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Suggested Citation:"1. Introduction and Backgrounds." Institute of Medicine. 1993. Nutritional Needs in Hot Environments: Applications for Military Personnel in Field Operations. Washington, DC: The National Academies Press. doi: 10.17226/2094.
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1
Introduction and Background

THE COMMITTEE'S TASK

The Committee on Military Nutrition Research (CMNR) of the Food and Nutrition Board (FNB), Institute of Medicine (IOM), National Academy of Sciences (NAS), was asked by the Division of Military Nutrition, U.S. Army Institute of Environmental Medicine (USARIEM), U.S. Army Medical Research and Development Command (USARMRDC), to review current research pertaining to nutrient requirements for working in hot environments and to comment on how this information might be applied to military nutrient standards and military rations. The committee was thus tasked with providing a thorough review of the literature in this area and with interpreting these diverse data in terms of military applications. In addition to a focus on specific nutrient needs in hot climates, the committee was asked to consider factors that might change food intake patterns and therefore overall calories. The CMNR was presented with this problem as a direct result of the movement of the Armed Forces into Saudi Arabia in Operation Desert Shield in the autumn of 1990; the committee was organizing the workshop that resulted in this report while the American Armed Forces were actively engaged in Operation Desert Storm in early 1991. Although concern for adequate nutrition for U.S. soldiers in Saudi Arabia prompted the initiation of this project, its scope was defined as including the nutrient needs of individuals who may be actively working in both hot-dry and hot-moist climates.

Suggested Citation:"1. Introduction and Backgrounds." Institute of Medicine. 1993. Nutritional Needs in Hot Environments: Applications for Military Personnel in Field Operations. Washington, DC: The National Academies Press. doi: 10.17226/2094.
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The CMNR was asked to address the following questions:

  1. What is the evidence that there are any significant changes in nutrient requirements for work in a hot environment?

  2. If such evidence exists, do the current Military Recommended Dietary Allowances provide for these changes?

  3. Should changes be made in military rations that may be used in hot environments to meet the nutrient requirements of soldiers with sustained activity in such climates?

  4. Specifically, are the meals, ready-to-eat (MREs) good hot-weather rations? Should the fat content be lower? Should the carbohydrate content be higher?

  5. What factors may influence food intake in hot environments?

  6. To what extent does fluid intake influence food intake?

  7. Is there any scientific evidence that food preferences change in hot climates?

  8. Are there special nutritional concerns in desert environments in which the daily temperature may change dramatically?

  9. Is there an increased need for specific vitamins or minerals in the heat?

  10. Does working in a hot climate change an individual's absorptive or digestive capability?

  11. Does work at a moderate to heavy rate increase energy requirements in a hot environment to a greater extent than similar work in a temperate environment?

To assist the CMNR in responding to these questions, a workshop was convened on April 11–12, 1991, that included presentations from individuals familiar with or having expertise in digestive physiology, energetics, macro-nutrients, vitamins, minerals, appetite, psychology, sociology, and olfaction. The invited speakers discussed their presentations with committee members at the workshop and submitted the content of their verbal presentations as written reports. The committee met after the workshop to discuss the issues raised and the information provided. The CMNR later reviewed the workshop presentations and drew on its collective expertise and the scientific literature to develop the following summary, conclusions, and recommendations.

MILITARY RECOMMENDED DIETARY ALLOWANCES

History

The history of the Military Recommended Dietary Allowances (MRDAs) is related to the history of both the Recommended Dietary Allowances (RDAs)

Suggested Citation:"1. Introduction and Backgrounds." Institute of Medicine. 1993. Nutritional Needs in Hot Environments: Applications for Military Personnel in Field Operations. Washington, DC: The National Academies Press. doi: 10.17226/2094.
×

and the Food and Nutrition Board of the National Academy of Sciences. The Food and Nutrition Division, Office of the Surgeon General, U.S. Army, was established in 1917 to (1) safeguard the nutritional interests of the Army; (2) inspect food supplied to the Army to ensure the proper amount and distribution of nutrients; and (3) obtain data on which to base intelligent alterations of military rations. During World War I, the Food and Nutrition Division of the Army conducted nutrition surveys at Army training camps to determine food consumption and wastage. Based on these early surveys, the first recommended nutrient requirements for the training of soldiers were developed in 1919. They were listed as follows: protein, 12.5 percent kcal; fat, 25 percent kcal; and carbohydrate, 62.5 percent kcal (Murlin and Miller, 1919).

During World War II the responsibilities for nutrition of the Office of the Surgeon General were expanded to provide more direct nutrition guidance. In 1940 the Food and Nutrition Board (FNB) of the National Academy of Sciences was organized in conjunction with the defense program to help the Army establish a satisfactory standard for operational rations. From 1943 until 1947 the Surgeon General's Office accepted diets as nutritionally adequate if they met the recommended allowances of the FNB. Beginning with Army Regulation (AR) 40-250 Nutrition (October 28, 1947), the Office of the Surgeon General initiated the first use of a specified ''Minimum Nutrient Intake'' for military personnel. These standards incorporated an adjusted caloric standard for the extreme cold.

The military nutrient standards were patterned after the current FNB Recommended Dietary Allowances (RDAs) with modifications to meet the needs of Army personnel beginning with AR 4-564 (February 9, 1956). The first Tri-Service regulation (AR 40-25, 1968) based on the RDAs with modifications was issued on July 2, 1968. The military nutrition standards were first termed "Military Recommended Dietary Allowances" with the May 15, 1985, revision of AR 40-25. The CMNR provided commentary to the Army during the revision process. This regulation also designated the Army Surgeon General as the Department of Defense (DOD) Executive Agent for Nutrition for the military. The 1985 MRDAs are adapted from the ninth edition of the RDAs (NRC, 1980) and are the current standard for all branches of the military.

Current MRDAs

The MRDA regulation (AR 40-25, 1985) is presently under revision.1 The revised standards will reflect changes in the nutrition knowledge base,

1  

At the request of the Army Medical Research and Development Command representative, the Committee on Military Nutrition Research held a meeting on November 27, 1990, at the

Suggested Citation:"1. Introduction and Backgrounds." Institute of Medicine. 1993. Nutritional Needs in Hot Environments: Applications for Military Personnel in Field Operations. Washington, DC: The National Academies Press. doi: 10.17226/2094.
×

changes in the RDAs based on the tenth edition (NRC, 1989b), and military nutrition initiatives for the twenty-first century. AR 40-25 (1985) not only lists the nutrient standards but includes definitions of terminology, guidelines for healthful diets, and clarification of the use of the MRDAs for menu planning, dietary evaluations, nutrition education, and food research and development in the military. A separate table provides nutritional standards for operational and restricted rations. AR 40-25 is included in full in Appendix A.

One purpose of the present study was to comment on the applicability of the current MRDAs for work in hot environments. Table 1-1 is a comparison of the nutrient recommendations in the latest edition of the RDAs (NRC, 1989b) and those in AR 40-25 (1985). Table 1-2 compares the estimated safe and adequate ranges for selected vitamins and minerals from the same two sources. These tables provide a reference for the physiological and nutrient-by-nutrient discussion that follows.

PHYSIOLOGICAL CHANGES ARISING FROM EXERCISE AND HEAT

For the most part, reported studies in the areas of physiology and gastrointestinal function have examined the effect on physiological function of an increased core temperature, whether as a result of exercise or increased ambient temperature. In only a few cases are the effects of exercise on body core temperature compared with the effects of a hot environment alone, whether in exercising or resting people. A few studies are described in the historical perspective in Chapter 6. Important physiological considerations related to performance are reviewed below.

Exercise

Muscular exercise can increase metabolism by up to 15 times the basal rate (see discussion in Chapter 3). Most of the heat resulting from this level of energy expenditure needs to be removed to maintain thermostasis. Heat loss occurs through both insensible (evaporative) and sensible (radiative and convective) mechanisms. These are controlled by a thermoregulatory center in the hypothalamus; this center, through the autonomic nervous system, controls heat transfer from the body core to the skin primarily via

   

National Academy of Sciences in Washington, D.C., to discuss the status and the direction of the revision of the MRDAs. Dietitians and representatives from the Army, Navy, Air Force, Marines, and Coast Guard attended and discussed specific service-based concerns regarding MRDA revisions and issues related to military nutrition initiatives for the future. They also covered garrison menu planning and general implementation of the MRDAs in various nongarrison military settings.

Suggested Citation:"1. Introduction and Backgrounds." Institute of Medicine. 1993. Nutritional Needs in Hot Environments: Applications for Military Personnel in Field Operations. Washington, DC: The National Academies Press. doi: 10.17226/2094.
×

blood circulation. The increased blood flow to the surface raises the temperature of the skin and allows sensible heat loss by radiative and convective mechanisms. Evaporative heat loss occurs through sweating (see Chapters 3, 4, and 5).

Heavy exercise at increased ambient temperatures decreases the skin-to-ambient-temperature gradient, thus substantially decreasing sensible heat loss. Under these conditions, most heat loss by the body will occur through evaporative cooling (i.e., sweating). As is well known, heat loss by this mechanism can be greatly decreased under conditions of high humidity. The resulting dehydration from excess sweating can reduce blood volume and cardiac filling. If compensatory circulatory and cardiac changes are insufficient, skin and muscle blood flow will be impaired, thus reducing sensible heat loss and physical performance. A state of adequate hydration is therefore important in maintaining the effectiveness of the physiological mechanisms involved in heat dissipation.

Heat Stress

Thermoregulation can be defined as the summation of the mechanisms by which the body adapts to a heat stress in order to maintain thermoneutrality. Body core and skin temperatures have been used as indices of the ability of the body to thermoregulate, along with cardiovascular changes in heart rate, blood volume (see Harrison, 1985, for a comprehensive review) and blood pressure, with sweat rate as a visible mechanism of adaptation. Acclimatization is the process of adapting to prolonged exposure to a new environment, so that the mechanisms that result in initial responses are modified to allow increased endurance with less strain on body functions.

