the Potable Water Planning Guide), published by the 

US Army Combined Arms Support Command,93 recommends that each 

soldier be provided with 11.4 L of water per day (ie, intake for each 

soldier should be no more than 11.4 L of water per day93a). Therefore, 

in an established battlefield situation in which water supply is achieved, 

sufficient water should be available to sustain the water balance of each 

individual soldier.

Impact of Hot Environments on Fluid Requirements | 25

Effect of Cold Weather 

Operations on Water 

Requirements

A number of factors can increase fluid requirements during cold weather 

operations. One factor is the development of cold-induced diuresis. This 

phenomenon, first described more than 200 years ago,94 occurs when 

people become chilled during either cold water or cold air exposure. It 

is an osmotic diuresis and can increase urine water losses 2-fold above 

basal conditions.95,96 Although the precise mechanism is not known, 

generally it occurs as a consequence of increased central vascular pressure caused by peripheral vasoconstriction. Fortunately, diuresis seems 

to be self-limiting, because the rate of urine production declines with the 

development of dehydration.

A second factor is increased respiratory water loss from breathing 

cold, dry air. The magnitude of water loss is dependent on both the 

ventilatory volume and water vapor in ambient air.97,98 Because cold 

environments increase the water vapor pressure gradient between the 

lungs and the atmosphere, more water is lost through respiration in cold 

environments than in warm environments. The effect of cold temperatures 

on respiratory water loss, however, does not greatly increase daily fluid 

requirements. Soldiers performing 12 hours of easy-to-moderate exercise, 

followed by 4 hours of hard exercise in very cold weather, would accrue 
only a 0.3 L greater daily fluid requirement than if they performed the 

same activities in a more temperate environment.

A third factor is that wearing bulky, cold weather clothing could 

contribute to water loss. Military physical training activities can generate 

substantial metabolic heat that must be dissipated to prevent excessive 

elevations in body temperature. If clothing insulation is too thick for the 

activity level, then substantial sweating can occur. For example, during rest 

and light work, persons wearing protective clothing from the US Army 

Extended Cold Weather Clothing System are thermally comfortable and 

are able to dissipate much of their body heat through nonsweating means 

(Exhibit 1). However, during more vigorous activities, sweating rates can 

achieve levels in excess of 1 L/h. To minimize these unnecessary sweat losses, 

soldiers should be trained to use layering and open ventilation options in their 

clothing level to minimize sweat production during cold weather training. 

A fourth factor that can affect water requirements is the added 

metabolic cost of movement in cold terrain. The addition of bulky clothing reduces mechanical efficiency and can increase the energy cost of a 

specific activity an additional 10% to 20%.99,100 The metabolic cost of 

movement in soft snow can be 2.5 to 4.1 times greater than performing 

the same activity on a blacktop surface. 

28 | Water Requirements and Soldier Hydration

Exhibit 1. Generation III Extended Cold Weather Clothing System

The Generation III Extended Cold Weather Clothing System consists of protective 

clothing developed by the US Army Natick Research, Development, and Engineering 

Center (Natick, Mass) to be functional in cold weather climates. This adjustable, 

layered insulation system is comprised of the following 12 components:

1. Lightweight undershirt 

2. Lightweight underwear

3. Midweight shirt

4. Midweight underwear

5. Fleece cold weather jacket

6. Wind cold weather jacket

7. Soft shell jacket 

8. Soft shell trousers

9. Extreme cold/wet weather jacket 

10. Extreme cold/wet weather trousers

11. Extreme cold weather parka 

12. Extreme cold weather trousers
Effect of Cold Weather on Thirst and Voluntary Dehydration

Investigations studying the effect of air temperatures on thirst and drinking behavior consistently report depressed voluntary drinking during 

cold weather activities.96,101,102 Experiments in rats demonstrate that this 

species will voluntarily dehydrate during cold exposure, but will manifest 

striking thirst when returned to a warm room.101 Similar observations 

have been made in humans.102 One strategy that has improved voluntary 

intake is the provision of warm drinks rather than cool drinks during 

cold exposure.103

An additional cold weather problem occurs when individuals purposefully refrain from drinking to avoid having to urinate in the cold. This 

