Appetite

Appetite refers to the urge to eat, often triggered by hunger, though appealing foods can spark this desire even when a person is not hungry. Satiety, or feeling full, can significantly dampen appetite. This drive exists across all higher life-forms, ensuring they take in enough energy to meet their metabolic needs. A complex interaction among the digestive tract, body fat, and the brain governs appetite, and it is closely tied to individual behavior. Only two types of behavior involve energy intake: appetitive (or approach) behavior and consummatory behavior; all other behaviors involve energy release. Stress can raise appetite, leading to increased food intake. A reduced desire to eat is called anorexia, while an increased desire is termed polyphagia or hyperphagia. Problems with appetite regulation are linked to conditions such as ARFID, anorexia nervosa, bulimia nervosa, cachexia, overeating, and binge eating disorder.

A limited or excessive appetite is not always pathological. Abnormal appetite can be defined as eating habits that cause malnutrition and related issues like obesity. Both genetic and environmental factors can influence appetite, and disruptions in either may lead to abnormalities. Poor appetite (anorexia) can stem from physical causes (infectious, autoimmune, or malignant disease) or psychological ones (stress, mental disorders). Excessive eating (hyperphagia) may result from hormonal imbalances, mental disorders such as depression, or other factors. Indigestion, or dyspepsia, can affect appetite by causing a feeling of being overly full soon after starting a meal. Changes in taste and smell, including a bad taste (dysgeusia) or their absence, can also impact appetite. Genetic links to abnormal appetite are suggested by Prader–Willi syndrome, a form of obesity from chromosome alterations discovered in the 1950s. Anorexia nervosa and bulimia nervosa occur more often in females than males, hinting at a possible connection to the X-chromosome.

Dysregulation of appetite is central to anorexia nervosa, bulimia nervosa, and binge eating disorder. Anorexia nervosa is a mental disorder marked by severe dietary restriction and an intense fear of weight gain, sometimes involving ritualistic exercise. People with anorexia have high levels of ghrelin, a hormone that stimulates appetite, meaning the body tries to induce hunger, but the person suppresses the urge to eat. Binge eating disorder (BED) involves eating excessively or uncontrollably at periodic intervals. The risk for BED can appear in childhood and most often manifests in adulthood, with studies suggesting its heritability in adults is about 50%. As with bulimia, some individuals may engage in purging and binging, such as vomiting after eating or using purgatives. Body dysmorphic disorder can involve food restriction to address a perceived flaw and may be linked to depression and social isolation.

Various inherited forms of obesity have been traced to defects in hypothalamic signaling, such as in the leptin receptor and MC-4 receptor, or remain uncharacterized, as in Prader–Willi syndrome. A reduced response to satiety may also promote obesity. Ghrelin-reactive IgG immunoglobulins have been found to affect ghrelin's appetite-stimulating response. Beyond genetic causes, physiological factors also play a role. Ghrelin, released from the stomach, stimulates hunger, while leptin, released from fat cells, stimulates feelings of fullness. Changes in normal production levels of these hormones can lead to obesity. Leptin production is triggered by body fat percentage; as body fat accumulates, excess leptin is produced, leading to a resistant hypothalamus and eventually little to no leptin effect. From that point, ghrelin production causes an insatiable appetite.

Eating issues like picky eating affect about 25% of children, but this number can be significantly higher among children with developmental disorders. In some cases, this may relate to how they process sounds, smells, and tastes, a condition known as sensory processing disorder.

The glycemic index was thought to affect satiety, but a study found that it did not predict effects on satiety or food intake.

Mechanisms controlling appetite are a target for weight loss drugs. These mechanisms strongly counteract undereating but appear weak at controlling overeating. Early appetite suppressants (anorectics) included fenfluramine and phentermine. A later one, sibutramine, increased serotonin and noradrenaline levels in the central nervous system but was withdrawn from the market due to an adverse cardiovascular risk profile. Similarly, the appetite suppressant rimonabant, a cannabinoid receptor antagonist, was withdrawn after being linked to worsening depression and an increased risk of suicide. Recent reports on recombinant PYY 3-36 suggest it may aid weight loss by suppressing appetite. Given the epidemic of obesity in the Western world and its rapid increase in some poorer countries, developments in this area are expected to accelerate.

Weight loss or loss of appetite (cachexia) is an effect of some treatments.