The ability to defend one's body temperature against heat stress is influenced by level of activity, acclimatization state, aerobic fitness, and hydration level. In heat-acclimatized2 individuals, the thermoregulatory mechanisms involved in dissipating heat become fully operative. Although some investigators report that to perform a given submaximal exercise task the metabolic rate is greater in a hot compared to a temperate environment (Consolazio et al., 1961, 1963; Dimri et al., 1980, Fink et al., 1975), other investigators report lower metabolic rates in the heat (Brouha et al., 1960; Petersen and Vejby-Christensen, 1973; Williams et al., 1962; Young et al., 1985). A person's state of heat acclimatization does not account for whether individuals demonstrate an increased or decreased metabolic rate during submaximal exercise in the heat; other mechanisms explain this discrep-

2  

The term heat acclimatization is used here to refer to the adaptive changes that occur due to exposure to a hot natural environment; heat acclimation will be used to refer to adaptive changes to a hot environment under controlled conditions, such as in an environmental chamber.

Suggested Citation:"1. Introduction and Backgrounds." Institute of Medicine. 1993. Nutritional Needs in Hot Environments: Applications for Military Personnel in Field Operations. Washington, DC: The National Academies Press. doi: 10.17226/2094.
×

TABLE 1-1 Comparison of the Current Military Recommended Dietary Allowances (MRDAs) (AR 25-40, 1985) That Are Based in Part on the Ninth Edition of the Recommended Dietary Allowances (RDAs) (NRC, 1980) with the Most Recent RDAs (NRC, 1989b)

 

 

MRDAsa

RDAsb

 

 

Men

Women

Men

Men

Women

Women

Nutrient

Unit

(17–50 y)

(17–50 y)

(19–24 y)

(25–50 y)

(19–24 y)

(25–50 y)

Energy

Kcal

3200 (2800–3600)c,d

2400 (2000–2800)c,d

2900e

2900e

2200e

2200e

 

MJ

13.4 (11.7–15.1)

10.0 (8.4–11.7)

 

 

 

 

Protein

g

100f

80f

58

63

46

50

Vitamin Ag

µg RE

1000

800

1000

1000

800

800

Vitamin Dh

µg

5–10i

5–10i

10

5

10

5

Vitamin Ej

mg TE

10

8

10

10

8

8

Ascorbic Acid

mg

60

60

60

60

60

60

Thiamin (B1)

mg

1.6

1.2

1.5

1.5

1.1

1.1

Riboflavin (B2)

mg

1.9

1.4

1.7

1.7

1.3

1.3

Niacink

mg NE

21

16

19

19

15

15

Vitamin B6

mg

2.2

2.0

2.0

2.0

1.6

1.6

Folacin

µg

400

400

200

200

180

180

Vitamin B12

µg

3.0

3.0

2.0

2.0

2.0

2.0

Calcium

mg

800–1200i

800–1200i

1200

800

1200

800

Phosphorus

mg

800–1200i

800–1200i

1200

800

1200

800

Magnesium

mg

350–400i

300i

350

350

280

280

Iron

mg

10–18i

18i

10

10

15

15

Zinc

mg

15

15

15

15

12

12

Iodine

µg

150

150

150

150

150

150

Sodium

mg

See notel

See notel

500m

500m

500m

500m

Suggested Citation:"1. Introduction and Backgrounds." Institute of Medicine. 1993. Nutritional Needs in Hot Environments: Applications for Military Personnel in Field Operations. Washington, DC: The National Academies Press. doi: 10.17226/2094.
×

a MRDA for moderately active military personnel, ages 17 to 50 years, are based in part on the Recommended Dietary Allowances, ninth revised edition, 1980. The MRDAs are currently under revision.

b For the RDAs, the allowances, expressed as average daily intakes over time, are intended to provide for individual variations among most normal persons as they live in the United States under usual environmental stresses. Diets should be based on a variety of common foods in order to provide other nutrients for which human requirements have been less well defined. See text for detailed discussion of allowances and of nutrients not tabulated. Values are taken from the RDAs, tenth edition (NRC, 1989b).

c Energy allowance ranges are estimated to reflect the requirements of 70 percent of the moderately active military population. One megajoule (MJ) equals 239 kcal.

d Dietary fat calories should not contribute more than 35 percent of total energy intake.

e From Table 3–5 from the RDAs, tenth edition (NRC, 1989b) by using the assumption of light to moderate activity for each age and gender group. These figures were calculated using the World Health Organization (WHO, 1985) equations for resting energy expenditure multiplied by an activity factor as described in the text (NRC, 1989b) pp. 25–33.

f Protein allowance is based on an estimated protein requirement of 0.8 g per kilogram (kg) desirable body weight. By using the reference body weight ranges for males of 60 to 79 kilograms and for females of 46 to 63 kilograms, the protein requirement is approximately 48 to 64 grams for males and 37 to 51 grams for females. These amounts have been approximately doubled to reflect the usual protein consumption levels of Americans and to enhance diet acceptability.

g One microgram of retinol equivalent (µg RE) equals 1 microgram of retinol, or 6 micrograms beta-carotene, or 5 international units (IU).

h As cholecalciferol, 10 micrograms of cholecalciferol equals 400 IU of vitamin D.

i High values reflect greater vitamin D, calcium, phosphorus, magnesium, and iron requirements for 17-to 18-year olds than for older ages.

j One milligram of alpha-tocopherol equivalent (mg TE) equals I milligram d-alpha-tocopherol.

k One milligram of niacin equivalent (mg NE) equals I milligram niacin or 60 milligram dietary tryptophan.

l The safe and adequate levels for dietary sodium intake of 1100 to 3300 mg published in the RDAs (NRC, 1980) are currently impractical and unattainable within military food service systems. However, an average of 1700 milligrams of sodium per 1000 kilocalories of food served is the target for military food service systems. This level equates to a daily sodium intake of approximately 5500 milligrams for males and 4100 milligrams for females. [Note: This comment is based on the ninth edition of the RDAs (NRC, 1980). The MRDAs are currently under revision in light of the 1989 publication of the tenth edition of the RDAs (NRC, 1989b).]

m Estimated minimum requirements for healthy persons. No allowance has been included for large, prolonged losses from skin through sweat. There is no evidence that higher intakes confer any health benefit.

SOURCE: Adapted from Table 2-1, MRDA for selected nutrients, p. 2-4 (AR 25-40, 1985) and National Research Council Recommended Dietary Allowances (1989b), p. 284, and Tables 3-4, 3-5, and 11-1; pp. 29, 33, and 253.

Suggested Citation:"1. Introduction and Backgrounds." Institute of Medicine. 1993. Nutritional Needs in Hot Environments: Applications for Military Personnel in Field Operations. Washington, DC: The National Academies Press. doi: 10.17226/2094.
×

TABLE 1-2 Comparison of the Estimated Safe and Adequate Daily Dietary Intake Ranges of Selected Vitamins and Minerals from the Military Recommended Dietary Allowances (AR 25–40, 1985) That Are Based in Part on the Ninth Edition of the Recommended Dietary Allowances (RDAs) (NRC, 1980) with the Values from the Tenth Edition of the RDAs (NRC, 1989b)

Nutrient

Unit

From MRDAsa

From RDAsb

Vitamins

Vitamin K

µg

70–140

65, 80c

Biotin

µg

100–200

30–100

Pantothenic Acid

mg

4–7

4–7

Trace Elementsd

Fluoride

mg

1.5–4.0

1.5–4.0

Selenium

µg

50–200

55, 70c

Molybdenum

mg

0.15–0.50

0.075–0.250

Copper

mg

2–3

1.5–3.0

Manganese

mg

2.5–5.0

2.0–5.0

Chromium

µg

50–200

50–200

Electrolytes

Potassium

mg

1875–5625

2000

Chloride

mg

1700–5100

750

a MRDAs = Military Recommended Dietary Allowances. Data in this portion of the table are based in part on the Recommended Dietary Allowances , ninth edition, 1980, Table 10, "Estimated Safe and Adequate Daily Dietary Intakes of Selected Vitamins and Minerals." Estimated ranges are provided for these nutrients because sufficient information upon which to set a recommended allowance is not available. Values reflect a range of recommended intake over an extended period of time.

b RDAs = Recommended Dietary Allowances. Because there is less information on which to base allowances, these figures were not given in the main table of RDA and were provided in the form of ranges of recommended intakes.

c First number is the RDA for women aged 19–50; the second number is the RDA for men of the same age range. With the publication of the tenth edition of the RDAs, vitamin K and selenium were moved into the summary chart for recommended, age and gender-based dietary allowances.

d Since the toxic levels for many trace elements may be only several times usual intakes, the upper levels for the trace elements given in this table should not be habitually exceeded.

SOURCE: MRDA values adapted from Table 2-2, p. 2-5 (AR 40-25, 1985); RDA values adapted from Table 11-1, p. 253; Summary Table: Estimated Safe and Adequate Daily Dietary Intakes of Selected Vitamins and Minerals, p. 284, and the Recommended Dietary Allowances, tenth edition, 1989, Summary Table, p. 285 (NRC, 1989b).

Suggested Citation:"1. Introduction and Backgrounds." Institute of Medicine. 1993. Nutritional Needs in Hot Environments: Applications for Military Personnel in Field Operations. Washington, DC: The National Academies Press. doi: 10.17226/2094.
×

ancy. Most investigators have only calculated the aerobic metabolic rate during submaximal exercise, ignoring the contribution of anaerobic metabolism to total metabolic rate. Although both increases and decreases have been observed in metabolic rate in the heat, it does not appear that the presence or absence of heat acclimatization has an effect on metabolic rate (see Chapters 3 and 6 for further discussion).

Muscular activity produces an enormous amount of heat, with the amount of heat production directly related to the intensity of exercise (Nadel et al., 1977). The amount of heat production generated by the increased energy metabolism of skeletal muscle during exercise may be as much as 100 times that of inactive muscle. The mechanisms for dissipating this heat are generally well regulated. Although heat loss occurs through evaporation of sweat and by conduction; convection, and radiation, evaporation of sweat is clearly the most effective avenue of heat loss during exercise. The sweat glands are capable of secreting up to 30 grams of sweat per minute, removing approximately 18 kcal of heat in the process. Sweat rate is directly associated with exercise intensity (Maughan, 1985; Nadel et al., 1977).

