One of the essentials for an omnivore faced with a variety of new and different foods is the capacity to learn. It is not possible for an inborn preference or aversion to guide the choice of every possible food. Therefore, we learn which foods are beneficial (and which are not) by eating them. This learning involves the association between the sensory and the postab-sorptive characteristics of foods. In this way the sensory characteristics of foods act as cues and come to predict the impact that foods will later have. Consequently, these cues should suppress hunger according to their relationship with subsequent physiological events.
It is possible to demonstrate experimentally how human beings adapt their eating to a food's energy content. A distinctively flavored food which contains 'extra' hidden energy, presented on several occasions, will result in a change in eating and in preference. When deprived of food, subjects' preference for the taste increases with gained experience. If presented when satiated, preference for the taste decreases. This process is also observable in young children, who eat smaller meals following a taste previously associated with a high-energy snack, and larger meals following a taste previously associated with a low-energy snack.
The idea that we can have conditioned hunger for specific nutrients is far more contentious. The concept of conditioned hunger suggests that the organism, faced with a diet deficient in a single important nutrient, will seek an alternative food source that contains the missing nutrient. However, earlier evidence from animals has largely been reinterpreted from the standpoint of conditioned aversions. Indeed, conditioned aversions are far more potent examples of the impact of learning on eating behavior than any examples of conditioned hunger. A conditioned aversion that will be familiar to many readers is the profound dislike that occurs in response to a food or drink that was eaten prior to vomiting or illness. An example of a conditioned taste aversion was famously described by learning theorist Martin Seligman. Steak with sauce
Bearnaise was Seligman's last meal before a bout of gastric flu. Yet knowing that it was the flu rather than the food that made him sick did not prevent the subsequent aversion to sauce Bearnaise. In fact, surveys show that conditioned taste aversions are commonplace and reported by 40-60% of people.
Conditioned taste aversions are important in the present context not because they represent a special form of one-trial learning that we are biologically pre-prepared to acquire. Rather, they show that the strength of cue-consequence learning in the area of food intake depends on the stability and reliability of the relationship between tastes (sensory cues) and physiological effects (metabolic consequences) of food. When there is distortion, variation, or extreme complexity in the relationship between sensory characteristics and nutritional properties, then the conditioned control of hunger is weakened or lost. In many respects, the variety of foods available to us represents a cacophony of different sensory characteristics and has the added complication of ingredients that preserve the sensory qualities while altering their nutritive value. Learned hunger therefore is a relatively less important factor when the food supply contains many food items with identical tastes but differing metabolic properties.
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