ancy. Most investigators have only calculated the aerobic metabolic rate during submaximal exercise, ignoring the contribution of anaerobic metabolism to total metabolic rate. Although both increases and decreases have been observed in metabolic rate in the heat, it does not appear that the presence or absence of heat acclimatization has an effect on metabolic rate (see Chapters 3 and 6 for further discussion).
Muscular activity produces an enormous amount of heat, with the amount of heat production directly related to the intensity of exercise (Nadel et al., 1977). The amount of heat production generated by the increased energy metabolism of skeletal muscle during exercise may be as much as 100 times that of inactive muscle. The mechanisms for dissipating this heat are generally well regulated. Although heat loss occurs through evaporation of sweat and by conduction; convection, and radiation, evaporation of sweat is clearly the most effective avenue of heat loss during exercise. The sweat glands are capable of secreting up to 30 grams of sweat per minute, removing approximately 18 kcal of heat in the process. Sweat rate is directly associated with exercise intensity (Maughan, 1985; Nadel et al., 1977).
It has been reported (see Chapter 4) that gastric emptying and intestinal motility decrease as core temperature increases during exercise and in hypohydration. Some, but not all, investigators have also observed reductions in intestinal absorption of nutrients under these conditions.
Most of the studies on the effects of exercise and heat on gastrointestinal function have been carried out in endurance athletes such as marathon runners. Gastrointestinal symptoms under these conditions are often severe, although transient. They include cramps, belching, gastrointestinal reflux, flatulence, bloody stools, vomiting, diarrhea, and nausea. Mechanisms for these effects are discussed in Chapter 4. The relevance of these findings to the range of physical activity in the military is not at all clear, and the findings appear transient when associated with extreme physical activity. Instances of levels of physical activity in the military approaching those of highly competitive endurance athletes would appear to be the exception rather than the rule.
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,