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2
Conclusions and Recommendations

CONCLUSIONS

As stated in Chapter 1, the Committee on Military Nutrition Research (CMNR) was asked to respond to 11 specific questions dealing with nutrient requirements for work in hot environments. The committee's responses to these questions appear below:

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

Sensible and insensible water losses are increased markedly by work in a hot environment, resulting in an increased need for water. In general, energy requirements decline somewhat in a hot environment, primarily because of the tendency to reduce activity. However, other factors, including the degree of acclimatization, may modify the body's energy requirement in the heat. In addition, there is considerable individual variation. Recent evidence suggests that slight increases in protein may be required for work in hot environments; however, the Military Recommended Dietary Allowance (MRDA) for protein already includes an amount sufficient to meet this increased level given adequate consumption of kilocalories. Significant losses of several minerals occur with profuse sweating; however, current methodology does not provide data that indicate the need for measurable increases in requirements. Based on losses in sweat and the potential for dehydration, people working in hot environments may require additional sodium and other electrolytes. Vitamin requirements do not appear to increase with ex-



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Nutritional Needs in Hot Environments: Applications for Military Personnel in Field Operations 2 Conclusions and Recommendations CONCLUSIONS As stated in Chapter 1, the Committee on Military Nutrition Research (CMNR) was asked to respond to 11 specific questions dealing with nutrient requirements for work in hot environments. The committee's responses to these questions appear below: What is the evidence that there are any significant changes in nutrient requirements for work in a hot environment? Sensible and insensible water losses are increased markedly by work in a hot environment, resulting in an increased need for water. In general, energy requirements decline somewhat in a hot environment, primarily because of the tendency to reduce activity. However, other factors, including the degree of acclimatization, may modify the body's energy requirement in the heat. In addition, there is considerable individual variation. Recent evidence suggests that slight increases in protein may be required for work in hot environments; however, the Military Recommended Dietary Allowance (MRDA) for protein already includes an amount sufficient to meet this increased level given adequate consumption of kilocalories. Significant losses of several minerals occur with profuse sweating; however, current methodology does not provide data that indicate the need for measurable increases in requirements. Based on losses in sweat and the potential for dehydration, people working in hot environments may require additional sodium and other electrolytes. Vitamin requirements do not appear to increase with ex-

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Nutritional Needs in Hot Environments: Applications for Military Personnel in Field Operations posure to a hot environment; however, few studies have examined this issue. In particular, the role of antioxidant vitamins (A, C, and E) in reducing lipid peroxidation induced by exercise in a hot environment should be examined. If such evidence exists, do the current Military Recommended Dietary Allowances (MRDAs) provide for these changes? The variations in nutrient requirements, including sodium, that may occur as a result of working—and sweating—in a hot environment are reasonably covered by the nutrient content of the MRDAs, because the MRDAs provide generous allowances over most nutrient requirements. If military rations are consumed in amounts that approximate energy expenditures, it is likely that the nutrient requirements of soldiers will be met. 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? Based on the evidence available at this time, the nutrient content of military rations does not need to be changed. Nevertheless, because appetite is depressed and food preferences and eating patterns are changed in response to short-term and long-term exposure to heat, changes should be made in ration components to enhance intake. Military feeding in hot environments needs to take into account what is known about these changes in food preferences and meal schedules. The components of the rations and field feeding environments should be adjusted to encourage consumption of military rations. Convenience, taste, and acceptability become all important. Specifically, are the meals, ready-to-eat (MREs) good hot weather rations? Should the fat content be lower? Should the carbohydrate content be higher? The nutritional composition of MRE rations is appropriate for use in a hot environment. There are no consistent data that suggest that the relative proportions of protein, carbohydrate, and fat should be altered. It is clear, however, that the experience gained during Operation Desert Storm regarding the acceptability of the various MRE rations and ration components needs to be evaluated. Significant components, including the entrees, in the MREs available in 1991 required heating to provide the most palatable meal. As noted in anecdotes from those conducting observations in the Persian Gulf area during hot weather, the shift in soldiers' food preferences to a desire for cooler items (salads, sandwiches, etc.) confirms that the MREs were not designed specifically for long-term consumption in hot climates. Data from animal studies show an increase in fat consumption in the heat, with a decrease in

