Before examining the available data on the effects of heat on appetite, some preliminary considerations require attention, including a definition of terms. Heat can be defined in number of ways. Environmental temperature varies seasonally in the moderate climates in which most of the research is done, so one may ask whether appetite differs in summer and winter. Even within the seasons, of course, there may be considerable variability in temperature; does appetite suffer during a summer heat wave as compared to normal summer weather? Even more acute variations in temperature are available for examination, owing to the prevalence of air-conditioning. If, during a summer heat wave, one eats in an air-conditioned dining room, is appetite controlled by the outdoor or indoor temperature? Aside from temperature changes in the normal environment, one might also want to look at changes of environment. A winter trip to the tropics probably represents a greater short-term shift in temperature than might be encountered if one stayed put; how does it affect appetite?
To further complicate matters, how hot one feels is not simply a matter of the environment; one's own activity may generate heat, so that being active may be functionally equivalent to raising the environmental temperature. Indeed, eating itself has thermogenic effects, so that not only does heat affect appetite, but appetite may affect heat.
The meaning of appetite should also be clarified. There is tremendous variability in what scientists mean when they use the term (Herman and Vaccarino, 1992). Ordinarily, to achieve some clarity, one must distinguish among three terms that are often used interchangeably and confusingly. Appetite refers to the subjective desire to eat, whereas hunger usually refers to a more objective deprivation state. These terms are not unrelated, but it is preferable to think of hunger as a true need that often produces a felt desire (appetite). Distinguishing between hunger and appetite becomes useful when considering the possibility that one may desire to eat something even in the absence of a need for it. Conceivably, one might also be hungry without recognizing it or feeling a desire to eat, as is allegedly the case with some anorexia nervosa patients.
The third appetite-related term requiring attention is intake. In the scientific literature, food intake is often taken to be an operationalization of appetite, especially in nonhuman species where the animal's desires must be inferred from its behavior. In humans, it is quite possible to distinguish between what the person wants (appetite) or needs (hunger), on the one hand, and what the person eats (intake), on the other. People sometimes eat when they experience neither hunger nor appetite. Conversely, people sometimes refrain from eating despite experiencing hunger or appetite or both. For instance, Rolls et al. (1990) found, in a study related to temperature that is reviewed below, almost no relationship between how hungry or sated people claimed to be, on the one hand, and how much they subsequently ate, on the