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Water Requirements and Soldier Hydration

Chapter 7

WATER REQUIREMENTS AND SOLDIER 

HYDRATION

SCOTT J. MONTAIN, PhD*; and MATTHEW ELY, MS†

INTRODUCTION

WATER-ELECTROLYTE BALANCE

THIRST AND SATIATION

PHYSIOLOGICAL CONSEQUENCES OF BODY WATER IMBALANCE 

FACTORS INFLUENCING DAILY WATER REQUIREMENTS 

IMPACT OF HOT ENVIRONMENTS ON FLUID REQUIREMENTS 

EFFECT OF COLD WEATHER OPERATIONS ON WATER REQUIREMENTS 

EFFECT OF HIGH-ALTITUDE OPERATIONS ON WATER REQUIREMENTS 

EFFECT OF WATER IMMERSION ON WATER REQUIREMENTS 

BARRIERS TO REHYDRATION

STRATEGIES TO SUSTAIN HYDRATION IN HARSH ENVIRONMENTS 

SUMMARY

*Research Physiologist, Military Nutrition Division, US Army Research Institute of Environmental Medicine, Building 42, Kansas Street, Natick, 

Massachusetts 01760-5007

†Researcher, US Army Research Institute of Environmental Medicine, Building 42, Kansas Street, Natick, Massachusetts 01760-5007
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Military Quantitative Physiology: Problems and Concepts in Military Operational Medicine

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 7-1). 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 7-2). It is the responsibility of medical 

personnel to educate unit leaders about the importance 

of hydration and the necessary measures to optimize 

Fig. 7-1. EF 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.

fluid intake and retention. This chapter discusses the 

effect of harsh environments on fluid requirements 

and presents methods to sustain water balance during 

deployment to environmental extremes. 

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, between-individual 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 7-13 illustrates the sources of water 

losses and production, 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 

7-2) 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 forces12; therefore, maintenance of 

total body water is dependent on total solute and distri-
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Water Requirements and Soldier Hydration

Fig. 7-2. World War II soldier using a 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: Reproduced from the National Archives and 

Records Administration, Washington, DC, photo no. 208-AA4NN-24.

TABLE 7-1

ESTIMATES OF MINIMAL DAILY WATER 

LOSSES AND PRODUCTION* 

 Production

Source Loss (mL/d) (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 — +250 to +350

production

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 with permission from: Food and Nutrition Board, Institute 

of Medicine. Dietary Reference Intakes for Water, Potassium, Sodium, 

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

2005.

bution 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.

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.

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 
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Military Quantitative Physiology: Problems and Concepts in Military Operational Medicine

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 

TABLE 7-2 

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 water from human milk 0.0 0.7 0.7 0.0 0.7 0.7

7–12 mos Average consumption of water from human milk 0.2 0.6 0.8 0.2 0.6 0.8

and complementary foods

1 yr Median total water intake from NHANES III 0.4 0.9 1.3 0.4 0.9 1.3

4–8 yrs Median total water intake from NHANES III 0.5 1.2 1.7 0.5 1.2 1.7

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

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

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

Pregnancy Same as median intake for nonpregnant women 0.7 2.3 3.0

14–50 yrs from NHANES III

Lactation Same as median intake for nonlactating women 0.7 3.1 3.8

14–50 yrs 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 with permission 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.
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Water Requirements and Soldier Hydration

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

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 and sensitivity alterations increased as a function of body water loss, which 

is in agreement with the observation that temperature 
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Military Quantitative Physiology: Problems and Concepts in Military Operational Medicine

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 exercise-heat 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 dehydration can be (Figure 7-3). 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. 

Fig. 7-3. (Left) Man falls out of formation as other troops keep marching. He has been overcome by heat exhaustion and 

collapses. (Right) 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: Reproduced from the National Archives and Records Administration, Washington, DC, photo nos. SC-391634 

(left) and SC-391635 (right).
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Water Requirements and Soldier Hydration

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 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 waterelectrolyte 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,

	 •	 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 
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Military Quantitative Physiology: Problems and Concepts in Military Operational Medicine

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 saltcontaining beverages (eg, commercial sports drinks), 

or using commercially available salt tablets are viable 

options to replace salt losses. 