Gastrointestinal Functioning

It has been reported (see Chapter 4) that gastric emptying and intestinal motility decrease as core temperature increases during exercise and in hypohydration. Some, but not all, investigators have also observed reductions in intestinal absorption of nutrients under these conditions.

Most of the studies on the effects of exercise and heat on gastrointestinal function have been carried out in endurance athletes such as marathon runners. Gastrointestinal symptoms under these conditions are often severe, although transient. They include cramps, belching, gastrointestinal reflux, flatulence, bloody stools, vomiting, diarrhea, and nausea. Mechanisms for these effects are discussed in Chapter 4. The relevance of these findings to the range of physical activity in the military is not at all clear, and the findings appear transient when associated with extreme physical activity. Instances of levels of physical activity in the military approaching those of highly competitive endurance athletes would appear to be the exception rather than the rule.

CHANGES IN NUTRIENT REQUIREMENTS FOR HOT ENVIRONMENTS

Fluid and Dehydration

The requirement for water in a hot environment depends on the amount of fluid loss, which in turn depends on such factors as exercise intensity,

Suggested Citation:"1. Introduction and Backgrounds." Institute of Medicine. 1993. Nutritional Needs in Hot Environments: Applications for Military Personnel in Field Operations. Washington, DC: The National Academies Press. doi: 10.17226/2094.
×

exercise duration, environmental conditions (dry heat versus humid heat), state of training and heat acclimatization, sex, and age (see Chapter 5). The increased heat production of exercise, an increased sweat rate, and inadequate hydration predispose soldiers in hot environments to dehydration.

Along with exercise intensity, sweat rate is related to environmental conditions, clothing, and acclimatization state (Shapiro et al., 1982). In hot, dry conditions, water loss from the skin and respiratory surfaces can be as much as 2 to 3 liters per hour (Wenger, 1988). In hot, moist (humid) conditions, sweat losses are measurably less than in hot, dry conditions. In a study that measured physiologic changes and sweat losses in healthy young men during hyperthermia induced by humid heat in an environmental chamber, total sweat losses averaged 7 liters per 24 hours (Beisel et al., 1968). However, humidity per se does not appear to affect core (rectal) temperature (Morimoto, 1967). In terms of military apparel, the nuclear-biological-chemical (NBC) protective clothing worn by many military personnel prevents the normal dissipation of body heat because of the cloth's lack of moisture permeability and its insulating properties. As a result, body temperature may rise excessively, producing high levels of sweat (1 to 2 liters per hour) that cannot evaporate effectively because air turnover is reduced, and caution must be taken (Muza et al., 1988; Pimental et al., 1987).

If the fluid involved in excessive sweat loss is not replaced, total body water, along with the total blood volume, will be decreased. A water loss as small as 1 percent of body weight will induce changes such as increased heart rate during rest and exercise, and decreased performance. However, a 1 percent loss is difficult to discern relative to what might be regarded as initial water balance. Thus, it is hard to attribute physiological changes to a 1 percent loss, but such changes can be readily observed at losses of 2.0 to 2.5 percent. A 10 percent loss of body weight through dehydration3 is life-threatening (Adolph, 1947). Water loss from the blood leads to a decrease in sweat rates and skin blood flow (Sawka and Pandolph, 1990; Wyndham, 1977), which results in less evaporative cooling and a risk of heat stroke (Wyndham, 1977). The normal compensatory response to exercise and heat stress is increased peripheral blood flow to maximize heat dissipation and prevent hyperthermia. However, in dehydrated individuals with greatly diminished blood volume, skin blood flow is reduced to maintain cardiac output and blood pressure.

Reductions in blood volume can result in a reduced flow of blood to organs during exercise and reduced venous flow in return. This reduced venous return to the heart decreases stroke volume and causes a compensatory increase in the heart rate to maintain cardiac output and blood pressure.

3  

The term dehydration is used here to refer to the process of losing body water, while the term hypohydration will be used to denote the result of the dehydration process.

Suggested Citation:"1. Introduction and Backgrounds." Institute of Medicine. 1993. Nutritional Needs in Hot Environments: Applications for Military Personnel in Field Operations. Washington, DC: The National Academies Press. doi: 10.17226/2094.
×

This reflex increase in heart rate, however, is not sufficient to compensate for the decrease in stroke volume (Rowell et al., 1966); consequently, maximal cardiac output is reduced.

Several studies have shown that cardiovascular performance is compromised following thermal or exercise-induced hypohydration ≤(2 percent body weight loss) (Armstrong et al., 1985; Costill et al., 1976; Pitts et al., 1944; Saltin, 1964). Cardiac output is reduced by almost 2 liters per minute with decreased blood volume (Fortney et al., 1983; Nadel et al., 1980). This reduction in cardiac output can almost entirely account for decreases in as a result of hypohydration (Rowell et al., 1966; Saltin, 1964). Significant reductions in physical work capacity have been seen in wrestlers after hypohydration-caused weight loss (Herbert and Ribisl, 1972), as well as in runners after diuretic-induced weight loss (Armstrong et al., 1986).

The acute heat stress in hot climates that causes and is caused by dehydration has been associated with several factors. It can be precipitated by an increase in resting and submaximal exercise metabolic rates (Consolazio et al., 1961, 1963; Dimri et al., 1980; Fink et al., 1975), increases in plasma or muscle lactate levels (Dill et al., 1930; Dimri et al., 1980; Fink et al., 1975; Nadel, 1983; Robinson et al., 1941; Young et al., 1985), and glycogenolysis during submaximal exercise.

Effect of Gender

Early studies that investigated dehydration and exercise in heat and humidity found differences in sweat rate and endurance, with women sweating less than men for a given thermal stress (Fox et al., 1969; Wyndham, 1965). These studies were initially interpreted as evidence that women were not as capable as men in coping with heat stress. More recent studies comparing the effects of exercising in heat and humidity in men and women continue to find differences in sweat rate. Gender differences in response to thermal stress (body core temperature, acclimatization, etc.) however, appear to result from differences in aerobic power, due to disparities in body weight-to-mass ratio or level of physical fitness (Armstrong et al., 1990; Avellini et al., 1980; Dill et al., 1977; Grucza et al., 1985; Havenith and van Middendorp, 1990; O'Toole, 1989; Paolone et al., 1978; White et al., 1992; Chapter 5, this volume).

Avellini et al. (1980) compared acclimation to work in humid heat in an environmental chamber in men and women with similar aerobic capacities and surface-area-to-mass ratios. The women were tested both pre-and postovulation. Prior to acclimation, the women sweated less than the men, their endurance was greater, and their rectal temperature and heart rate did not increase to the level seen in men. After acclimation, rectal temperature and heart rates were similar, although there was an increased difference in sweat

Suggested Citation:"1. Introduction and Backgrounds." Institute of Medicine. 1993. Nutritional Needs in Hot Environments: Applications for Military Personnel in Field Operations. Washington, DC: The National Academies Press. doi: 10.17226/2094.
×

rates between the two groups. Women had the greatest tolerance to the exercise and lowest rectal temperature prior to acclimation if they were in the pre-ovulatory phase, whereas post-ovulation, rectal temperature was similar to men while sweat rate and heart rate continued to be significantly lower. In women there was also a lag period before sweating began in the post-ovulatory phase, resulting in core temperatures rising above those seen in the pre-ovulatory phase. The differences seen in sweat rate pre-and post-ovulation in women, however, were not of the same magnitude as those seen when they were compared to men (Avellini et al., 1980).

Studies comparing men to women in hot environments have shown that women acclimated to the same work load as men demonstrate decreased sweat rates, but similar core (rectal) temperatures (Avellini et al., 1980; Wyndam, 1965). Other studies comparing men and women exercising in hot environments (Dill et al., 1977; Morimoto et al., 1967; Weinman et al., 1967) have consistently demonstrated less elevation of total body sweat rates (in milliliters per meter squared per hour) among women. Although heat and dehydration affect thermoregulatory responses such as sweating in men more severely than in women (Grucza et al., 1987), it is not gender but an individual's surface area, fitness or aerobic capacity, and acclimatization status that determine the relative heat strain in a given environment (Havenith and van Middendorp, 1990).

Overall, therefore, women do not seem to have less heat tolerance than men when they are exercising at equivalent intensities in relation to their aerobic capacities. Whereas women sweat less, they rely on circulatory cooling to a greater extent for heat dissipation. Therefore well-trained, heat acclimatized women show similar responses to hot-humid and hot-dry environments as do men.

Effect of Age

Apparent heat intolerance among the aged has been attributed to a reduction in sweating capacity and a decline in aerobic fitness (see Kenney and Gisolfi, 1986, for a review). It appears that the development of the decline in heat tolerance normally associated with men and women beginning around age 50 to 60 can be attributed to reduced cardiovascular fitness and a lack of prior heat exposure that would allow for heat acclimatization. One study (Robinson, et al., 1986) cited by Gisolfi (Chapter 5) demonstrated decreased sweating capacity in four men age 44 to 60 compared with measurements made 21 years earlier. However, this decline did not affect the ability of the older men to become acclimated to a hot-dry environment (as defined by a decreased body core temperature after 6 to 8 days) and to work at the same level and intensity as they had previously worked.

In contrast, a study done with five older men (aged 61 to 67), in which

Suggested Citation:"1. Introduction and Backgrounds." Institute of Medicine. 1993. Nutritional Needs in Hot Environments: Applications for Military Personnel in Field Operations. Washington, DC: The National Academies Press. doi: 10.17226/2094.
×

they were compared to six younger men (aged 21 to 29) who were matched for height, weight, and body surface area, but not percent body fat, demonstrated that the older men were less able to respond to a single 3-hour period of thermal dehydration than the younger men (Miescher and Fortney, 1989). Rectal temperatures in the older men increased more rapidly while sweat rates were not significantly different. In addition, plasma volume decreased and plasma osmolality increased to a larger extent in the older men. Within 30 minutes of rehydration, plasma volume and osmolality had returned to normal in the young men, while the older men took 60 minutes to restore plasma osmolality, and 90 minutes to restore plasma volume. Since these older subjects were not considered ''extremely'' fit, did not have similar ratios of body fat compared to the younger subjects, and since the protocol did not call for work or exercise during the 3 hour period, it is possible that the differences noted were due to these factors and not age per se.