behavior typically occurs late in the day when soldiers try not to leave 

a warm tent or sleeping bag to urinate. If the mission scenario requires 

soldiers to rehydrate late in the day, their behavioral decision to refrain 

from drinking adequate quantities of fluid can lead to persistent dehydration during the next day’s activities. One solution is to ensure that soldiers 

drink regularly during cold weather activities to avoid development of 

large water deficits. Snack breaks can also stimulate additional fluid 

intake. This particular strategy will help soldiers to avoid rehydration 

with large quantities of water before bedtime and reduce the likelihood 

of their having to get up and urinate during set sleep periods.

Effect of Cold Weather on Water Delivery

The most important factor regarding fluid intake in the cold is the logistical constraint of potable water delivery. In arctic regions, melting snow 

and ice will provide only enough water for emergency use by individuals and small units. Because of the extensive fuel requirements to melt 

snow and ice, this is an impractical water source for larger military units. 

However, the primary water source is unfrozen water underlying frozen 

rivers, lakes, and wells constructed by civilians and military personnel. 

The limited availability of water sources greatly increases distribution 

requirements, and storage and distribution elements must be augmented 

with specialized equipment to prevent or retard freezing of packaged 

water. If small units must rely on melting snow and ice for their water 

supply, the provision of adequate fuel is essential. In addition, the water 

must be adequately treated to ensure that it is potable. 

Effect of Cold Weather Operations on Water Requirements | 29
Daily Water Requirements

Studies examining the voluntary intake of water by soldiers during 

field maneuvers report daily fluid intakes ranging from 2 to 4 L/d.104–106

However, when energy expenditures are high, human water needs can 

range from 4 to 6 L/d. Accordingly, the Water Consumption Planning 

Factors study report recommends the daily provision of 7.6 L of drinking water per soldier for tactical theater and field training exercises in 

arctic environments.93

30 | Water Requirements and Soldier Hydration
Effect of High-Altitude 

Operations on Water 

Requirements

Military operations in mountainous areas can affect short- and long-term 

water balance. For example, acute exposure to moderately high altitudes 

can often produce a self-limited reduction of body water resulting from 

reduced fluid intake and increased urine volume. Other factors can also 

affect voluntary water intake and daily water requirements, including 

the following: (a) symptoms of anorexia and hypodipsia; (b) increased 

respiratory water loss from breathing cold, dry air; and (c) cutaneous 

evaporative losses with physical activity.

Effect of High-Altitude Operations on Water Balance

Unacclimated soldiers exposed to moderate, high-altitude hypoxia 

(3,500–5,000 m) typically become somewhat dehydrated (losing approximately 1–2 L of water from the body, which equals 3% to 5% 

of total body water) during the first 1 to 3 days of exposure.107,108 This 

water deficit results in decreased extracellular volume, decreased plasma 

volumes, and some loss of intracellular water. It occurs as a consequence 

of arterial chemoreceptor activation, which produces a self-limited 
natriuresis and diuresis, as well as suppressed thirst and reduced voluntary water intake.107 Attempting to overdrink to prevent the water deficit 

is not recommended. Diuresis and natriuresis are considered beneficial 

adaptations because (a) increased plasma hemoglobin concentration 

increases arterial oxygen content and improves tissue oxygen delivery; 

and (b) people who develop natriuretic and diuretic responses have less 

severe symptoms and less incidence of acute mountain sickness.107

The relative dehydrated state produced by moderate-to-high altitude 

exposure can persist for several weeks. Studies examining water balance 

in humans exposed to elevations of 3,500 to 5,334 m report persistent 

reductions in total body water for up to 14 days.108,109 Over time, however, blood volume increases as the volume of red blood cells increases at 

altitude, with plasma volume remaining reduced or returning to sea-level 

values.107 Observation of increased plasma volume over time suggests 

some restoration of total body water with chronic hypoxic exposure. 

