Appetite

Dysregulation of appetite lies at the root of anorexia nervosa, bulimia nervosa, and binge eating disorder. Anorexia nervosa is a mental disorder characterized as severe dietary restriction and intense fear of weight gain. Furthermore, persons with anorexia nervosa may exercise ritualistically. Individuals who have anorexia have high levels of ghrelin, a hormone that stimulates appetite, so the body is trying to cause hunger, but the urge to eat is being suppressed by the person.[5] Binge eating disorder (commonly referred to as BED) is described as eating excessively (or uncontrollably) between periodic time intervals. The risk for BED can be present in children and most commonly manifests during adulthood. Studies suggest that the heritability of BED in adults is approximately 50%.[6] Similarly to bulimia, some people may be involved in purging and binging. They might vomit after food intake or take purgatives. Body dysmorphic disorder may involve food restriction in an attempt to deal with a perceived fault, and may be associated with depression and social isolation.[7]

Various hereditary forms of obesity have been traced to defects in hypothalamic signaling (such as the leptin receptor and the MC-4 receptor) or are still awaiting characterization – Prader-Willi syndrome – in addition, decreased response to satiety may promote development of obesity.[8] It has been found that ghrelin-reactive IgG immunoglobulins affect ghrelin's orexigenic response.[9]

Other than genetically-stimulated appetite abnormalities, there are physiological ones that do not require genes for activation. For example, ghrelin and leptin are released from the stomach and adipose cells, respectively, into the blood stream. Ghrelin stimulates feelings of hunger, whereas leptin stimulates feelings of satisfaction from food.[10] Any changes in normal production levels of these two hormones can lead to obesity. Looking at leptin, the more cells present in a body, the more adipose tissues there are, and thus, the more leptin would be produced. This overproduction of leptin will cause the hypothalamus to become resistant to leptin and so, although the adipose cells are producing leptin, the body will not understand that it should stop eating.[11] This will produce a perpetual cycle for those that are obese.

Eating issues such as "picky eating" affects about 25% of children, but among children with development disorders this number may be significantly higher, which in some cases may be related to the sounds, smells, and tastes (sensory processing disorder).[12]

Mechanisms controlling appetite are a potential target for weight loss drugs. Appetite control mechanisms seem to strongly counteract undereating, whereas they appear weak to control overeating. Early anorectics (appetite suppressants) were fenfluramine and phentermine. A more recent addition is sibutramine which increases serotonin and noradrenaline levels in the central nervous system, but had to be withdrawn from the market when it was shown to have an adverse cardiovascular risk profile. Similarly, the appetite suppressant rimonabant (a cannabinoid receptor antagonist) had to be withdrawn when it was linked with worsening depression and increased risk of suicide. Recent reports on recombinant PYY 3-36 suggest that this agent may contribute to weight loss by suppressing appetite.

Given the epidemic proportions of obesity in the Western world and the fact that it is increasing rapidly in some poorer countries, observers expect developments in this area to snowball in the near future.

Weight loss or loss of appetite ("cachexia") is an effect of some diseases, and a side effect of some prescription drugs. Stimulants such as methylphenidate commonly reduce appetite in patients,[14] and have been prescribed off-label for weight loss.[15] Three agents are approved for appetite stimulation in the United States: megestrol acetate - a progesterone available as an oral tablet, oxandrolone - an oral anabolic steroid, and dronabinol - a cannabinoid available in an oral capsule.[16]

Ghrelin, a gut hormone recognized as affecting appetite, is under investigation.[17] Ghrelin itself must be delivered parenterally[17]^:2178 and research has therefore focused on substances that can be taken orally. Rikkunshito, a traditional Japanese Kampo medicine, is under preliminary research for its potential to stimulate ghrelin and appetite.[17]