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When discussing preferences for sweet and fat tastes, it should be emphasized that although a preference for sweet foods may be universal, a preference for fat appears to be specific to the individual and therefore a learned response. Some preferences for combinations of fat and carbohydrate foods have been examined (typically dessert type foods), but preferences for foods high in protein and fat have not been examined in detail. Gender differences in preferences for different macronutrients also have not been well studied. In addition, the intake of specific foods during different seasons, such as fresh corn during the summer months, appears to be primarily a function of availability and learning. (See additional discussion of seasonality of food intake below).

Hotter-temperature foods generally are rated as having greater intensity of taste and smell. Further studies with various types of drinks and foods are necessary to clarify whether temperature or mode of presentation can, in fact, influence satiety. Largely unexamined is the degree to which variations in the temperature of foods (rather than preloads) or variations in ambient temperature influence the intake of specific meals or the intake of subsequent meals.

Appetite

It is important to distinguish among appetite, hunger, and intake. "Appetite" will be used here to refer to the subjective desire to eat, whereas "hunger" usually refers to a more objective deprivation state. In humans, it is possible to distinguish between what a person wants (appetite) or needs (hunger) and what a person eats (intake). These distinctions are useful because large-scale or clinical human studies often involve combinations of measures, including choice or preference ratings, in discussions of food intake. These clearly are not always the same. For example, preference ratings do not accurately predict food intake.

Thermoregulation

One of the body's major physiological concerns is thermoregulation, the maintenance of body thermoneutrality. Eating appears to be a major contributor to maintaining body heat. The "thermostatic" hypothesis of feeding is that the body experiences a temperature-dependent variation in energy needs that should be reflected in appetite. If normal food intake continues under conditions of heat stress, the additional heat that must be dissipated as a result of the amounts ingested may lead to a breakdown in the body's heat mechanisms (see Chapter 15).

In a series of studies by Hamilton (1963a), rats exposed to a temperature of 35°C ate only 2 grams of food during the first 24 hours, compared



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