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Nutritional Requirements for Work in Hot Environments

As a direct result of the movement of the Armed Forces into Saudi Arabia in the autumn of 1990 for Operation Desert Shield (which became Operation Desert Storm in 1991) and the deployment of military personnel in the harsh desert environment of the Middle East, the Committee on Military Nutrition Research was asked by the Division of Military Nutrition, U.S. Army Institute of Environmental Medicine (USARIEM) 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 scope of this project was defined beyond Desert Shield to include the nutrient needs of individuals who may be actively working in both hot-dry and hot-moist climates.

Relatively few studies over the past half-century have focused on the influence of heat on nutrient requirements and work performance that are relevant to the military. The Committee decided that the best way to review the state of knowledge in this diverse area was through a workshop with invited experts. These experts could provide an update on current knowledge and identify gaps in the knowledge base that might be filled by future research. A subgroup of the committee met in December 1990 to plan the workshop. The invitational workshop was held at the National Academy of Sciences on April 11–12, 1991.

The CMNR was asked to address 11 questions dealing with nutrient requirements for work in hot environments:



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Committee on Military Nutrition Research: Activity Report Nutritional Requirements for Work in Hot Environments As a direct result of the movement of the Armed Forces into Saudi Arabia in the autumn of 1990 for Operation Desert Shield (which became Operation Desert Storm in 1991) and the deployment of military personnel in the harsh desert environment of the Middle East, the Committee on Military Nutrition Research was asked by the Division of Military Nutrition, U.S. Army Institute of Environmental Medicine (USARIEM) 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 scope of this project was defined beyond Desert Shield to include the nutrient needs of individuals who may be actively working in both hot-dry and hot-moist climates. Relatively few studies over the past half-century have focused on the influence of heat on nutrient requirements and work performance that are relevant to the military. The Committee decided that the best way to review the state of knowledge in this diverse area was through a workshop with invited experts. These experts could provide an update on current knowledge and identify gaps in the knowledge base that might be filled by future research. A subgroup of the committee met in December 1990 to plan the workshop. The invitational workshop was held at the National Academy of Sciences on April 11–12, 1991. The CMNR was asked to address 11 questions dealing with nutrient requirements for work in hot environments:

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Committee on Military Nutrition Research: Activity Report What is the evidence that there are any significant changes in nutrient requirements for work in a hot environment? If such evidence exists, do the current Military Recommended Dietary Allowances provide for these changes? 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? Specifically, are the Meals, Ready-to-Eat (MREs) good hot-weather rations? Should the fat content be lower? Should the carbohydrate content be higher? What factors may influence food intake in hot environments? To what extent does fluid intake influence food intake? Are there special nutritional concerns in desert environments in which the daily temperature may change dramatically? Is there an increased need for specific vitamins or minerals in the heat? Does working in a hot climate change an individual's absorptive or digestive capability? 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? The Committee's report, Nutritional Needs in Hot Environments (Marriott, 1993), provides responses to the questions the CMNR was asked to address and includes recommendations for future research. The report also includes the 12 invited papers presented at the workshop on topics such as the effects of exercise and heat on gastrointestinal function and nutrient metabolism and requirements; effects of heat on appetite and taste perceptions, smell, and oral sensations; and situational influences on food intake. CONCLUSIONS AND RECOMMENDATIONS In a hot environment, water needs are increased due to marked increases in both sensible and insensible losses. Protein requirements may be increased slightly. Based on losses in sweat, additional sodium and other electrolytes may be needed. The few studies examining vitamin needs in hot environments suggest that requirements do not increase. The Committee concluded that the variations in nutrient requirements in such environments are covered reasonably by the current Military Recommended Dietary Allowances (MRDAs, AR 40-25, 1985) and, therefore, the nutrient content of military rations does not need to be changed.

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Committee on Military Nutrition Research: Activity Report Studies show that appetite is depressed and food preferences and eating patterns are changed in response to short-term and long-term exposure to heat. The reasons for the depressed appetite may be both physical decreased intake to reduce the thermic effect of food and thereby keep body temperature from rising, and psychological, caused by stress and the lack of desire to eat hot foods in hot environments. Adequate hydration appears to be necessary for depressed food intake to return to normal. Therefore, to enhance food intake in hot environments, it may be necessary to make changes in ration components as well as the social situation during meals and time of day for meal service, and ensure that the soldiers are well hydrated. Fortunately, well-trained individuals who are acclimatized to heat and accustomed to endurance exercise experience fewer symptoms of gastrointestinal distress (which would further reduce food intake) than those who are not as well conditioned. Working at a moderate to heavy rate in a hot environment does not appear to increase energy requirements to a greater extent than similar work in a temperate environment. RECOMMENDATIONS FOR RESEARCH The Committee made several recommendations for future research within the military related to nutrition for soldiers working in hot environments. Does heat enhance satiety or impair hunger? With the decreased food intake in hot environments and a previous lack of research emphasis, one important need is further investigation of factors that affect food intake in a hot environment. Another is to evaluate whether the reduction in food intake serves a protective metabolic effect, as suggested by animal studies of hyperthermia. More generally, a study is recommended to determine why soldiers don't consume adequate amounts of food under operational conditions regardless of environmental climate, and to evaluate steps that may be taken to ensure consumption of sufficient rations. Additional research needs include the development and validation of appropriate functional indicators of nutritional status, with an emphasis on vitamins and minerals for which sweat losses are significant. With the lipid peroxidation induced by exercise in a hot environment, the potential role of higher dietary intakes of zinc, vitamin C, and other antioxidants could be explored. Also, studies that focus on gastrointestinal function in the heat are important. Finally, more research is needed to evaluate the impact of adequate mineral intake on physical performance in the heat.

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Committee on Military Nutrition Research: Activity Report * * * * * The full conclusions and recommendations from this report are included in Appendix F.