Military researchers have measured thermoregulatory responses and acclimation in two groups of nine men who were matched for body weight, surface area, percent body fat, and maximal aerobic power, but with average ages of 21 and 46 (Pandolf et al, 1988). Initially, the older group demonstrated increased performance time with decreased rectal and skin temperatures and increased body sweat loss. After acclimation (measured after 10 days), no differences were seen between the two groups in thermoregulatory responses, including sweating rate, or performance time. The authors also noted that those in the older group who engaged in regular weekly aerobic activity were better able to initially respond to the thermal stress, although such differences were not evident after heat acclimation.

An additional study, compared a group of eight sedentary men, average age 34, with six "moderately active" older men, average age 57 (Smolander et al., 1990). The men in both groups walked on a treadmill at 30 percent for up to 3.5 hours in thermoneutral, warm-humid, and hot-dry environments. There was little difference in the ability of the older men to tolerate the protocol when compared with the younger men. The authors concluded that the ability to exercise in hot environments may not necessarily be associated with calendar age but more importantly with factors such as physical activity habits and aerobic capacity.

Based on these studies, it appears that for the age group of the active military, it is important to take into consideration the level of fitness of military troops regardless of age, particularly when going into a hot environment in which significant work is initially expected.

Effect on Electrolyte Balance

Although electrolytes are lost with sweat, these losses, except in some extreme cases, are usually not large enough to affect performance capacity

Suggested Citation:"1. Introduction and Backgrounds." Institute of Medicine. 1993. Nutritional Needs in Hot Environments: Applications for Military Personnel in Field Operations. Washington, DC: The National Academies Press. doi: 10.17226/2094.
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(Costill et al., 1976; Koslowski and Saltin, 1964). (Plasma concentrations may even rise as a result of the relatively larger fluid losses.) Renal retention of electrolytes during exercise can compensate for some of these electrolyte losses; following exercise, normal dietary intake can replenish these losses. In extreme cases in which sweat loss is great enough to result in a significant electrolyte deficit, the dehydration itself may cause debilitating conditions.

Summary

Increased physical activity in hot environments can result in severe hypohydration. This is particularly true when fluids are in short supply or not very palatable. Hypohydration can cause large decrements in performance and can greatly increase the risk of heat casualties. The risk of hypohydration is reduced in individuals who have been acclimatized to the heat and who are physically fit. The papers presented in this volume (see, in particular, Chapters 35 and 1214) provide the scientific information necessary for understanding both acute and long-term adaptations to heat stress, particularly when combined with exercise. The physiological mechanisms that lead to increased water loss during heat exposure and the adaptability of such mechanisms to chronic heat exposure must be well understood to begin to make nutritional recommendations for soldiers subjected to these conditions for long periods. As Gisolfi (Chapter 5) concludes, sweat rates, proportional to metabolic rates, can reach as much as 10 liters per day. Training and heat acclimatization can increase the rate of sweating (and therefore the ability to work in a hot environment) by 10 to 20 percent or 200 to 300 milliliters per hour. Although men sweat more than women and require more water, well-trained, heat-acclimatized women can adapt to heat as effectively as men. Within the age range of the active-duty military force, there is no predicted decrement in sweating with increasing age; therefore, the water requirement during exercise in the heat is unchanged.

Sodium Levels for Work in the Heat

During the past several years there has been an emphasis on reducing the sodium content of foods and the sodium intake of the U.S. population. The Surgeon General's Report on Nutrition and Health (U.S. Department of Health and Human Services, 1988) recommended a reduction in sodium intakes, the latest version of the Recommended Dietary Allowances (RDAs) (NRC, 1989b) lowered the estimated minimum sodium requirements for healthy adults to 500 milligrams (mg) per day, and the Food and Nutrition Board's Diet and Health (NRC, 1989a) also recommended significant reductions in sodium in all diets. The 1989 RDAs contain a footnoted caution,

Suggested Citation:"1. Introduction and Backgrounds." Institute of Medicine. 1993. Nutritional Needs in Hot Environments: Applications for Military Personnel in Field Operations. Washington, DC: The National Academies Press. doi: 10.17226/2094.
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however, that "no allowance has been included for large, prolonged losses from the skin through sweat."

The military has endeavored to reduce the sodium intake of its personnel through modifications of garrison and operational ration guidelines. Dietary surveys conducted at various military facilities have documented changes in the dietary intakes of several nutrients, but the sodium intake of personnel eating in military dining halls has remained relatively stable at 1500 to 1850 mg of sodium per 1000 kcal of diet (IOM, 1991). The MRDAs set forth a goal of 1700 mg of sodium per 1000 kcal of diet for foods served in military dining halls. This level is estimated to equal a daily sodium intake of approximately 5500 mg for men and 4100 mg for women. For operational rations4 the MRDAs specify a range of 5000 to 7000 mg of sodium per day, excluding the additional salt packets that are packed with the rations. Restricted rations5 have sodium levels, as established by the MRDAs, of 2500 to 3500 mg of sodium per day.

In an earlier report (IOM, 1991), the CMNR evaluated the sodium content of military rations. The committee urged caution in arbitrarily reducing sodium intake drastically from current levels and noted that studies were needed to evaluate the impact of reductions on personnel who might not be heat acclimatized and who were routinely consuming diets that provided sodium in the range of 1700 to 1850 mg per 1000 kcal of diet. The committee recommended that the "total daily intake of salt should be limited to 10 grams or less (4000 mg sodium) except under conditions in which salt requirements exceed values due to large salt losses such as those associated with heavy physical work in hot environments."

Chapters 12 through 14 in Part III summarize the details of studies by researchers from the USARIEM who investigated the impact of reducing intake from 8 grams to 4 grams of sodium chloride per day (3200 to 1600 mg sodium) for individuals working in a hot environment. The primary concern of the CMNR in including this study as part of this report was a consideration of the possible detrimental effects on troops of being suddenly deployed from a temperate environment to a desert or a jungle without an opportunity for acclimatizing to the heat. A mobilization of this kind would also result in soldiers' consuming combat rations that would provide significantly lower levels of sodium than they had been consuming prior to deployment. The committee's concern centers on the ability of troops to

4  

Operational rations typically are composed of nonperishable items that are designed for use under actual or simulated combat conditions.

5  

Restricted rations are designed for use under more specific operational scenarios such as long-range patrol, assault, and reconnaissance when troops are required to subsist for short periods (up to 10 days) on an energy-restricted ration. These rations require no further preparation; because they are intended for short-range patrols, they provide suboptimal levels of energy and nutrients.

Suggested Citation:"1. Introduction and Backgrounds." Institute of Medicine. 1993. Nutritional Needs in Hot Environments: Applications for Military Personnel in Field Operations. Washington, DC: The National Academies Press. doi: 10.17226/2094.
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perform immediately in a combat situation without a period of heat acclimatization. Furthermore, it should be noted that most of the sodium consumed by troops is derived from the consumption of food. Therefore a reduction in food, which is frequently observed during deployment, will likely result in a significant drop in sodium intake.

The data presented in Chapters 12 to 14 show that soldiers acclimated fairly rapidly to the hot environment and adapted to the lowered salt intake over the 10-day study period. However, there were increased symptoms of heat exhaustion during the first two days, which could be a significant problem for troops involved in military operations. These symptoms might have been even more severe had the subjects not been following a careful fluid intake schedule to maintain hydration during the study. It is also possible that the tendency toward heat illness would have been greater if these subjects had not been adapted to 8 grams per day of sodium chloride rather than the levels found in garrison dietary surveys (approximately 12 to 13 grams per day). Therefore, although the CMNR supports the goals of reducing the sodium intake of the U.S. population as well as military personnel, the committee does not recommend a reduction in the sodium content of operational rations at this time. As stated in the committee's report Military Nutrition Initiatives (IOM, 1991), it is not reasonable to expect the dietary sodium intake of military personnel in garrison to be different from that of the civilian population, which for adults is estimated to range from 1800 to 5000 mg per day in some reports (NRC, 1989a,b), and at a slightly higher level of 4000 to 6000 mg of sodium per day in other reports (U.S. Department of Health and Human Services, 1988). In addition, reducing sodium levels in operational rations must follow the efforts to reduce sodium intake in the general population to minimize the potential for compromising soldier performance in the days following deployment to hot environments.

Macronutrients

Chapter 6 provides a review of the influence of heat on macronutrient needs and soldier performance. A summary of this information is provided below.

Protein

Various authors over the past 40 years have reviewed the protein requirements for individuals working in the heat. Mitchell and Edman (1951) stated that "considering all evidence, it may be concluded that protein requirements may be slightly increased in the tropics by some 5–10 grams daily." They postulated that the slight increase in requirements may be due to a stimulation of tissue catabolism if pyrexia occurs and to compensation

Suggested Citation:"1. Introduction and Backgrounds." Institute of Medicine. 1993. Nutritional Needs in Hot Environments: Applications for Military Personnel in Field Operations. Washington, DC: The National Academies Press. doi: 10.17226/2094.
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for sweat losses of nitrogen by diminished losses in the urine. Consolazio and Shapiro (1964) found that protein intakes of men exercising in a hot climate exceeded the then National Research Council (NRC) recommended allowances of 100 grams per day. They felt that the increased protein intake in the heat was not due to an innate desire for protein but to the relatively greater caloric intake that the men were consuming. Paul (1989) has suggested that because protein and amino acids contribute 5 to 15 percent of the energy for prolonged exercise, with the higher value perhaps associated with glycogen depletion, adequate protein intake is important when exercising in the heat. In Chapter 6, Buskirk concludes nevertheless, that there appears to be no evidence that protein intakes in excess of 1 to 1.5 grams per kilogram (kg) of body weight offer any advantage to the mature military person. Indeed, higher protein intakes may be a disadvantage, given the obligatory urine volume required to excrete the products of protein breakdown. The generous protein level of the MRDAs would suggest that somewhat lower levels might reduce body heat production while maintaining nutritional adequacy under conditions of high ambient temperatures. It should also be kept in mind that the matter of the relative proportions of protein, carbohydrate, and fat in hot environments is not yet entirely resolved.

