Nutritional Needs in Hot Environments Applications for Military Personnel in Field Operations (1993) / Chapter Skim
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1. Introduction and Backgrounds
1. Introduction and Backgrounds
Pages 3-44
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, 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.
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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?
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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.
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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)
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, 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., 19851. 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 discrep2The 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.
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Nutrient Unit From MRDAsa From RDAsb Vitamins Vitamin K ,ug 70-140 65, 80C Biotin leg 100-200 30-100 Pantothenic Acid mg 4-7 4-7 Trace Elementsd Fluoride mg 1.5-4.0 1.5-4.0 Selenium lug 50-200 55, 70c Molybdenum mg 0.1 5-0.50 0.075-0.250 Copper mg 2-3 1.5-3.0 Manganese mg 2.5-5.0 2.0-5.0 Chromium ,ug 50-200 50-200 Electrolytes Potassium mg 1875-5625 2000 Chloride mg 1700-5100 750 aMRDAs = 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.
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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,
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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~.
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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)
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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)
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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.
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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.
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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.
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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.
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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.
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Clarkson (Chapter 81. 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.
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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.
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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.
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Z nc Sweat losses of zinc can present a significant problem for military personnel in a hot environment, whether they are exercising or not.
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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.
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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.
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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 151.
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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.
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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 (Belier, 1977~.
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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~.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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Kamon, and J.T. Krajewski 1980 Physiological responses of physically fit men and women to acclimation to humid heat.
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Bove, and E.R. Nadel 1983 Effect of blood volume on forearm venous and cardiac stroke volumes during exercise.
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van Middendorp 1990 The relative influence of physical fitness, acclimatization or state, anthropometric measures and gender on individual reactions to heat stress.
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1989b Recommended Dietary Allowances, Tenth Edition. Washington, D.C.: National Academy Press.
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Conn, and F Kusumi 1966 Reductions in cardiac output, central blood volume and stroke volume with thermal stress in normal men during exercise.
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Stunkard, A., and A.J. Mayer NUTRITIONAL NEEDS IN HOT ENVIRONMENTS Induced by preoptic heating 1978 Smorgasbord and obesity.
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Wilbur, and J.L. Pinsky 1980 Influence of ecologic events on cafeteria food selections; Understanding food habits.
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