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Nutritional Needs in Hot Environments: Applications for Military Personnel in Field Operations
with a previous intake of more than 20 grams at 24°C; mild (32°C) and severe (35°C) heat stress over 21 days resulted in a continued lower level of food intake. At 40°C, rats stop eating altogether; if force-fed by intubation, they suffer heat stress and occasionally die (Hamilton, 1967). Studies in a number of experimental animals demonstrate cessation of eating at high temperatures, with the possibility that continued eating would probably lead to hyperthermia. The marked decrease in food intake is followed by a decrease in body weight and fat (Jakubczak, 1976). Reduced intake in the heat would thus seem to be adaptive. Keys and coworkers (1950) found that their semistarved volunteers complained of the cold even in warm summer weather. This indicates that a reduction in food intake may actually be a mechanism to cope with hot environments. There is thus significant research in various models to support the observation that food intake drops as the environmental temperature increases from normal to hot ambient temperatures, followed by a decrease in body weight.
Heating the preoptic and anterior hypothalamic regions in animals appears to act in much the same fashion as external cues to inhibit eating (Andersson and Larsson, 1961). Opposite results, however, were obtained by Spector and colleagues (1968). Heating of the preoptic medialis region caused increased eating when the temperature of the area was raised to 43°C; decreased eating occurred when the ambient temperature was raised to 35°C. Local temperature in the anterior hypothalamic area reportedly drops at the onset of eating in the monkey, which is the opposite of what would be expected (Hamilton, 1963b). It appears that the effect of brain temperature on eating may be more a result of external ambient temperature than of localized temperature changes. In addition, it may be due to the rate of heat flow from the body's core to the periphery or vice versa, as no single temperature uniquely governs the level of food intake (Spector et al., 1968).
Thermogenic Effect of Food
Studies of the theory that animals stop eating to prevent hyperthermia have noted differences in the resulting thermic effect of the food ingested (dietary induced thermogenesis, or specific dynamic action) as a possible triggering mechanism (Chapter 15). The caloric intake of rats that were fed special diets during mild heat stress was inversely related to the thermogenic effect of the diet selected (Hamilton, 1963a). It appears that fats may be the preferred energy source in heat stress (Salganik, 1956) and that in conditions of severe heat, rats avoid protein because of the comparatively high amount of heat it creates (Hamilton, 1963a). Under this theory, body temperature should be highly correlated with hunger and satiety, yet there appears to be no consistently observed relationship between them. LeBlanc