Energy

Caloric requirements of troops are largely determined by the physical activities in which troops are engaged. The higher caloric intakes recommended for cold environments are largely due to the need to maintain thermal balance. It is interesting that studies of troops who operated in cold, moderate, and hot environments doing moderate work had essentially the same caloric requirements when calculated on the basis of body weight plus clothing and equipment being manually transported. In addition, a study that examined the performance of well-fed troops who were actively exercising in a hot environment with a group who experienced moderate energy restriction over a 12-day period found no difference in task performance between the groups, with both groups exhibiting weight loss (Crowdy et al., 1982). Buskirk (Chapter 6) concludes that for troops working in a hot environment, the submaximal exercise they perform has a far greater impact on their physiological functioning than if they performed the same tasks in a more comfortable environment. He also concludes that acclimatization plays a valuable role in physiological adaptation but that the process has only a minor part in modifying energy turnover and caloric requirements.

A major factor in meeting macronutrient requirements is the tendency for appetites to be adversely affected when unacclimatized personnel are suddenly exposed to a hot environment. Therefore, careful attention should

Suggested Citation:"1. Introduction and Backgrounds." Institute of Medicine. 1993. Nutritional Needs in Hot Environments: Applications for Military Personnel in Field Operations. Washington, DC: The National Academies Press. doi: 10.17226/2094.
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be paid to those factors that will encourage adequate ration consumption to minimize the potential for reduced nutrient intake over time.

It is also fitting to consider the quote from Dill (1985) cited by Buskirk:

In the hot desert even a well trained human can sprint only about half the distance one would guess before collapsing. One should respect the incredible intensity of the desert, protect oneself with shade, spare water, slow movement, equally-minded partners, then enjoy and relish its beauty.

Buskirk continues:

Unfortunately, military personnel engaged in combat or under the threat of combat may not have the luxury of contemplating beauty, but they nevertheless must deal with the "incredible intensity of the desert."

Vitamins

There has been considerable research dealing with the effects of temperature and exercise on vitamin requirements, particularly requirements for the B vitamins and vitamin C. A review of this published literature was presented at the workshop by Priscilla M. Clarkson (Chapter 8).

B Vitamins

Although there is limited evidence of small increases in the loss of some B vitamins in sweat during work in hot environments, these losses are not sufficient to increase the requirements beyond the intakes recommended in the current MRDAs. Because the vitamins thiamin, riboflavin, niacin, and vitamin B6 are important in energy metabolism, their intake should be related to energy intake. As noted earlier, the MRDAs are based on the RDAs and are revised periodically to reflect the regular revision of the RDAs. For these vitamins, the current MRDAs (see Table 1-1) are based directly on the amounts given in the ninth edition of the RDAs (NRC, 1980) (vitamin B6) or are based on the amounts given in the ninth edition of the RDAs with a higher assumed caloric intake (thiamin, riboflavin, and niacin). The MRDAs are currently undergoing revision to reflect the changes in the tenth edition of the RDAs (NRC, 1989b), current scientific knowledge, and the demands of military tasks. Thus, the recommendations contained in the present MRDAs for these B vitamins appear sufficient to satisfy requirements for hot environments as long as the rations are consumed in adequate amounts. Furthermore, consideration can be given to decreasing the MRDAs for these nutrients in the revised edition of this regulation, in keeping with the recommendations of the tenth edition of the RDAs and on the basis of caloric intake.

There is no evidence that the levels of folic acid and B12 required for

Suggested Citation:"1. Introduction and Backgrounds." Institute of Medicine. 1993. Nutritional Needs in Hot Environments: Applications for Military Personnel in Field Operations. Washington, DC: The National Academies Press. doi: 10.17226/2094.
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work in the heat are increased beyond the levels recommended in the 1989 RDAs. The folate allowance was lowered in the tenth edition of the RDAs because it was recognized that diets containing approximately half the RDA listed in the ninth edition maintained both an adequate folate status and ample liver stores (NRC, 1989b). Similarly, the RDA for vitamin B12 in the tenth edition was reduced by one-third for the adult age groups. The committee that wrote the RDAs commented that this was a conservative approach, which left the recommendation at approximately twice the level deemed safe by the Food and Agricultural Organization of the United Nations (FAO, 1988). The MRDAs (see Table 1-1) directly reflect the values of the ninth edition and presumably can be revised downward in a similar fashion without undue concern about the levels needed for work in hot environments.

Vitamin C

There is some evidence that increased intake of vitamin C may help to reduce heat stress during acclimatization, particularly in those individuals who may have low intakes that are nevertheless considered to be in the adequate range. There is some limited evidence that excess vitamin C may adversely affect the absorption of vitamin B12. The recommended dietary allowance for vitamin C remained at 60 mg per day in the tenth edition of the RDAs (NRC, 1989b). Vitamin C levels in the MRDAs directly reflect the RDAs for this nutrient (see Table 1-1). More research is needed before any conclusions can be drawn.

Fat-Soluble Vitamins

At present there is no evidence that requirements for fat-soluble vitamins increase for people working in hot environments. Vitamin D levels appear adequate for work in hot environments, and the exposure to sunlight in these climates would likely be adequate to meet any increased need that might exist.

Vitamins A and E, as well as vitamin C, function as antioxidants and may be useful in the reduction of lipid peroxidation induced by exercise stress. However, there are no studies that have adequately examined this issue. Intakes of vitamins A, D, and E, as recommended in the 1989 RDAs, appear adequate to meet the requirements for military personnel performing their duties in hot environments. The MRDAs are based directly on the 1980 RDAs; the 1989 revision of the RDAs for these vitamins did not change appreciably (see Table 1-1). Further studies on whether these vitamins will be important as antioxidants for those living and working in a hot environment are warranted.

Suggested Citation:"1. Introduction and Backgrounds." Institute of Medicine. 1993. Nutritional Needs in Hot Environments: Applications for Military Personnel in Field Operations. Washington, DC: The National Academies Press. doi: 10.17226/2094.
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Summary

It appears that the intake of vitamins at levels recommended in the 1989 RDAs and in the current MRDAs is adequate for military personnel working in hot environments. Operational rations are the primary source of nutrients during the early stages of military deployment and may be used for extended periods, as apparently was the case in Operations Desert Storm and Desert Shield. It is important therefore that operational rations be formulated in accordance with the MRDAs and that the acceptance of all ration components—particularly those that may be the principal carrier of vitamin fortification—be such that these intakes are achieved over extended periods of use.

Minerals

Chapter 7 presents a review of the effects of exercise and heat on mineral metabolism and requirements. Current data are not adequate to determine under what conditions strenuous exercise or heat, or both, increase mineral requirements beyond the levels set by the MRDAs. A major difficulty is that past research on this topic has focused on sweat losses, plasma changes, and mineral balances rather than on biochemical indicators of nutritional status and functional indicators such as performance, immunity, antioxidant defense, resistance to injury, and recovery from illness or trauma.

There is no doubt that during profuse sweating (>5 liters per day), mineral losses can be substantial. In some experiments, the concentrations of minerals in plasma increased with intense exercise, whereas in other cases plasma mineral levels decreased, accompanied by substantial tissue redistribution (see Chapter 7). The significance of plasma changes is not clear. Likewise, reduced urinary excretion, which has also been observed following profuse sweating, can reflect tissue conservation or reduced tissue stores, or both. In most of the studies carried out to date, the effect of exercise has not been separated from the effects of increased ambient temperature. Moreover, there are few data that demonstrate beneficial effects from mineral supplements on either biochemical or functional indicators.

Sodium, Potassium, and Chloride

As discussed extensively in a previous report of the CMNR, Fluid Replacement and Heat Stress (Marriott and Rosemont, 1991), sweat losses of sodium, potassium, and chloride can be substantial under conditions of profuse sweating. For sodium, the problem is particularly acute for people who are not heat acclimatized (see section: Sodium Levels for Work in the Heat in this chapter). The committee previously concluded that there are circum-

Suggested Citation:"1. Introduction and Backgrounds." Institute of Medicine. 1993. Nutritional Needs in Hot Environments: Applications for Military Personnel in Field Operations. Washington, DC: The National Academies Press. doi: 10.17226/2094.
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stances in which the performance of military personnel would be improved by the use of electrolyte-carbohydrate beverages (see Marriott and Rosemont, 1991). In these cases, such beverages should provide approximately 20 to 30 milliequivalents (mEq) of sodium and 2 to 5 mEq of potassium per liter, with chloride as the only anion.

Iodide, Chromium, and Selenium

Sweat losses of iodide, chromium, and selenium are appreciable with intense exercise or in a hot environment. In the case of iodide, the losses that occur during profuse sweating make the use of iodized salt highly desirable. With intense exercise, plasma chromium increases, urinary chromium decreases, and plasma selenium decreases. As discussed above, the significance of these changes for the mineral status of an individual is not clear.

Iron

Iron deficiency can reduce physical performance; it has also been reported to result in a defect in thermoregulation. Losses of iron during heavy sweating can be considerable. Although anemia (i.e., "sports anemia") may occur during training, it is transitory and due in part to plasma volume expansion. Iron deficiency anemia is not commonly seen with chronic intense exercise, although low serum ferritin levels have been observed. Low serum ferritin levels are an indication that iron stores are not high and that an acute loss of iron or a decrease in intake will almost certainly result in anemia. However, caution must be employed when using iron supplements because of their reported adverse effect on zinc absorption and the potential for creating iron overload in some individuals if used for a prolonged period.

