Sensory And Perceptual Factors Flavor

The term "flavor" is intended to encompass both gustatory (taste) and olfactory (smell) properties of food. Palatability is a matter of both aspects of flavor; when people say that a food tastes good, they tend to underestimate the contribution of smell. The palatability of food, which corresponds to how positively its flavor is rated and also includes certain secondary properties of the food such as its texture, is a major determinant of intake. Organisms are basically finicky, avoiding indiscriminate consumption in favor of selective consumption (where possible) of preferred foods. The finding that nonhuman animals consume more highly palatable food is compromised by the fact that palatability must often be inferred from intake itself; but humans also show a strong tendency to consume more of foods that they independently rate as more pleasant.

Palatability has a direct effect on intake, but whether palatability overrides other potentially competing considerations in controlling intake remains an active research question. It has been observed that the duration of the meal is largely a matter of the palatability of the available food but the likelihood of a rat initiating feeding is more closely tied to caloric deprivation (20). In short, palatability may affect some meal parameters more than others. In humans, extremes of food deprivation or satiety mute the effects of flavor; starving people tend to ignore the flavor of food and eat a great deal of unpalatable food if that is all that is available, whereas fully sated people eat very little of even quite palatable food. Interestingly, research indicates that at least under certain qualifying conditions hungry animals (including humans) do not display reduced finickiness; indeed, finickiness may be exaggerated both with respect to acceptance of good-tasting food and rejection of bad-tasting food (21,22). An apparent paradox has been identified in which "the animal eats ... for taste when he needs calories [ie, when deprived]" (23). This paradox may be more apparent than real, in that the flavor of food is ordinarily confounded with its caloric density (ie, good-tasting foods are calorically denser, leading one humorist to describe the calorie as a term that scientists use to measure how good something tastes). Accordingly, the hungry animal may become more finicky as a way of maximizing energy intake in an energy deficiency crisis.

The potency of palatability as a determinant of eating may vary in different types of individuals. Obese people, for instance, have long been regarded as prone to overeat highly palatable foods (rather than food per se); this view was crystallized in the externality theory of obesity, which argues that obese people's eating is controlled virtually entirely by environmental cues, including taste, whereas normal-weight people's eating is more responsive to physiological cues of hunger and satiety (24). The observation that obese people are especially responsive to taste (24) may be meshed with the observation that hungry organisms are especially responsive to taste (23), if it is postulated that most obese people are chronically hungry as a result of their partially successful attempts to reduce their weight (25).

Other eating problems have likewise been associated with extreme partiality toward highly palatable foods. Although the evidence is conflictual (26), binge eating in normal-weight individuals is often interpreted as targeted specifically on highly palatable carbohydrates (27), which elevate serotonin levels and, ultimately, mood (19). Conceivably, this view might be extended in the direction of proposing that some flavors are deemed palatable precisely because of their central hedonic consequences.

Sensory Factors

If eating offset were associated solely with the accumulation of particular nutrient stores, then it would be difficult to account for the "dessert effect." That is, despite a satiating meal (fully repleting nutrient stores), people can still find room for dessert. One way of accounting for this effect is in terms of sensory specific satiety, which refers to an animal becoming satiated to a particular flavor without showing satiation to other normally preferred flavors (28). Thus the individual may become satiated to the sensory characteristics of the main course while not being satiated to the sensory characteristics of the dessert. (It should be noted that there may be other contributors to the "dessert effect," including social facilitation, as described later.)

Collateral support for the role of nongut sensory factors in satiety is found in studies of sham-feeding (esoph-agotomized) animals. In these animals, food is chewed and swallowed but does not reach the gut, and the nutrients are therefore not absorbed. Of special interest here is the fact that although these animals eat larger than normal meals (providing evidence for the involvement of post-ingestional factors in regulating eating offset), they do eventually stop eating. This has been interpreted to indicate that sensory aspects of the food (taste, olfaction, and texture) are sufficient to induce satiety-like effects on eating. A full explication of satiety will eventually have to incorporate and reconcile sensory and postingestional factors.

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