Effect of Hypoxia on Thirst and Voluntary Water Intake

It is not uncommon for voluntary water intake to decrease approximately 

1 L/d during the first 1 to 2 days of moderate, high-altitude exposure.108

Suppressed voluntary water intake does not persist because, generally, 

daily water intake returns to predeployment levels within the first week 

of altitude exposure.

Other Factors Associated With High-Altitude Operations 

That Affect Water Requirements

There is a general belief that increased respiratory evaporative water losses 

accompanying mountainous operations significantly affect daily water 

requirements.110 This may be true at high-altitude extremes; for example, 

Pugh111 reported that climbers at 5,500 m had respiratory evaporative 

water losses of 2.9 g of water per 100 L (body temperature and pressure, 

saturated with water) of ventilation. However, at more modest elevations, the combined effects of colder air temperatures, lower humidity, 

and the higher ventilation rates relative to energy expenditure do not add 

appreciatively to daily water requirements. Over a broad range of energy 

expenditures (3,500–7,000 kcal/d), the estimated net respiratory water 

32 | Water Requirements and Soldier Hydration
losses at 2,200 to 3,100 m are very similar to the estimated respiratory 

water losses at sea level.110 Deployment to 4,300-m altitude will increase 

daily water requirements approximately 200 to 300 mL/d to compensate 

for added respiratory water loss.

Cutaneous sweat losses in soldiers working at low, moderate, or high 

altitudes depend on clothing insulation, ambient weather conditions, and 

absolute exercise intensity and duration. As in cold weather operations, 

clothing insulation at high altitudes must be matched to heat production 

to keep soldiers warm while preventing excessive heat storage and sweat 

losses. However, vigorous physical activity can produce relatively high 

rates of sweating and increase daily water requirements. 

Effect of High-Altitude Operations on Water Delivery

Water supply in mountainous regions can come from lakes, streams, and 

local water supplies. Because mountain operations are often associated 

with cold weather, the same restraints can exist as those described for 

cold weather operations. An added factor is that high altitude increases 

the time required to purify water by boiling it. Therefore, if military 

units rely on melted snow to acquire water, this can have a significant 

effect on the time required to obtain potable water, as well as on the fuel 

requirements of small units. 

Water Requirements

Military groups operating in high-altitude environments have generally 

sustained hydration by drinking 4 to 5 L/d less when engaged in heavy 

work than when performing primarily sedentary activities.104,112 The 

Water Consumption Planning Factors study report recommends the 

provision of 5.6 to 7.6 L/d of drinking water per soldier.93 Therefore, if 

supply lines are maintained as planned, the water supply should be sufficient to sustain hydration during military maneuvers.

Effect of High-Altitude Operations on Water Requirements | 33

Effect of Water Immersion on 

Water Requirements

Water immersion increases central venous pressure and produces a 

transient diuresis and natriuresis. The magnitude is dependent on water 

depth, temperature of the water, and the initial state of hydration.113

The quantity of water lost, however, is not sufficient to greatly increase 

daily water requirements. However, because immersion blunts the thirst 

response,16–18 it may also affect voluntary fluid intake and the ability to 

sustain hydration during prolonged operations in water environments. 

Whether prolonged partial immersion would sufficiently affect drinking 

behavior and the ability to sustain hydration have not been studied.

Barriers to Rehydration

Water Availability

Lack of potable water is probably the greatest barrier to sustaining 

hydration. If water is not available, rehydration cannot take place and 

soldiers will suffer. Just having potable water, however, is not sufficient. 

Water must be made available to the troops. Soldiers at work are more 

likely to refrain from drinking water if they have to travel some distance to obtain it. This is especially true in desert and arctic conditions, 

because both weather extremes diminish the desire to do additional 

work.

