(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.
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 3–5 and 12–14) 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.
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,