Leptin is a 16 kDa, non-glycosylated polypeptide that regulates adipose tissue mass and energy balance. It is a hormone made by fat cells which regulates the amount of fat stored in the body by adjusting both the sensation of hunger, and adjusting energy expenditures. The effect of leptin is
opposite to that of ghrelin. Leptin acts on receptors in the hypothalamus, where it inhibits hunger by counteracting the effects of neuropeptide Y and anandamide, and promoting the synthesis of α-MSH. It controls food intake and energy expenditure by acting on receptors in the mediobasal, hypothalamus. Although regulation of fat stores is deemed to be the primary function of leptin, it also plays a role in other physiological processes, as evidenced by its multiple sites of synthesis other than fat cells, and the multiple cell types beside hypothalamic cells which have leptin receptors. Leptin deficiency also influences the immune system, depressing Th1 responses and causing increased frequency of infections. Leptin also regulates puberty, blocking the onset of puberty, or of menses if Leptin deficiency exists due to excessive thinness, such as results from starvation, extreme exercise-induced weight loss, anorexia or cancer-induced cachexia.