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Nutritional Needs in Hot Environments: Applications for Military Personnel in Field Operations
other. In this chapter, owing to the fact that the literature largely ignores the distinctions just suggested, the effect of heat on appetite, hunger, and intake is considered collectively, with the awareness that heat may affect one of these without affecting the others. Use of the term appetite in a general or unqualified sense should be understood to refer to all three constructs.
One final preliminary caution. The mandate here was to examine the effect of heat on appetite in humans. For perhaps understandable reasons, a high proportion of the scientific research on appetite and on heat effects has been conducted on nonhumans. In what follows, preference will be given to human studies. Caution is urged in extrapolating from rats and other species to humans, but the paucity of human data requires reference to animal studies for clues as to how human appetite is affected by heat.
ENVIRONMENTAL TEMPERATURE, EATING, AND THERMOREGULATION
Any discussion of the effects of heat on eating must begin with a recognition that eating represents the basic means of securing energy for humans. Most analyses of heat and eating go one step further and point out that a major physiological concern of humans is thermoregulation—the maintenance of an appropriate body temperature—and that eating provides a major contribution to maintaining body heat (Brobeck, 1948). Indeed, it may be that "the important factor in regulation of food intake is not its energy value, but rather the amount of extra heat released in its assimilation" (Strominger and Brobeck, 1953). Thus, this "thermostatic hypothesis" of feeding argues that the total energy content of the food is not the determining factor in regulation. Energy that becomes stored as fat does not control feeding; rather, it is the direct heating effect of food intake that is monitored and that provides a regulatory mechanism.
According to this view, if the environment is cold, the resultant heat loss demands compensatory strategies, including notably increased food intake for its thermic effect. By extension, if the ambient temperature is warm, and heat loss is not an issue, there ought to be a reduced caloric demand. And should the environment become significantly hot—which changes the concern from how to obtain energy to how to dissipate it—a suppression of caloric intake should be expected. "At a high temperature where loss of heat is difficult, food intake should be low, lest by eating and assimilating food the body acquire more heat than it can dispose of" (Brobeck, 1948). This temperature-dependent variation in energy needs should, in principle, be reflected in appetite. Brobeck (1948) claims that "everyone knows ... that appetite fails in hot weather."
The inverse relation of appetite to environmental temperature may be examined in a number of different ways. Clearly, one might manipulate (or