The collapsible bladder with drink-tube hydration systems provides 

a practical example of how availability increases voluntary fluid replacement. With these systems, the soldier can drink on the go in a near 

hands-free manner (Figure 5). When carrying a traditional 0.9-L rigid 

plastic canteen, the soldier cannot easily access the canteen on the go—

particularly when carrying an approach load—and this increases the need 

to take rest breaks to drink. Although historically the collapsible-bladder 

hydration systems suffered from poor durability and were difficult to 

keep clean, advances in the manufacturing process have lessened these 

deficiencies.
Figure 5. Hands-free drinking. Competitor Spc. Heyz Seeker gets a drink from a hydration 

device during the Urban Warfare Orienteering Course event of the Department of the Army 

Best Warrior competition at Fort Lee, Va.

Photograph: Courtesy of T. Anthony Bell, Fort Lee Public Affairs Office.

38 | Water Requirements and Soldier Hydration
Palatability of Water

If water has an unpleasant taste or odor, voluntary intake will be 

reduced. Efforts should be made to make water as palatable as possible. Factors that affect palatability include odor, taste, temperature, 

color, and turbidity of the fluid. The odors and tastes found in water 

are most commonly caused by algae, decomposed organic matter, and 

dissolved gases. Chlorination and iodination both add taste to water 

and can decrease palatability. Water color is derived from colored 

substances (eg, vegetative matter that is dissolved from roots, leaves, 

and humus) or from inorganic compounds (eg, iron and manganese 

salts). Turbidity is caused by suspended clay, silt, organic and inorganic 

matter, and plankton and other microorganisms. Warm water tastes 

flat and augments any odors and flavors in it. Water treatment and 

disinfection remove much of the particulate matter and reduce water 

color. Cooling water further suppresses odors and tastes, and makes 

water more palatable.

During the Persian Gulf War (1990–1991), the US military introduced 

bottled water to combat nonpalatable water. Although bottled water is 

expensive to buy and deliver to the troops, particularly in warm weather 

conditions, it avoids the taste issues associated with mass-produced water 

containing residual chlorine. Flavor additives can mask the unpalatable 

taste of chlorine, but they should be added to the water just prior to 

consumption because these additives can completely destroy the effectiveness of the chlorine. 

Gastric Emptying

A potential barrier to sustaining hydration is gastric emptying and intestinal absorption. Evidence to date, suggests that, under most conditions 

faced by the fighting soldier, neither gastric emptying nor intestinal 

absorption should hinder the ability to sustain hydration during deployment. The stomach and small intestine function extremely well under 

conditions of exercise and environmental stress.114 Furthermore, neither 

gastric emptying nor intestinal absorption are delayed or impaired 

by exercise intensities ranging up to 70% maximum oxygen uptake, 

even during prolonged, sustained physical activity.114,115 It should be 

recognized, however, that there is a great deal of individual variability 

Barriers to Rehydration | 39
in rates of gastric emptying and intestinal absorption. Therefore, some 

soldiers may be less tolerant of drinking large volumes of fluid, and there 

are other soldiers who are so tolerant that they can actually drink fast 

enough to make themselves sick (see section on Water Intoxication or 

Hyponatremia). Scheduled rest breaks and adequate mealtimes should 

be included to provide time to rehydrate.

Lost Solutes

Complete restoration of a fluid volume deficit cannot be achieved without electrolyte replacement (primarily sodium) in food or beverages.89

Electrolytes, primarily sodium chloride and, to a lesser extent, potassium, are lost through sweat during physical activity. The concentration 

of sodium in sweat averages approximately 30 to 50 mEq/L, but can 

vary, depending on an individual’s state of heat acclimatization, diet, 

and hydration.89 Furthermore, the amount and composition can also 

vary, contingent on exercise intensity and environmental conditions. 

Potassium concentration in sweat is approximately 4 to 5 mEq/L. In 

temperate conditions, sodium chloride concentration in sweat can range 

from 2 to 13 g/d, averaging approximately 6 g/d for the active soldier. 

Potassium losses will range from 0.1 to 0.9 g/d, averaging approximately 

0.5 g/d for the active soldier. In hot environments, sodium chloride and 

potassium losses can exceed 17 and 1.9 g/d, respectively. Considering 

that three Meals, Ready-to-Eat contain 12.6 g of sodium chloride and 

2.7 g of potassium, and that typically only two Meals, Ready-to-Eat are 

provided during field operations, there are many situations in which 

supplemental salt may be required to offset the sodium chloride lost in 

sweat. Typically, this deficit can be managed by salting food to taste. 

