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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.



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