Stomach distension and the detection of macro-nutrients such as fat or protein within the gut are all powerful satiety cues. They bring a meal to an end and for a time inhibit further consumption. Eventually, hunger again prevails and food intake follows. The flux between hunger and satiety is episodic and underpins the expression of our eating behavior throughout the day. However, it is not just the absence of episodic satiety cues (e.g., stomach distension and intestinal or absorbed nutrients) that influence the expression of hunger. Reduction in blood glucose levels or in levels of the circulating adipose tissue hormone leptin indicates a deficit in available energy and in energy reserves. Fluctuation of these factors indicates the metabolism and storage of the body's energy reserves. These are a tonic class of physiological signals that also influence the expression of appetite. Like episodic satiety signals, these tonic signals normally act on inhibitory mechanisms with the hypothalamus (anorexogenic circuits). Their absence elicits an active feeding response. Other tonic factors that indicate the body's energy status, such as adiponectin, cytokines, and gonadal hormones, also appear to act on energy regulator centres within the brain, particularly the hypothalamus, mainly to suppress hunger.
However, not all physiological signals, episodic or tonic, inhibit hunger. For instance, blood levels of the recently discovered gut hormone ghrelin have been shown to increase prior to a meal. Subsequent intake has been shown to suppress ghrelin release. Further studies have shown that ghrelin infusions increase food intake. Thus, this is a hormone that acts to promote food intake. Interestingly, ghrelin receptors are found in various hypothalamic locations that form part of the orexogenic circuits promoting food intake. These circuits contain many neuropeptides, such as neuropeptide Y, orexins, melanocortin concentrating hormone, and galanin, which all stimulate food intake. The precise nature of the physiological and neurobiological regulation of appetite is discussed elsewhere in this encyclopedia. Finally, it should be noted that the biological mechanisms critical to the expression of hunger are not independent of psychological ones. Indeed, the sensory and cognitive cues that stimulate hunger produce physiological changes that anticipate the ingestion and metabolism of energy and subsequently aid these processes. This brings on the psychological factors critical in the expression of appetite.
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