Figure 6 provides estimates of expected daily sodium losses consequent 

to working in different environments. 

40 | Water Requirements and Soldier Hydration
Figure 6. Estimates of daily sodium requirements for varied weather conditions and energy 

expenditures. 

WBGT: wet-bulb globe temperature

Data source: Sawka MN, Wenger CB, Montain SJ, et al. Heat Stress Control and Heat Casualty Management. Washington, DC: Headquarters, US Department of the Army and Air Force; 

2003: 13. TB MED 507/AFPAM 48-152. Daily Sodium Requirements, grams/day

Daily Means WBGT, °F

Daily Means WBGT, °C

50 60 70 80 90 100 110

5 1 10 5 20 25 30 35 40

5,500 kcal/day

4,500 kcal/day

3,500 kcal/day

2,500 kcal/day

12

10

8

6

4

2

0

Barriers to Rehydration | 41

Strategies to Sustain Hydration 

in Harsh Environments

Do Not Skip Meals

Studies on soldiers working in the desert1 clearly demonstrate that the 

majority of daily fluid intake occurs at mealtime. As illustrated in Figure 

7, soldiers working in the heat voluntarily dehydrated themselves during 

work periods, but made up the water deficit during mealtime. This same 

pattern of drinking has since been confirmed by others.10,11 This observation is important because meals are a valuable time for rehydration 

and maintenance of fluid balance (Figures 7 and 8). They also provide 

a source of solutes to offset the electrolytes lost during perspiration. As 

discussed previously, if missing solutes are not replaced, complete rehydration cannot be achieved.

Make Time to Drink

Soldiers will often underdrink voluntarily relative to the perspiration they 

produce during work. Rest breaks provide an opportunity for their fluid 

intake to catch up with their deficit. This is especially relevant when work 

and weather conditions produce sustained, profuse sweating. To achieve 
high rates of rehydration, it is advantageous to maintain a high gastric 

volume, because the speed in which a bolus of fluid leaves the stomach 

falls exponentially as the stomach volume decreases. A relatively full 

stomach and high rates of gastric emptying can be achieved by frequently 

drinking small amounts of fluid during work.114

Maximize Palatability 

How a drink tastes is a major determinant of the amount of drink consumed. If the provided water tastes disagreeable, soldiers will drink less 

and will endure greater dehydration.1 Many factors play a role in palatability, including cultural background, previous experience with the drink, 

and time of day. Even per individual, palatability is not constant. The 

decrease in pleasantness can be specific to a particular drink. Therefore, 

Figure 7. Changes in body weight of one researcher during a day in the desert. The loss was 

more rapid in warmer hours and fastest during an afternoon walk, reflecting change in evaporative water loss with conditions of activity and exposure. Most of the fluid was taken with 

meals.

Data source: E. F. Adolph and associates. Physiology of Man in the Desert. New York, NY: 

Interscience; 1947: 39–40. Chap 3.

44 | Water Requirements and Soldier Hydration

Urine

Urine

Walk

Food

Food

Thirst

AcuteThirst

Drin

Drink

k

Drink

Drink

65

64

63

62

6 8 10 12 14 16 18 20 22

Body Weight, Kg

Hour of Day
Figure 8. US Army Air Force in Tunisia consuming their rations from defensive positions 

(March 7, 1943). The largest daily fluid intake is generally associated with consumption of 

meals.

Photograph: Courtesy of the National Archives, photo no. 208-AA-2U-7.

if the goal is to increase voluntary fluid intake, then switching to a different drink can help to stimulate fluid intake (Figure 9). Rolls and Rolls116

found that persons offered a choice of three different flavors drank 22% 

more than when no flavored drinks were provided, and they consumed 

more of the flavored beverage than the nonflavored beverage. 

