Feeding is needed for the maintenance of ENERGY BALANCE and requires the integrated activity of many neural and behavioural systems. In order to feed, animals must know about their internal and external environments (when is it time to feed? what is edible, what inedible?), they must be able to search effectively for food (see FORAGING) and be able to activate the MUSCLES necessary to capture it once found (to gather plants or catch and kill other animals) and ingest it. Effective monitoring of internal state is imperative for proper regulation of feeding. Signals indicating both depletion and loss, and repletion and satisfaction are analyzed by the brain. The brain is able to monitor, via analysis of BLOOD compo-sition and receipt of neural input (from the VAGUS NERVE for example) such things as circulating levels of GLUCOSE, activity of HORMONES such as INSULIN and GLUCAGON in the PANCREAS, metabolic activity in the LIVER, gut activity (through CHOLECYSTOKININ concentration for example) and the state of FAT stores (through analysis of LEPTIN concentration). In addition, there are specific mechanisms for the detection of critical diet components such as sodium (see SODIUM APPETITE). The analysis of taste (see GUSTATION) is an important part of feeding: taste serves to give warning of ‘badness’ or ‘goodness’ in food and acts as a stimulant to intake. The brain mechanisms involved in this are distributed widely, from, for example, the NUCLEUS OF THE SOLITARY TRACT in the brainstem, the HYPOTHALAMUS in the diencephalon to the FRONTAL CORTEX. Accounts of the neurobiology of feeding have in the past centred on the hypothalamus, which was thought to have specific centres regulating feeding (and other motivated behaviours) but this is seen as over-simplified now (see DUAL CENTRES HYPOTHESIS; LATERAL HYPOTHALAMIC SYNDROME; VENTROMEDIAL HYPOTHALAMIC SYNDROME).
Current accounts of the neurobiology of feeding emphasize the distributed nature of its processing.
However, it has to be borne in mind that feeding is not simply a matter of energy regulation and that feeding occurs in response to a wide variety of states and stimuli. In animals it is evident that provision of the SUPERMARKET DIET can lead to excessive intake. This of course can be accounted for by the fact that animals which normally have seasonal variation in food availability should be prepared to overeat when the opportunity presents. But stimuli completely unrelated to nutrition or deprivation, such as TAIL-PINCHING, can also reliably elicit feeding in rats, while in humans it is clear that psychological states of AROUSAL or ANXIETY can trigger feeding (see Slochower, 1983). Evidence such as this points to two conclusions: first, that eating can be triggered by non-specific activation; and second, that in humans at least, LEARNING to attribute (see ATTRIBUTION) the label HUNGER only to appropriate physiological states may be a lot more difficult than is commonly sup posed. The most startling evidence of this is presented by Hilde Bruch (1973). She cites (pp.60–61) the developmental history of a grossly obese boy who was fed inappropriately in response to whatever EMOTION he expressed. In consequence he learned that feeding was the appropriate response to a wide variety of psychological and physiological states and ate indiscriminately. Humans generally have mechanisms for monitoring body state and regulating feeding: whether they learn to use them properly is another matter.