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Nutritional Needs in Hot Environments: Applications for Military Personnel in Field Operations protein consumption. As a result of the organoleptic changes in fat within foods in conditions of extreme heat, however, food products that contain significant amounts of fat may be deemed unacceptable by soldiers and thus may not be consumed. The requirement for sustained physical activity in hot environments might result in the need for a modified ration that would encourage food consumption, for example, one lower in fat and higher in carbohydrate that could be consumed with little preparation. Heat-stable food products that are similar to those available in the private sector appear to be preferred by soldiers in terms of appetite. In designing MRE rations for use in hot environments, information from the experience gained during Operations Desert Shield and Desert Storm should be combined with what is known about how food preferences change in the heat. Moreover, factors other than ration composition that may influence food intake need to be considered. These include the availability of potable liquids in generous supply, the eating situation of troops (i.e., alone or in groups), the time of day when food may be offered, and the convenience of consuming the rations. Nonnutritional factors such as these can have a significant influence on ration intake. What factors may influence food intake in hot environments? The major factors that appear to influence food intake in hot environments are the need to maintain body temperature (through decreased intake to reduce the thermic effect of food) and the apparent relationship between decreased body weight and decreased body temperature. With the hydration regimens in place in the military, which appear to encourage adequate fluid intake, and the awareness among military personnel of potential heat stroke, the observation in laboratory animals of markedly decreased food intake to prevent hyperthermia is probably not a significant concern within the military population. Other factors such as psychological stress may further depress food intake. In addition, the lack of a desire in hot environments to eat hot foods (even though their palatability may be greater than that of cold foods) and the concomitant increased desire to consume cold foods are documented somewhat subjectively in nationwide surveys of food intake of individuals from households in the U.S. general population during various seasons. The intake of food by humans in a hot environment may be further influenced by the availability of cool potable water, the time of day, the psychosocial environment, and ration components. To what extent does fluid intake influence food intake? Animal studies demonstrate that dehydration markedly decreases voluntary food intake and that forcing foods during dehydration results in increased mortality. Although there have been a few human studies of this

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Nutritional Needs in Hot Environments: Applications for Military Personnel in Field Operations question, it appears that rehydration is necessary in humans before depressed food intake returns to normal. To maximize the energy intake of military personnel in hot environments in which significant physical activity is required, maintenance of adequate hydration status should be a primary objective of all policies related to soldier readiness. Maintenance of states of proper hydration was also identified as the most critical issue facing soldiers in desert environments in an Army report on food management issues written during Operation Desert Storm (Norman and Gaither, 1991). The recent CMNR report Fluid Replacement and Heat Stress (Marriott and Rosemont, 1991) thoroughly addressed this issue. Is there any scientific evidence that food preferences change in hot climates? Several animal studies document changes in food preferences in hot environments. There are also a limited number of studies that show decreased caloric intake in humans when working in a hot environment. Most of these studies did not allow for acclimatization of subjects to the hot climate. In the one major study that did, food intake decreased less markedly in the acclimatized individuals, with no change in percent distribution of kilocalories from fat, carbohydrate, or protein. In the summer season food choices do change, but whether this is due to environmental temperature or other factors such as price and availability has not been well established. Thus, to date, most information on changes in food preferences in humans is limited to anecdotal observations or studies that were not balanced with respect to temporal adaptation to climatic change. Are there special nutritional concerns in desert environments in which the daily temperature may change dramatically? If rations are consumed in adequate amounts, no specific nutrient concerns need be addressed as a result of the dramatic changes in temperature that frequently occur in the desert. Adequate intake of fluid to avoid dehydration and help maintain food intake is obviously important. The levels of nutrients specified by the MRDAs appear to be adequate to meet the nutrient needs of soldiers if rations are consumed in appropriate amounts. Is there an increased need for specific vitamins or minerals in the heat? Although small increases may occur in the losses of B vitamins in sweat during work in hot environments, these losses do not appear to be great enough to justify increasing the requirement over that established in the MRDAs. There is limited evidence that vitamin C may have an effect in reducing heat stress during periods of acclimatization, particularly if the individual has had a low vitamin C intake prior to exposure to the heat.