Adding sweeteners to drinks leads to increased fluid intake.117 Adding too much sugar can decrease acceptability during physical activity. 

Strategies to Sustain Hydration in Harsh Environments | 45
Trained endurance athletes reported more symptoms of nausea and 

fullness when drinks contained 12% glucose compared with drinks 

containing 6% glucose or water.118

Carbonated beverages can be pleasurable with meals or when sedentary, but they make a poor fluid replacement beverage during activity. 

Sohar and colleagues119 found that soldiers drank less of the carbonated 

beverages than either water or flavored beverages. Soldiers who drank 

the carbonated beverages complained that carbonation made it difficult 

to drink large quantities of fluid. 

Temperature of the preferred drink depends on a number of factors, 

including culture and learning,120 climatic conditions, and the physiological state of the individual.121 In hot and temperate environments, cool 

Figure 9. Nutrient on-the-move delivery system attached to standard Army hydration devices. 

This new system, developed by researchers at the US Army Research Institute of Environmental Medicine (USARIEM), provides the opportunity for soldiers to flavor their water, 

combined with various drink mixes, and add nutrients (eg, carbohydrate mixes).

Photograph: Courtesy of the US Patent and Trademark Office (US Patent No. 7,533,786 B2), 

Alexandria, Va.

46 | Water Requirements and Soldier Hydration
drinks increase voluntary intake.89,122 Drink temperatures of 15°C to 

20°C seem optimal when persons need to drink large volumes of fluid.89

In cold environments, voluntary intake is enhanced when drinks are 

warmed.103 In both environmental extremes, the dehydration-induced 

drive to drink will overcome any initial resistance to drink warm or 

very cold water.123

Minimize Body Water Losses 

There are several behavioral actions that soldiers can take to reduce 

sweat losses and maximize fluid retention. For example, in hot, arid environments, sweating rates can be reduced by working at night. During 

daylight hours, sweating rates can be reduced by covering the skin with 

light, vapor-permeable clothing. In cold environments, matching clothing insulation and/or clothing ventilation to the rate of heat production 

reduces the need for sweating to dissipate heat. Drinking small quantities 

of fluid frequently results in less urine production than drinking large 

quantities of fluid less frequently. Similarly, adding small quantities of 

salt to fluid-replacement beverages improves fluid retention compared 

with drinking water only, particularly when many hours of labor separate meals. There is no evidence that heat acclimation promotes water 

conservation; in fact, acclimation leads to earlier onset and more profuse 

sweating. As such, behavioral strategies are the best solution for minimizing water turnover.

Provide Education

Troop education on the importance of hydration is essential to deployed 

soldiers. Military planners and logistics personnel must ensure that 

adequate quantities of potable water are available. Small unit leaders 

should maximize delivery and availability of potable water to the soldiers. 

Therefore, these soldiers must understand the importance of drinking, 

making time to drink, and drinking the right amount.

As previously discussed, dehydration in excess of 2% body weight can 

have detrimental effects on troop performance. Thus, it is important that 

soldiers make time to drink during physical activity and drink sufficient 

quantities to minimize the likelihood of dehydration. It is not essential 

Strategies to Sustain Hydration in Harsh Environments | 47
Table 3. Fluid Replacement and Work/Rest Guidelines for Warm Weather Training Conditions*

Easy Work Moderate Work Hard Work

Heat WBGT†,‡ Work/Rest§,¥ Water¶,** Work/Rest Water Work/Rest Water

Category Index (°F) (min) Intake (qt/h) (min) Intake (qt/h) (min) Intake (qt/h)

1 78–81.9 NL†† 1⁄2 NL 3⁄4 40/20 3⁄4

2 82–84.9 NL 1⁄2 50/10 3⁄4 30/30 1

(green)

3 85–87.9 NL 3⁄4 40/20 3⁄4 30/30 1

(yellow)

4 88–89.9 NL 3⁄4 30/30 3⁄4 20/40 1

(red)

5 >90 50/20 1 20/40 1 10/50 1

(black)

