The term appetite does not have a single widely accepted meaning. Colloquially, it refers to a motivational state or desire, ordinarily for food but more generally for almost any reinforcer. Often the desire is not for food per se but for a specific food or nutrient (eg, salt). Hunger and appetite are often regarded as synonyms, although some investigators have distinguished between hunger as an objective depletion of energy or a specific nutrient and appetite as a subjective desire reflecting psychological factors more than physiological depletion. When people report a loss of appetite in conjunction with, say, an episode of clinical depression, the decline in the subjective desire for food is assumed to correspond to a suppression of the physiological correlates implicated in objective hunger processes; but it remains possible that subjective appetite and objective hunger might diverge under certain circumstances. Certainly the expression of an intense desire for an attractive dessert that appears unexpectedly at the end of a large meal cannot easily be reconciled with a simple depletion model of hunger.

People may report on appetite as a subjective state, but in accepting such reports at face value, one must proceed at one's own risk. First, people do not readily distinguish between their objective need for calories or nutrients (hunger) and their subjective desires (appetite). Second, few if any people can make sophisticated quantitative distinctions between different gradations of appetite or hunger. Indeed, it has been argued that people often infer the intensity of these internal states only retrospectively, by observing the amount of food they have consumed (1). Direct intro-(or viscero-)spection of hunger or appetite, except at the extremes of deprivation or satiation, may not be possible for the untrained subject. Thus, people may assume that they ate as much as they did because they were correspondingly hungry, rather than accept the less obvious or plausible explanation that they ate because of social influence or simply because the food was there (2). The possibility that intake might not be directly reflective of hunger or appetite also eludes many physiological researchers (behavioral neuroscientists), who accept as an axiom that all eating is driven by hunger and that the cessation is eating is necessarily a matter of the onset of physiological satiety.

To the extent that we rely on the amount consumed as an index of hunger or appetite, we may be misled by the fact that people often desire food but do not consume it even when it is available (as happens with dieters) and, obversely, people often consume food for which they have little or no reported desire (as occasionally happens under conditions of social pressure). Finally, much of the research on appetite is conducted on nonhuman animals, necessitating the substitution of physiological or consummatory assessments for verbal assessments. We readily assume the amount that an animal eats corresponds to its "subjective desire" for food, but of course we cannot be sure. Like humans, nonhuman animals often eat more in the presence of conspecifics (3), and we cannot simply infer that the presence of others increases hunger/appetite; it may well be that the presence of others increases intake directly, without affecting hunger/appetite.

Although we cannot complacently assume a direct correspondence between appetite and hunger, or between appetite and intake, the paucity of research on appetite (or hunger) as a phenomenon separate from intake demands that we suspend our concerns about the correspondence between desire and behavior and examine the factors that influence eating. Whether all of these factors operate through the mediator of subjective appetite is doubtful, but at this stage in the development of the research, we have little choice but to make that assumption. None of the research on nonhuman animals has connected appetite to intake, and even in humans, the majority of the research has focused on what is consumed, with a relative neglect of the corresponding subjective state. We are thus forced to shift our attention to the determinants of intake, on which a significant body of research has accumulated. These determinants may be classified as arising from the internal (physiological) and external environments. The role of appetite qua subjective state in this research is largely unexplored, but we may indulge ourselves with the probably only partly incorrect belief that eventually researchers will connect the subjective state of appetite to the more objective determinants of intake, both in humans and (somehow) even in nonverbal animals.

The factors that affect eating have not been systematically cataloged, let alone integrated into a complete systems approach to eating. We have elected to divide the influences on eating into internal and external categories, corresponding roughly to physiological versus environmental factors. Some factors, such as the palatability of the available food, depend jointly on the properties of food, the history of the organism, and the acute physiological state of the organism; such factors, obviously, do not fit neatly into the internal-external categorization. The internal and external distinction is also threatened by findings that demonstrate that through conditioning, external signals can stimulate hunger, or at least feeding (4). Likewise, the perception of attractive food triggers (cephalic) anticipatory digestive reflexes that in turn may affect feeding (5). By the same token, deprivation or other physiological manipulations can alter receptivity to external influences (6). Any comprehensive analysis of eating must contend with these mutual, interactive influences, but current research only hints at some of these complexities. In the absence of such a fully integrated view of eating, this article will be confined to a less satisfactory and somewhat arbitrary listing of influences.

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