Zinc

Sweat losses of zinc can present a significant problem for military personnel in a hot environment, whether they are exercising or not. Intense exercise has been observed in some instances to increase, and in others to decrease, plasma zinc concentrations. The low plasma zinc values could also result from an acute metallothionein-induced sequestration of zinc within hepatic cells. Such interleukin-1 (IL-1) generated zinc redistribution occurs during many stress situations, and does not cause loss of zinc from the body. The frequent observation of lowered plasma zinc levels during chronic and prolonged exercise, when considered together with the high sweat losses that have been observed, suggests the appropriateness of zinc supplements

Suggested Citation:"1. Introduction and Backgrounds." Institute of Medicine. 1993. Nutritional Needs in Hot Environments: Applications for Military Personnel in Field Operations. Washington, DC: The National Academies Press. doi: 10.17226/2094.
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under these conditions. However, no clear evidence indicates whether zinc in excess of the MRDAs should be recommended. Once again, caution is required in considering supplementation: zinc supplements reportedly lower the absorption of copper, a nutrient that already may be marginal in many diets (NRC, 1989b).

Magnesium and Copper

Sweat losses of magnesium and copper, like those of other trace elements, can be appreciable. Negative nitrogen, potassium, and magnesium balances were produced in young men by diminished dietary intake, increased urinary excretion, and sweat losses during hyperthermia induced by humid heat in an experimental chamber (Beisel et al., 1968). In some studies involving intense exercise the plasma levels of these elements have increased and in other studies they decreased. It should be noted however, that plasma levels of both elements are not a useful measure of body stores.

Calcium and Phosphorus

Calcium and phosphorus were not addressed specifically at the workshop. Based on current knowledge, the existing MRDAs for these nutrients appear to be sufficient for nutritional needs even during profuse sweating in a hot environment.

Summary

It is unclear whether mineral losses resulting from chronic heat exposure or exercise, or both, result in compromised health and performance (endurance capacity, immune defense, antioxidant defense, or recovery from illness or trauma). This information is essential before the applicability of the MRDAs can be fully assessed for military personnel working in hot environments.

FACTORS THAT MAY INFLUENCE EATING PATTERNS, FOOD PREFERENCES, AND FOOD INTAKE IN HOT ENVIRONMENTS

Olfaction and Taste

Chapter 9 reviews flavor effects (taste-gustation, smell-olfaction) and trigeminal sensation including touch, temperature, and pain, as well as color and psychological factors that affect sensory aspects of food consumption. Subject variables (age, gender, ethnic group, disease state, etc.) were not explored.

Suggested Citation:"1. Introduction and Backgrounds." Institute of Medicine. 1993. Nutritional Needs in Hot Environments: Applications for Military Personnel in Field Operations. Washington, DC: The National Academies Press. doi: 10.17226/2094.
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The effects of changes in temperature on sensory perception and preferences have been examined by using several different approaches. On the one hand, the perceived intensity of sucrose solutions has been reported as greater (sweeter) at higher temperatures (Bartoshuk et al., 1982); alternatively, taste thresholds for salt at cold (0°C) and hot (55°C) temperatures did not differ markedly (Pangborn et al., 1970). The effects of increased temperature on suprathreshold intensity estimates are unclear. The threshold for detecting the four basic tastes reportedly varies in a U-shaped function with the minimum at 20° to 30°C. Thus, when food or beverages of low or threshold concentrations are heated to 30°C (86°F) or above, taste thresholds become more difficult to detect.

Cooling the tongue reduced the perceived intensity of the sweetness of sucrose and the bitterness of caffeine test solutions (Green and Frankmann, 1987). However, the perceived saltiness of sodium chloride (NaCl) and the sourness of citric acid were not affected. The temperature of the tongue reportedly was the critical factor in decreasing the sweetness and bitterness. Again, the measured responses for the four basic tastes after changes in tongue temperature were not the same.

Warming a familiar food reportedly ''enhances'' flavor and aroma, which suggests that for certain foods, warm temperatures can enhance immediate consumption (Trant and Pangborn, 1983). However, studies examining the effects of warming on subsequent intake have not been conclusive. Further studies with other types of foods and drinks are desirable to clarify relationships among environmental temperature and mode of presentation, familiar and novel foods, hot or cold temperatures, and immediate or delayed effects.

Most experiments have primarily employed model chemosensory stimuli rather than real foods and have manipulated stimulus temperature alone rather than stimulus temperature plus environmental temperature. In addition, other aspects of sensory responsiveness, such as the physical properties of smoothness, creaminess, and thickness, need to be examined in the context of stimulus and oral (tongue) temperature differences. Capsaicin and other chemical irritants appear to increase the sensory impact of foods (Rozin et al., 1982). It is worth noting that these foods tend to be consumed predominately in hot environments.

It is unclear whether a dry-hot versus a humid-hot environment produces differential sensory responses or food consumption. The degree to which subject differences (for example, weight or fatness, age, and gender) affect responses is unknown. Under experimental conditions, decreased sweating responses have been demonstrated in older individuals at 50 percent relative humidity (Robinson et al., 1986; see previous discussion in this chapter). A question relevant to this report therefore is, How do these subject variables affect sensory responses?

Suggested Citation:"1. Introduction and Backgrounds." Institute of Medicine. 1993. Nutritional Needs in Hot Environments: Applications for Military Personnel in Field Operations. Washington, DC: The National Academies Press. doi: 10.17226/2094.
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When discussing preferences for sweet and fat tastes, it should be emphasized that although a preference for sweet foods may be universal, a preference for fat appears to be specific to the individual and therefore a learned response. Some preferences for combinations of fat and carbohydrate foods have been examined (typically dessert type foods), but preferences for foods high in protein and fat have not been examined in detail. Gender differences in preferences for different macronutrients also have not been well studied. In addition, the intake of specific foods during different seasons, such as fresh corn during the summer months, appears to be primarily a function of availability and learning. (See additional discussion of seasonality of food intake below).

Hotter-temperature foods generally are rated as having greater intensity of taste and smell. Further studies with various types of drinks and foods are necessary to clarify whether temperature or mode of presentation can, in fact, influence satiety. Largely unexamined is the degree to which variations in the temperature of foods (rather than preloads) or variations in ambient temperature influence the intake of specific meals or the intake of subsequent meals.

Appetite

It is important to distinguish among appetite, hunger, and intake. "Appetite" will be used here to refer to the subjective desire to eat, whereas "hunger" usually refers to a more objective deprivation state. In humans, it is possible to distinguish between what a person wants (appetite) or needs (hunger) and what a person eats (intake). These distinctions are useful because large-scale or clinical human studies often involve combinations of measures, including choice or preference ratings, in discussions of food intake. These clearly are not always the same. For example, preference ratings do not accurately predict food intake.

Thermoregulation

One of the body's major physiological concerns is thermoregulation, the maintenance of body thermoneutrality. Eating appears to be a major contributor to maintaining body heat. The "thermostatic" hypothesis of feeding is that the body experiences a temperature-dependent variation in energy needs that should be reflected in appetite. If normal food intake continues under conditions of heat stress, the additional heat that must be dissipated as a result of the amounts ingested may lead to a breakdown in the body's heat mechanisms (see Chapter 15).

In a series of studies by Hamilton (1963a), rats exposed to a temperature of 35°C ate only 2 grams of food during the first 24 hours, compared

Suggested Citation:"1. Introduction and Backgrounds." Institute of Medicine. 1993. Nutritional Needs in Hot Environments: Applications for Military Personnel in Field Operations. Washington, DC: The National Academies Press. doi: 10.17226/2094.
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with a previous intake of more than 20 grams at 24°C; mild (32°C) and severe (35°C) heat stress over 21 days resulted in a continued lower level of food intake. At 40°C, rats stop eating altogether; if force-fed by intubation, they suffer heat stress and occasionally die (Hamilton, 1967). Studies in a number of experimental animals demonstrate cessation of eating at high temperatures, with the possibility that continued eating would probably lead to hyperthermia. The marked decrease in food intake is followed by a decrease in body weight and fat (Jakubczak, 1976). Reduced intake in the heat would thus seem to be adaptive. Keys and coworkers (1950) found that their semistarved volunteers complained of the cold even in warm summer weather. This indicates that a reduction in food intake may actually be a mechanism to cope with hot environments. There is thus significant research in various models to support the observation that food intake drops as the environmental temperature increases from normal to hot ambient temperatures, followed by a decrease in body weight.

Heating the preoptic and anterior hypothalamic regions in animals appears to act in much the same fashion as external cues to inhibit eating (Andersson and Larsson, 1961). Opposite results, however, were obtained by Spector and colleagues (1968). Heating of the preoptic medialis region caused increased eating when the temperature of the area was raised to 43°C; decreased eating occurred when the ambient temperature was raised to 35°C. Local temperature in the anterior hypothalamic area reportedly drops at the onset of eating in the monkey, which is the opposite of what would be expected (Hamilton, 1963b). It appears that the effect of brain temperature on eating may be more a result of external ambient temperature than of localized temperature changes. In addition, it may be due to the rate of heat flow from the body's core to the periphery or vice versa, as no single temperature uniquely governs the level of food intake (Spector et al., 1968).

Thermogenic Effect of Food

Studies of the theory that animals stop eating to prevent hyperthermia have noted differences in the resulting thermic effect of the food ingested (dietary induced thermogenesis, or specific dynamic action) as a possible triggering mechanism (Chapter 15). The caloric intake of rats that were fed special diets during mild heat stress was inversely related to the thermogenic effect of the diet selected (Hamilton, 1963a). It appears that fats may be the preferred energy source in heat stress (Salganik, 1956) and that in conditions of severe heat, rats avoid protein because of the comparatively high amount of heat it creates (Hamilton, 1963a). Under this theory, body temperature should be highly correlated with hunger and satiety, yet there appears to be no consistently observed relationship between them. LeBlanc

Suggested Citation:"1. Introduction and Backgrounds." Institute of Medicine. 1993. Nutritional Needs in Hot Environments: Applications for Military Personnel in Field Operations. Washington, DC: The National Academies Press. doi: 10.17226/2094.
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and Cabanac (1989) recently demonstrated that the postprandial thermogenic effect of food intake has both a cephalic and a gastrointestinal phase. The cephalic effect (which was evident in subjects who did not even swallow the food but merely chewed and spit it out) was stronger than the subsequent gastrointestinal effect following consumption. Some researchers (Penicaud et al., 1986) argue that temperature control has primacy over food intake control.