Easy Work Moderate Work Hard Work

•	 Weapon maintenance	 •	 Walking in loose sand 	 •	 Walking on hard

•	 Walking on hard surface	 	 at 2.5 mph, no load	 	 surface at 3.5 mph,

	 at 2.5 mph, <30-lb load	 •	 Walking on hard surface 	 	 ≥40-lb load

•	 Manual of arms	 	 at 3.5 mph, <40-lb load	 •	 Walking in loose sand at

•	 Marksmanship training	 •	 Calisthenics	 	 with load

•	 Drill and ceremony	 •	 Patrolling	 •	 Field assaults

	 	 •	 Individual movement

 techniques (ie, low 

 crawl, high crawl)

	 	 •	 Defensive position

 construction

NL: no limit; MOPP-4: mission-oriented protective posture 4; WBGT: wet-bulb globe temperature

*Applies to average size and heat-acclimated soldier wearing battle dress uniform in hot weather.

†If wearing body armor, add 5°F to WBGT index in humid environments.

‡If wearing nuclear, biological, and chemical clothing (MOPP 4), add 10°F to WBGT index.

§Work/rest times and fluid replacement volumes will sustain performance and hydration for at least 4 hours of work in the specified 

heat category. Fluid needs can vary based on individual differences (±1⁄4 qt/h) and exposure to full sun or full shade (±1⁄4 qt/h).

¥Rest means minimal physical activity (sitting or standing), accomplished in shade, if possible.

¶CAUTION: Hourly fluid intake should not exceed 11⁄2 quarts.

**Daily fluid intake should not exceed 12 quarts.

††No limit equals no limit to work time per hour (up to four continuous hours).

Adapted from: Sawka MN, Wenger CB, Montain SJ, et al. Heat Stress Control and Heat Casualty Management. Washington, 

DC: Headquarters, US Department of the Army and Air Force; 2003: 13. TB MED 507/AFPAM 48-152.

48 | Water Requirements and Soldier Hydration
Strategies to Sustain Hydration in Harsh Environments | 49

that soldiers fully offset sweat losses during physical activity. They can 

make up for any water deficit during rest breaks and at mealtime. The 

best method to prevent excessive dehydration during physical activity is 

to begin drinking early and continue drinking small amounts of water 

every 15 to 20 minutes of exercise. 

Table 3 provides current Army hot weather guidance for fluid replacement in troops in training environments where work:rest ratios are 

practical for preventing heat injury.124 The amounts in each cell reflect the 

expected hourly fluid intake to fully replace all sweat lost during activity. As such, the numbers in each cell are the average predicted sweating 

rates for each activity level and environmental condition. Because there 

is considerable variability between individuals, the numbers in each cell 

must be considered as approximates. As noted in Table 3 footnotes, 

actual sweating rates (and 100% fluid replacement) may vary between 

individuals by ±1⁄4 quart per hour. Similarly, the work:rest ratios are a 

guide to approximately how much time should be dedicated each hour 

to work versus rest (assuming work will be sustained for several hours) 

to prevent body core temperatures from reaching levels associated with 

premature fatigue and heat exhaustion. Table 3 also specifies an upper 

limit for hourly (1.5 quarts) and daily (12 quarts) water intake to provide a safeguard against overdrinking and developing water intoxication 

(hyponatremia) during training.

Summary

It has long been known that individuals can survive much longer without 

food than they can without water. Only in recent decades has science 

been able to quantify the factors that influence the body’s ability to 

maintain optimal water levels and the physiological consequences of 

water imbalance. This chapter has presented information to assist the 

caregiver—whether physician, medic, unit leader, or fellow soldier—in 

understanding the influence of environment, physical activity, body size 

and gender, and load carriage in maintaining water balance. In the notso-distant past, a lack of understanding of these complex systems led 

to the deaths of thousands of military personnel in various campaigns. 

Until science (through the development of biometrics) can accurately 

and instantly evaluate the water balance of every soldier, it remains the 

responsibility of each individual—as well as support personnel—to be 

vigilant in recognizing the potential for and the development of water imbalance situations and to effectively institute rehabilitative measures.

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