Learned Appetites Satieties and Feeding Behavior

Animals and humans learn (or become conditioned) to associate a given food with the physiological consequences of having ingested it. They associate certain proximal stimuli such as the smell, color, taste, or texture of a food (the conditioning stimulus) with a set of sensations that are directly felt (sensory afferent inputs), in relation to the external stimulus and to the endogenous changes such as physiological and neuroendocrine responses to food. The physiological changes that occur as a result of ingesting the food are termed the 'unconditioned stimulus.' The subject forms a learned or 'conditioned association' between the conditioning stimulus and the unconditioned stimulus (the detectable consequence of eating), which informs them of the sensory and physiological consequences of ingesting that food. This process is summarized in Figure 1. Conditioned or learned associations are most efficiently established if the food is sensorially distinct, if there is a significant detectable postin-gestive consequence of ingesting the food and if a training or learning schedule is encountered (e.g., by repeated exposure to the food under similar conditions). Learning is facilitated by social interaction.

As regards appetite 'regulation' a problem arises when foods are constructed to look and taste like foods with a different composition. For some time after the initial exposure to the food subjects will respond to it in a manner that is determined not by immediate exposure to the food but by what they have learned during the previous period of exposure to the similar foods upon which the learning was originally based. Only if the food produces a very large unconditioned stimulus will this previously learned response be instantly over-ridden. This raises the possibility that the use of food mimetics (e.g., artificial sweeteners) may disrupt stable patterns of learned feeding behavior in consumers at large.

The above view of the nature of feeding behavior has implications for the way the appetite system functions in lean and overweight people.

Environment

Environment

Figure 1 The process by which the subject learns to associate the postingestive consequences of eating with the food eaten and the environment in which it was eaten. Environmental influences can vary in strength from negligible effect to, in extremis, influences so strong that they can constitute the major factor determining a subject's subsequent response to that food.

Figure 1 The process by which the subject learns to associate the postingestive consequences of eating with the food eaten and the environment in which it was eaten. Environmental influences can vary in strength from negligible effect to, in extremis, influences so strong that they can constitute the major factor determining a subject's subsequent response to that food.

Physiologists have expended considerable time and effort in attempting to understand how feeding behavior is geared to the regulation of a stable body weight. Obesity is therefore seen as a consequence of defects in this regulation. The evidence from behavioral studies suggests that feeding behavior is inherently more responsive to decreases rather than increases in body weight. Second, current secular trends in body weight suggest that, over time, it is relatively easy to increase body weight, which infers body weight is not tightly regulated, at least with reference to weight gain. For instance, according to the National Health and Nutrition Examination Survey (NHANES) ongoing dietary surveys of American adults, average weight change amounts to a gain of 0.2-2.0 kg year-1. Third, while obese subjects exhibit differences in their feeding behavior and physiology, the literature is remarkably short of clear lean-obese differences in feeding behavior of a type that suggest defects in a regulatory system. For example, evidence suggests that the obese tend to select a diet rich in fat, which in itself facilitates over consumption. However, the tendency to select fat cannot be viewed as a defect in physiological regulation. It may be far more profitable to attempt to understand how feeding responds to environmental and endogenous stimuli, and which of these responses are functional and/or adaptive and which are not. This consideration influences the methodological approaches that attempt to investigate how feeding behavior responds to endogenous and environmental influences.

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