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Nutritional Needs in Hot Environments: Applications for Military Personnel in Field Operations However, there is insufficient evidence at this time to recommend an increase in vitamin C beyond that currently supplied by the MRDA. Prolonged moderate-to high-intensity activity in hot environments will result in a significant loss of electrolytes (sodium, potassium, magnesium), particularly among troops who are not adapted to hot environments. However, if fluid intake is maintained to prevent dehydration and consumption of military rations is at or near energy requirements, sufficient intake of electrolytes should occur. Does working in a hot climate change an individual's absorptive or digestive capability? There is evidence that gastric emptying may be reduced during heat stress. Although the mechanisms responsible for this observation are unclear, they may be associated with dehydration, which frequently occurs when working in the heat, and with reduced splanchnic blood flow. Studies have also demonstrated that elevations in core body temperature can reduce stomach and intestinal motility. It is apparent that maintaining adequate fluid intake is important as an aid in reducing heat stress from working in a hot environment. Limited evidence suggests that net calcium absorption may be reduced as a result of increased fecal losses during profuse sweating while working in hot environments. Some investigators have reported reduced intestinal absorption during exercise. However, other studies, by using more direct techniques of segmental perfusion, have shown no effect of either exercise intensity or duration on fluid absorption. In short, individuals who are well trained, acclimatized to heat, and accustomed to endurance exercise seem to experience fewer symptoms of gastrointestinal stress than less well conditioned and acclimatized individuals. 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? Uncertainty exists about the influence on energy requirements of working in the heat (see Chapter 6). Submaximal exercise in a hot environment does not appear to have an impact greater than that occurring in a more comfortable environment. Maintaining adequate food intake in the temperature extremes of hot environments to meet caloric needs is a higher priority than concern over small differences in energy requirements. RECOMMENDATIONS On the basis of the papers presented by the invited speakers, discussion at the workshop, and subsequent committee deliberations, the Committee on

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Nutritional Needs in Hot Environments: Applications for Military Personnel in Field Operations Military Nutrition Research finds that the nutritional requirements for work in hot environments are not significantly different from those needed in more moderate conditions. The nutrient content of the military's operational rations is adequate to provide for any variation that may occur as a result of work in the heat. There are, however, significant concerns about inadequate intakes of soldiers engaged in field operations, exercises, or combat in that the nutrients actually consumed may be less than the amounts specified in the MRDAs. Special attention should be given to ensuring that the intake of rations by soldiers is adequate to meet caloric needs, thereby ensuring that each individual's nutrient requirements are met. Of primary consideration is maintaining adequate fluid intake to avoid dehydration and consequent decreased food intake. This topic has been addressed in a previous CMNR report, Fluid Replacement and Heat Stress (Marriott and Rosemont, 1991). The committee wishes to reiterate that water is the most important nutrient for maintaining the performance of the soldier. The committee offers the following recommendations regarding nutrient requirements for work in hot environments. The maintenance of adequate hydration should be the major objective of efforts to maintain the sustained performance of troops in hot environments. As recognized by current Army doctrine, water is an essential nutrient. Maintaining an adequate intake of operational rations should also be an important objective, particularly in hot environments, to ensure that troops will meet their nutritional needs over the course of extended military operations. Based on observations of the decreased food intake in hot environments, changes should be made in rations and their components to enhance appetite and food intake. These changes should include ensuring the delivery of a variety of ration options to avoid menu fatigue. Delivery systems and feeding situations should be designed to enhance intake and take into account the environmental factors, including psychosocial factors, that influence food consumption. The following should be considered: availability of cool, flavored, potable water, a cooling environment such as shade, time of day for meal service, the social situation during meals, ration preparation requirements, use of familiar commercial food products, and ethnic food preferences. Variations in ration components for different environments (hot-dry, hot-humid, moderate, and cold) should be evaluated.