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 

7-4). 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
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Water Requirements and Soldier Hydration

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 march 

at the same speeds without the packs.85 In temperate 

environments, the added energy cost of carrying the 

Fig. 7-4. 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 Departments 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

10 15 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

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 

twofold 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

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. 
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Military Quantitative Physiology: Problems and Concepts in Military Operational Medicine

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 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. 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.

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 selflimiting, 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 7-1). 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 
191

Water Requirements and Soldier Hydration

cost of movement in soft snow can be 2.5 to 4.1 times 

greater than performing the same activity on a blacktop surface. 

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. 

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

EXHIBIT 7-1 

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 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 
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Military Quantitative Physiology: Problems and Concepts in Military Operational Medicine

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, highaltitude 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. 

Effects 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 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 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.
193

Water Requirements and Soldier Hydration

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 has not been studied.

Fig. 7-5. Hands-free drinking. Competitor Specialist 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.

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 7-5). 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.

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.
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Military Quantitative Physiology: Problems and Concepts in Military Operational Medicine

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 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 7-6 provides estimates of expected 

daily sodium losses consequent to working in different 

environments. 

Fig. 7-6. 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 Departments 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

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 7-7, 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 
195

Water Requirements and Soldier Hydration

rehydration and maintenance of fluid balance (Figures 

7-8 and 7-9). 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 

Fig. 7-7. 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: EF Adolph and associates. Physiology of Man in 

the Desert. New York, NY: Interscience; 1947: 39–40. Chap 3. Urine Urine

Walk

Food

Food

Thirst

AcuteThirst

Drink

Drink

Drink

Drink

65

64

63

62

6 8 10 12 14 16 18 20 22

Body Weight, Kg

Hour of Day

Fig. 7-8. US Army Air Force personnel in Tunisia consuming their rations from defensive positions (March 7, 1943). 

The largest daily fluid intake is generally associated with 

consumption of meals.

Photograph: Reproduced from the National Archives 

and Records Administration, Washington, DC, photo no. 

208-AA-2U-7.

Fig. 7-9. 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, 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.
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Military Quantitative Physiology: Problems and Concepts in Military Operational Medicine

decrease in pleasantness can be specific to a particular drink. Therefore, if the goal is to increase voluntary fluid intake, then switching to a different drink 

can help to stimulate fluid intake. 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. 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 drinks increase voluntary intake.89,122 Drink temperatures of 15°C to 20°C seem optimal when persons 

TABLE 7-3

FLUID REPLACEMENT AND WORK/REST GUIDELINES FOR WARM WEATHER 

TRAINING CONDITIONS*

Easy Work Moderate Work Hard Work

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

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

1 78–81.9 NL†† 1/2 NL 3/4 40/20 3/4

2 (green) 82–84.9 NL 1/2 50/10 3/4 30/30 1

3 (yellow) 85–87.9 NL 3/4 40/20 3/4 30/30 1

4 (red) 88–89.9 NL 3/4 30/30 3/4 20/40 1

5 (black) >90 50/20 min 1 20/40 1 10/50 1

Easy Work Moderate Work Hard Work

	 •	 Weapon maintenance

	 •	 Walking on hard surface

at 2.5 mph, <30-lb load

	 •	 Manual of arms

	 •	 Marksmanship training

	 •	 Drill and ceremony

	 •	 Walking in loose sand

at 2.5 mph, no load

	 •	 Walking on hard surface

at 3.5 mph, <40-lb load

	 •	 Calisthenics

	 •	 Patrolling

	 •	 Individual movement

techniques (ie, low 

crawl, high crawl)

	 •	 Defensive position

construction

	 •	 Walking on hard surface

at 3.5 mph, ≥40-lb load

	 •	 Walking in loose sand

at 2.5 mph with load

	 •	 Field assaults

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

DC: Headquarters, US Departments of the Army and Air Force; 2003: 13. TB MED 507/AFPAM 48-152.
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Water Requirements and Soldier Hydration

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. Therefore, 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 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 7-3 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. Thus, 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 7-3 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 7-3 

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 not-so-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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Military Quantitative Physiology: Problems and Concepts in Military Operational Medicine

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