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Nutritional Needs in Hot Environments: Applications for Military Personnel in Field Operations
exploit naturally occurring variations in) ambient temperature and examine indices of appetite. One might manipulate (or exploit naturally occurring variations in) body temperature independent of environmental temperature—as in fever—and determine whether this form of hyperthermia suppresses appetite. Alternatively, one might manipulate the need to acquire or dissipate energy more indirectly, through exercise; exercise, by providing a short-term boost in internal heat, ought to reduce the need for further energy—in short, appetite should be suppressed. In the long-term, by depleting short-term energy stores, exercise should increase the cumulative demand for energy repletion.
A final research strategy involves examining the influence of eating itself. Food intake itself creates heat, in addition to providing stored energy for future use. The heat attendant on eating and digestion (the thermic effect of food) as well as the heat produced through the processes of postprandial thermogenesis, which is experienced when humans perspire or become flushed after overeating, ought to reduce appetite acutely. Eating-induced thermogenesis presumably combines with environmental heat, so that appetite will be more suppressed after a given mean in a hot environment than in a cold environment, all things being equal.
An inferential caution: Considerable research has been devoted to the effects of cold environments on physiology and behavior, including appetite. If cold exposure increases appetite compared to appetite under normal conditions, one might be tempted to conclude that heat exposure above normal levels should have the opposite effect. For example, if people eat more than normal when the ambient temperature drops from 70° to 55°F, one might be inclined to infer that appetite would be reduced below normal if the temperature were raised to 85°F. This temptation might be justified by the data, but in the absence of specific research on heat exposure, one should be cautious about extrapolating from research on cold exposure. Does unusual heat have the opposite effect from unusual cold? That is an empirical question.
Another inferential caution: The immediately foregoing analysis assumes that body temperature is regulated and that the heat generated by eating represents a major element in the regulatory equation. But temperature may not be the only important regulated variable. It is now widely believed that body weight or body fat or some associated variable is also regulated (for a review see Mrosovsky, 1990). Moreover, there is reason to believe that the level at which body weight/fat (hereafter BW) is regulated may shift in response to various inputs, perhaps including environmental temperature. What if the set-point for BW drops in the heat? (This would be a reasonable adaptive strategy physiologically, because humans require less insulation when the environmental temperature is high in order to maintain a comfortable body temperature. Note that the supposition of a BW set-point