Dehydration

Osmotic factors have also been shown to affect food intake (see discussion in Chapter 15). Ingestion or intubation of hypertonic solutions results in decreased food intake in rats (Ehman et al., 1972; Kozub, 1972). This reduction in intake is a protective mechanism that is demonstrated under conditions of total water deprivation, which drastically reduces eating in most species (Thompson, 1980). It appears that, to a large extent, decreased food intake in unacclimatized subjects in tropical climates may be mediated by hypertonicity associated with initial dehydration and may improve as acclimatization occurs (Bass et al., 1955).

Influence of Physique

Chapter 10 provides a discussion of the evolutionary aspects of survival in hot environments. For example, a bulkier shape minimizes heat loss, because the bulkier animal has a relatively smaller ratio of skin surface to metabolically active bulk and skin surface determines heat dissipation (Belief, 1977). Physical anthropologists (see Belief, 1977, for a review) have long noted a correspondence between physique and climate. The fact that linear physiques generally do better in the heat may be seen as an evolutionary selection principle. The endomorphy of a population, however, is not correlated with mean annual temperature so much as with mean January temperature (in northern latitudes) (Beller, 1977). It is quite possible that adaptation to one sort of challenge may prove to be contra-adaptive in some other sense. Animals and people who maintain a body weight below the set-point show aberrant eating patterns, hyperemotionality (including irritability), distractibility, and a reduced sex drive (Nisbett, 1972).

Heat as a Stressor

Body temperature increases under acute stress, which may elevate the thermoregulatory set-point—or simply add metabolic heat. Normal eaters in both laboratory and field settings respond to stress by decreasing their food intake. Not only does the stress of a hot environment involve the need for

Suggested Citation:"1. Introduction and Backgrounds." Institute of Medicine. 1993. Nutritional Needs in Hot Environments: Applications for Military Personnel in Field Operations. Washington, DC: The National Academies Press. doi: 10.17226/2094.
×

thermoregulation and maintenance hydration, but it encompasses psychological stress as well. It is difficult to ascertain the difference between appetite and hunger in animals; in humans, however, for whom other factors, such as situational stress, may affect hunger and appetite differently, it may be important to differentiate the two. Both, singly or in combination, will affect food intake. Given the stress expected with military excursions into hot and tropical environments and the rapid deployment that troops often experience, any evaluation of food intake and habits in hot environments should include all possible stressors to determine the potential combined effects of these factors.

Food Preferences in Hot Environments

In studies to determine the preferred types of foods for consumption in hot environments, palatability per se has not been measured in hot versus cold environments. In temperate environments, studies show that humans have an expressed preference for fats and sweets (Drewnowski et al., 1989). There is currently a dearth of solid experimental research on human food consumption in response to variations in heat. In terms of the proportions of various macronutrients in the diet, protein as a percentage of energy remained constant in military nutrition studies conducted during different seasons over the course of World War II (Edholm et al., 1964; Johnson and Kark, 1947). These data were supported by animal studies (Donhoffer and Vonotzky, 1947). Rolls and others (1990), however, found almost no relation between how hungry or satiated people claimed to be and how much they subsequently ate.

One classic study in food intake changes among military personnel was conducted by Edholm and Goldsmith (1966). Two similar groups of military men were followed in carefully controlled conditions. One group had spent a year in Bahrain prior to the experiment; the second group was first studied for 12 days in the United Kingdom and then flown to Bahrain, where it joined the first group. Both groups of subjects spent the first 4 days engaged in hard work, the next 4 days engaged in lighter work, and the final 4 days engaged in hard work in tents and outside. Both groups then returned to the United Kingdom for a repeat of the 12-day protocol.

In Bahrain the daytime temperature rarely went below 30°C (86°F), with a relative humidity of 40 to 90 percent. The mean food intake in Bahrain was approximately 25 percent less than that in the United Kingdom; however, the percentages of calories from fat and carbohydrate were similar, as was the percentage of calories from protein. While in Bahrain, the unacclimatized group lost an average body weight of 2.5 kg over 12 days, and the acclimatized group lost 1.1 kg. The lost weight was not quickly recovered upon the groups' return to the United Kingdom. This result led

Suggested Citation:"1. Introduction and Backgrounds." Institute of Medicine. 1993. Nutritional Needs in Hot Environments: Applications for Military Personnel in Field Operations. Washington, DC: The National Academies Press. doi: 10.17226/2094.
×

researchers to believe that the caloric deficit, rather than the state of hydration, was responsible for the majority of the weight lost in the hot environment.

A number of military studies conducted by the U.S. Department of Defense have looked at garrison feeding, food choices, and food waste; they have also conducted tests of the rations that have been developed. In each case these studies have been conducted during only one season, usually fall or spring; thus comparative information regarding summer food choices is not available.

A few studies have investigated the relationship between seasonal changes in body composition and seasonal changes in caloric intake and body weight. Some of these studies have also evaluated nutrient intake in adults by season of the year in hot environments. Decreased intake of several nutrients, such as vitamins A and C (Aldashev et al., 1986) and protein, vitamin C, and total energy (Mommadov and Grafova, 1983), has been reported. However, these studies did not evaluate changes in food preferences or appetite.

Empirical data, based on observations and practices in food service in both the military and the commercial sector, indicate a change in food preferences during seasons associated with elevated mean environmental temperatures. Few basic studies have attempted to specifically address food patterns that change according to season in self-selected diets. In a study of seasonal variations in self-selected lunches in a large employee cafeteria in Maryland, Zifferblatt and colleagues (1980) found a decreased selection of starches and cooked vegetables with increased purchases of fruits, salads, yogurt, and cottage cheese as the noontime temperature rose. As the temperature increased, average caloric purchases also tended to decrease. The workplace cafeteria, along with the work areas of most of the employees, was kept at 72°F; and thus, the environmental temperature of the location at which the food was ingested may have only moderately influenced workers' appetites.

National surveys have investigated the food consumption patterns of Americans, but they have not gathered recent data on the same individuals or on individuals in similar geographic areas at different times of the year. Such data would help determine whether changes of season, and thus changes in environmental temperatures, affect appetite (resulting in changes in food intake) or food selection patterns. The 1977–1978 Nationwide Food Consumption Survey reported three-day food intake information for about 36,100 individuals from a sample of households in 48 states that included four seasonal samples. The seasonal differences in average intakes of 10 major food groups were low—11 percent or less. For the major food groups, average intakes were typically higher in one season than in the three others with intakes of vegetables, fruits, and beverages increasing in the summer. Intake

Suggested Citation:"1. Introduction and Backgrounds." Institute of Medicine. 1993. Nutritional Needs in Hot Environments: Applications for Military Personnel in Field Operations. Washington, DC: The National Academies Press. doi: 10.17226/2094.
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of legumes, sugars and sweets, meats, and eggs was at a seasonal low in the summer. Data on the intake of food subgroups appear to mirror the seasonal availability. With the subgroups of fruits, for example, more noncitrus fruit was eaten in the summer than in other seasons, the intake of citrus fruits and juices was highest in the winter months, while apple intake was highest in the fall with a drop progressively from winter through summer. In contrast, the average intake of bananas was the same in all four seasons. Intake of fats and oils varied little across the seasons with a slight drop in reported intake of table fats in the summer but this corresponded with a slight increase in the use of salad dressings in spring and summer months. Thus, people do alter their eating behavior during the year, but to some extent these alterations are based on availability and prices of food items. Whether changes also occur in appetite (considered to be the desire to eat) is unknown from these data.

There was no discussion at the workshop or in the literature surveyed by the committee of the interaction of ethnic food preferences with appetite and intake or related variables in hot environments. This topic is undoubtedly one that should be considered for future research in light of the changing ethnic composition of the military services.

Summary

Herman's presentation in Chapter 10 raises several questions about appetite and provides a philosophical base from which to study the problem of changes in appetite in the heat. The studies cited suggest that the thermoregulatory value of decreased food intake in hot environments should be stressed. The percentage of body weight lost and the nutritional adequacy of the diet are thus major concerns. Studies that have documented voluntary decreased food intake in individuals in hot environments and animal studies that have supported the concept that decreased food intake is an adaptive mechanism to ameliorate the increased need for thermoregulation, make it clear that optimal nutrition is compromised if intake decreases to the extent of consuming inadequate levels of key nutrients.

SOCIAL AND PSYCHOLOGICAL INFLUENCES ON FOOD INTAKE DURING MILITARY OPERATIONS

Situational Influences on Food Intake

A common observation is that food consumption is reduced in battle situations. In attempting to address this issue, the Army is confronted by a set of complex interacting variables (the lack of palatable or familiar foods, environmental stress, time of meals, fatigue, etc.) that could lead to reduced

Suggested Citation:"1. Introduction and Backgrounds." Institute of Medicine. 1993. Nutritional Needs in Hot Environments: Applications for Military Personnel in Field Operations. Washington, DC: The National Academies Press. doi: 10.17226/2094.
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food intake. Another difficulty is the problem encountered trying to simulate combat conditions in the laboratory. In Chapter 11, Hirsch and Kramer ask, What are the limiting factors that lead to this drop in food intake? These authors report that the meals, ready-to-eat rations (MREs) are not actively disliked by troops and thus conclude that unpalatability must be only one limiting factor. Their data suggest that when normal volunteers are fed a consistent diet of MREs, food intake, which initially is no different from that of controls, drops by the sixth week. Lack of convenience, difficulty of preparation, poor palatability, and lack of menu variety are all factors that could contribute to this decreased intake. Situational influences—for example, meal location (eating in the field versus in dining facilities)—appear to have primary importance in determining the amount of food eaten.

Research indicates that the environmental factors impinging on food intake are often confounded with social factors. Social influences, time of day (breakfast versus dinner), and ease of preparation or accessibility of the food (see Chapter 11) also appear to be important influences on the amount of food consumed. Field troops in Operations Desert Shield and Desert Storm reportedly ate more food when they were served hot meals from the kitchen than when they ate self-prepared meals or MREs. Laboratory studies suggest that food intake can be increased when new foods are offered after satiation with familiar ones, when variety is increased, and when individuals eat together in small groups. The stimulation afforded by the sounds associated with eating in social settings also leads to increased meal size (de Castro and de Castro, 1989; Klesges et al., 1984).