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Nutritional Needs in Hot Environments: Applications for Military Personnel in Field Operations AREAS FOR FUTURE RESEARCH The Committee on Military Nutrition Research suggests a number of areas for future research within the military related to nutrition for soldiers working in hot environments. The CMNR believes that the military services, through their pool of volunteer personnel, offer an excellent and often unique opportunity to generate research data and statistics on the nutrition, health, and well-being of service personnel. These findings can be directly applied to improve both the health of military personnel and that of the general U.S. population. Future Research Needs The observed decreases in food intake in hot environments and the previous lack of research emphasis on this subject urge the investigation of factors that affect food intake in a hot ambient environment. Such factors include but are not limited to the following: environmental conditions in the dining situation such as meal setting, menu item variability, food item temperature, social setting, and meal timing and frequency; ethnic and gender differences in food preferences; the relationship of food preferences to climate, with a focus on carefully controlled studies of the same individuals in temperate and hot environments (both dry and humid); chemosensory perception of foods and menus in relation to climate; and composition of the ration, that is, proportion of fat, carbohydrates, etc. In addition to the application of current biochemical indicators, an important area of research is the development and validation of appropriate functional indicators of nutritional status, with an emphasis on vitamins and minerals for which sweat losses are significant. These functional indicators should relate to endurance, immunity, antioxidants, nutrient deficiencies, and recovery from illness/trauma. A particular concern would be the iron status of military women under conditions that produced significant sweating. The potential role in stress responses of higher dietary intakes of zinc, vitamin C, and other antioxidants could be explored further with emphasis on heat stress. Studies that focus on gastrointestinal function in the heat are important, in particular, the effects of various levels of militarily relevant physical activity and the interaction of physical activity with psychological stress and gastrointestinal function.

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Nutritional Needs in Hot Environments: Applications for Military Personnel in Field Operations More research is needed to evaluate the impact of adequate mineral intake on physical performance in a hot environment. Such research would allow the development of more specific recommendations concerning circumstances in which mineral supplements or food fortification is indicated. In particular, studies are needed that separate the effects of exercise from the effects of an elevated ambient temperature, and studies that evaluate the effects of higher levels of mineral intake on functional indicators. Does heat enhance satiety or impair hunger? These questions could be addressed through research that more specifically addresses whether the effect of heat on appetite suppression is expressed in terms of smaller meals-presumptive satiety effects—or less frequent meals—presumptive hunger effects. In light of animal studies of hypoxia, additional research appears warranted to evaluate whether the decreased human food intake in hot environments serves a protective metabolic effect. The committee has noted in a number of research projects presented for its review, that there is a decrease in food intake of military personnel under operational conditions regardless of environmental climate. Based on these results it is recommended that a study be conducted to determine why soldiers don't consume adequate amounts of food to maintain body weight under operational conditions, and to evaluate steps that may be taken to achieve adequate ration intake. The Committee on Military Nutrition Research is pleased to participate with the Division of Nutrition, U.S. Army Research Institute of Environmental Medicine, U.S. Army Medical Research and Development Command, in programs related to the nutrition and health of American military personnel. The CMNR hopes that this information will be useful and helpful to the Department of Defense in developing programs that continue to improve the lifetime health and well-being of service personnel. REFERENCES Marriott, B.M., and C. Rosemont, eds. 1991 Fluid Replacement and Heat Stress: Proceeding of a Workshop, 2nd printing. Washington, D.C.: National Academy Press. Norman, E.J., and R.M. Gaither 1991 Review of Army Food Related Operations in Hot Desert Environments. Technical Report Natick/TR-91/008. United States Army Natick Research, Development and Engineering Center. Natick, Mass.