There has been an enormous amount of experimental research conducted on both humans and animals that has sought to identify biological and behavioral mechanisms that regulate food intake. This work is grouped together here with the influences on food choice identified by it because most of this work is based on the assumption that food selection or food choice can be explained by internal physiological processes, and that the principal function served by food and eating is to satisfy nutritional requirements. The notion of choice implicit in such models is therefore not one of conscious decision making but, rather, of choice as the outcome of a physiological or behavioral response to a cue or stimulus that may be either internal or external. The notion of choice thus tends to be elided with that which is eaten, and the unit of analysis is generally that of the individual organism or species.
Many different biological models of food selection have been developed and most of these are based on homeostatic models (i.e., systems that work to maintain a balance between the intake of energy and/or specific nutrients and requirements for them) and are seen to operate via stimulus-response mechanisms of various kinds. Food choice is thus seen as driven either via some kind of internal biofeedback loop that responds to internal physiological states or stimuli, such as states of physiological need, feelings of hunger or satiety, and the energy and nutrient composition of meals, or via detection and response to external cues or influences, such as taste, smell, or the palatability of foods. Various mechanisms have been identified that act to regulate intake of energy as well as individual nutrients, although the evidence to support innate biological mechanisms governing food choice is probably strongest for overall energy intake. Some mechanisms also only appear to act in fairly extreme physiological circumstances, such as the craving for carbohydrates that follows administration of large doses of insulin. With perhaps the exception of salt and water, the evidence linking physiological needs states or cravings with the choice of specific foods is also equivocal.
The regulation of energy balance and appetite in particular has been the subject of a large amount of research. Much of this work has been carried out in relation to obesity and whether this can be linked to a faulty mechanism or genetic defect of some kind. This work is reviewed in detail elsewhere in this encyclopedia, but a number of different mechanisms have been proposed whereby energy intake and balance might be regulated. These include the adaptive thermogenesis theory (now largely discounted, this proposed that energy expenditure was flexible in some individuals and increased to expend excessive energy intakes); nutrient-based models of feeding in which the energy and/or nutrient composition of the diet is considered to lead to appetite suppression via complex gut-fill cues (e.g., the effect of carbohydrates on neurotransmitters and the central nervous system); and the glucostat, lipostat, and leptin theories, which are considered to operate via satiety effects. However, although experimental studies have shown that complex physiological changes are indeed associated with eating, that these vary with what is eaten, and that humans can respond to the covert manipulation of the energy and nutrient composition of our diets, these studies cannot explain the wide variation and flexibility in human food selection or the development of dysfunctional food habits. Humans appear to be more efficient at regulating up (i.e., eating more) when the energy content of the diet is reduced than regulating down. It has been hypothesized that this is the result of evolutionary adaptation to environments in which food supplies were scarce and/or unreliable and in which the ability to deposit energy stores would carry significant advantages. Thus, although there is evidence to support the idea that humans have some kind of appetite control knob (or even knobs) to regulate energy intake, this knob (or knobs) can be overridden, as evidenced by the increasing rates of obesity in many areas of the world.
In relation to external or behavioral cues influencing food choice, there has also been much work investigating relationships between the organoleptic or sensory properties of food, such as taste and palatability, and food preferences and choice. The palatability of food is a complex construct that combines both the sensory qualities of food (taste, smell, and texture) and our hedonic or pleasure response to that food. There has been much work on what makes certain foods more or less palatable. A high fat content, for instance, enhances the palat-ability of foods, and a liking for high-fat foods appears to be a universal biological disposition. A liking for sweetness and a dislike of bitter flavors also appear to be universal. Again, it has been argued that these carry evolutionary advantages because sweetness is often linked with good dietary energy sources and bitter tastes with foods containing alkaloids and other poisons. Beyond this, however, most likes and dislikes for specific tastes, flavors, textures, and food appear to be learned, often at an early age. Many food aversions, for instance, are very culturally specific.
In summary, although various stimuli, both internal and external, have been shown to influence certain aspects of human food choice, innate biological or behavioral mechanisms alone cannot explain the enormous diversity in human food choice that we see over time and place. As Rozin, a key researcher in the field of food choice, has pointed out, we need to remember that as a species humans are not specialized feeders; we can and do eat an enormous range of foods and diets that satisfy our nutritional requirements. Thus, although there is undoubtedly a physiological base to human nutrition, biology alone cannot explain the complexity of human food choice.
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