Animal data (Collier 1989) and human data (Levitz, 1975; Meyer and Puddel, 1977; Meyers et al., 1980) support the contention that even relatively minor changes in accessibility and the effort required to obtain food can lead to significant changes in food consumption. Other studies suggest that greater amounts of food are consumed when presented in a smorgasbord fashion (i.e., self-selection), compared with a more typical restaurant presentation (Stunkard and Kaplan, 1977; Stunkard and Mayer, 1978). Eating in small groups facilitates both increased food consumption and increased meal duration. In addition, individual group members can, by statement or example, influence the amount consumed by others (Engell et al., 1990; Polivy et al., 1979), which suggests that food acceptance can be increased by explicit examples.

In summary, evaluation of the suitability of the MREs as a hot-weather ration requires careful consideration not only of their nutrient content but of the social influences on eating. Modification of troop feeding practices based on the results of ration field trials can potentially increase the intake of MREs through the enhancement of effective social stimuli.

Suggested Citation:"1. Introduction and Backgrounds." Institute of Medicine. 1993. Nutritional Needs in Hot Environments: Applications for Military Personnel in Field Operations. Washington, DC: The National Academies Press. doi: 10.17226/2094.
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Field ObservatiOns of Food Intake

During the workshop, military personnel indicated that the environment in which food is provided, the soldier's understanding of the ration's nutrient content, and the form of the ration are as important to the soldier's dietary intake as the ration's actual nutrient composition. These comments were primarily based on anecdotal information provided by two short-term observers during Operation Desert Storm and should be considered in the context of other information that has been provided from other sources who were also present during the deployment.

Environmental Concerns

In a hot, dry environment, sand became an unwelcome but constant additive to all food items wherever food handling was involved. Protection from tents that had been set up decreased the amount of sand to some extent, but did not eliminate it entirely. As a result, some of the steps required for food preparation of field rations, such as heating and rehydration, may not be possible in a desert setting because of the introduction of sand as a contaminant.

It also appears that during the extremely hot part of the day, soldiers would not eat, although they would drink. When field kitchens became available, the time of meal service was adjusted as much as possible to coincide with cooler environmental temperatures. Even when they were hungry during the hotter periods of the day, soldiers often did not want to eat the kinds of food provided, but they did have ideas of what they would have liked to eat.

Dehydration and Constipation

A significant concern to soldiers during Desert Storm and Desert Shield was constipation; many held the belief that consuming the ration would result in constipation. In the recent Gulf War, constipation apparently was prevented by strict adherence to the water discipline regimen that had been established to prevent significant dehydration. However, the distant placement of sheltered latrines in the field resulted in decreased fluid consumption after dark to prevent having to get dressed, put on gear, and go through the dark to the latrine. Female soldiers in particular restricted their fluid intake; male soldiers could urinate in more convenient unsheltered latrines. In addition, some soldiers voluntarily restricted fluid intake prior to operations (i.e., when they were going on convoys, flying a mission, etc.). This practice resulted in the potential for fluid restriction both at night and dur-

Suggested Citation:"1. Introduction and Backgrounds." Institute of Medicine. 1993. Nutritional Needs in Hot Environments: Applications for Military Personnel in Field Operations. Washington, DC: The National Academies Press. doi: 10.17226/2094.
×

ing the day for the same individual. Some soldiers also reported self-medication to prevent defecation while on 3-to 4-day missions.

Food Preferences

As might be expected, foods that were commercially labeled—even though they were not as heat stable and in certain cases showed evidence of some deterioration—were preferred to the field ration (MRE). Apparently, soldiers had greater confidence—in terms of meeting their appetite and nutritional needs—in foods that seemed to be the same (including packaging) as those they had consumed at home. This was particularly true for flavored beverage powder. A significant concern of soldiers was the compatibility of foods in each MRE. For example, an MRE that contained a slice of ham as an entree did not come with cheese, which would have been preferable for making a sandwich. It instead was packaged with peanut butter. Likewise, peanut butter and jelly were not packed together in any MRE pouch because they were both considered "spreads." To overcome this problem, soldiers would "rob" one MRE pouch to obtain the other spread and then discard the remaining contents.

Because the Desert Shield operation lasted from summer to winter, it was necessary to provide foods appropriate to the prevailing climatic conditions. In the summer, the amount of beverage bases provided in each MRE pouch was not adequate to flavor all the water that was consumed. (Soldiers deemed it necessary to flavor the water because of its unpalatability.) Individuals were drinking from 8 to 9 bottles per day; thus, three beverage bases were needed to flavor 1 1/2 liters of water per bottle. Likewise, during the colder season, the soldiers all wanted cocoa, which had been discarded during the summer. Because cocoa was not included in every MRE pouch, often a hot drink choice was not available with each meal. Concomitantly, in order to have a hot drink, soldiers in the field who did not have access to kerosene heaters needed hot tabs.6 This had a direct effect on the acceptability of the rations. In 130°F weather, soldiers did not want a hot meal but rather the MRE entrees that were intended to be eaten without heating. In essence, they would have preferred entrees that were cool or cold.

Social/Psychological Aspects of Eating

The use of individual MREs decreased socialization because there was no need for a field kitchen and a common mess. To soldiers with little access to information about what was going on in the war, this practice

6  

Hot tabs are small portable elements for warming ration components. They are included only with rations that require heating.

Suggested Citation:"1. Introduction and Backgrounds." Institute of Medicine. 1993. Nutritional Needs in Hot Environments: Applications for Military Personnel in Field Operations. Washington, DC: The National Academies Press. doi: 10.17226/2094.
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decreased morale, because the opportunity for bringing the unit together on at least a daily basis was not available. Thus, the use of MREs, in decreasing social interaction, acted as a psychological stressor.

Caffeine consumption changed dramatically depending on the situation of the troops, and the use of smokeless tobacco increased as a result of light (fire) discipline and the fire hazards associated with smoking. It was reported by the two observers that these changes affected eating patterns but no quantitative information was available. In addition, the use of meal shifts changed normal times of meals and the types of food associated with certain meals, as well as the desire, among some soldiers, to have a meal.

Nutrition Understanding

The observations presented concerning Desert Shield and Desert Storm reinforced the committee's belief that a broad program of educating soldiers with regard to the ration and its contents, and how it would influence their desire to maintain or change body weight, was needed on the unit level. Many soldiers apparently read the packaging labels on their foods; this could be a vehicle for additional information and education. It may be appropriate to determine the need, if any, for a general policy regarding vitamin and mineral supplementation. Many soldiers reported their consumption of supplements from personal supplies or packages that were requested from home. The CMNR recommended in an earlier review of the MREs and T rations (NRC, 1986) that the distribution of vitamin and mineral supplements was unnecessary and ill-advised if the rations were well fortified by meeting the MRDAs and if the soldiers ate the rations in sufficient quantities to meet their caloric needs.

Summary

The following recommendations, gleaned from anecdotal comments of soldiers in the field during Operations Desert Shield and Storm, were discussed informally at the workshop: (1) pouch bread should be available at every meal, if at all possible; (2) more eat-on-the-go-type foods are necessary, such as cookie bars or snack items that could be saved and eaten later; (3) food items within the individual MREs should be packaged together so that they form complementary alternative foods, such as sandwich ingredients; (4) although salt packets are rarely used, other condiment packets such as pepper or mustard should be provided to add variety to the meal; (5) MREs should be unitized, along with sundry packs, supplement packs, and other such items, so that each pallet has a variety when it is moved forward to the field of operation. Although, it must be recognized that the decision to fortify certain foods within the MRE places the onus on the soldier to eat

Suggested Citation:"1. Introduction and Backgrounds." Institute of Medicine. 1993. Nutritional Needs in Hot Environments: Applications for Military Personnel in Field Operations. Washington, DC: The National Academies Press. doi: 10.17226/2094.
×

that specific item in order to meet the MRDA, practice in the field indicates that this was not always achieved.

CONCLUSION

The magnitude of the stress imposed by exercise in hot environments depends on an individual's nutritional status and his or her ability to regulate metabolic events and dissipate heat. Increased heat production, increased sweat losses, and inadequate hydration predispose soldiers in hot environments to dehydration. It is of paramount importance that hydration be preserved to maintain performance. Although it is generally recognized that some losses of minerals and vitamins occur during intense exercise in hot environments, available information suggests that the present MRDAs are adequate for achieving optimal work performance and preventing overt clinical deficiencies. The absence of sensitive, reliable indicators of many nutritional inadequacies limits the detection of subtle changes in dietary practices on health and performance. The interrelationships of exercise in hot environments and nutrient requirements, as influenced by eating behavior, age, gender, and body composition, are unclear. These factors clearly deserve additional investigation. There is substantial evidence that food intake decreases markedly as the environmental temperature increases, which probably reflects the need to control thermogenesis. It thus becomes prudent to provide palatable, nutrient-rich foods that reduce the monotony of eating during extremely hot conditions. Quoting E. R. Buskirk (Chapter 6): ''Finally, as the nutritional situation during the recent operations of Desert Shield and Desert Storm is reviewed, a comment by R. M. Kark (1954) comes to mind:

'Field studies have shown that physical deterioration in soldiers may be due to inadequate nutrition, but perhaps what is more important, they have shown that loss of military efficiency through inadequate nutrition is most often due to inadequate planning, catering or supply, and to inadequate training or indoctrination.... Maintaining good nutrition is like maintaining freedom of speech or democracy. You need eternal vigilance to make it work.'''

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×

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This volume examines the current state of knowledge concerning the influence of a hot environment on nutrient requirements of military personnel. A parallel concern is ensuring that performance does not decline as a result of inadequate nutrition.

The committee provides a thorough review of the literature in this area and interprets the diverse data in terms of military applications. In addition to a focus on specific nutrient needs in hot climates, the committee considers factors that might change food intake patterns and therefore overall calories. Although concern for adequate nutrition for U.S. soldiers in Saudi Arabia prompted the initiation of this project, its scope includes the nutrient needs of individuals who may be actively working in both hot-dry and hot-moist climates.

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