Appetite – the need to eat, gradually turning into a feeling of hunger. Despite the banality of this definition, behind it lies the most complex mechanism responsible for the regulation of the energy balance in the human body. It includes several levels: the nuclei of the hypothalamus, the brain stem, the pleasure center, which send and receive signals through biologically active substances. The source of the latter can be adipose tissue, pancreas, gastrointestinal tract, endocrine glands. They produce hormones that regulate appetite.
Thus, a decrease in the blood level of insulin, a hormone of the pancreas, leads to an increase in appetite (the phenomenon of an insulin crisis). Leptin is a hormone produced in adipose tissue. High concentrations of leptin in the blood lead to an increase in body weight due to fat. During starvation, the level of leptin in the blood plasma decreases, while there is a mobilization of fat and its splitting, a feeling of hunger appears.
Another hormone, ghrelin, which is produced in the lining of the stomach and intestines of a hungry person, tells the central nervous system that the gastrointestinal tract is ready to eat. The higher its level in the blood, the more you want to eat. Immediately after a meal, its plasma concentration begins to decline, reflecting the intake of nutrients in the body.
There are many similar hormones and hormone-like substances involved in the regulation of food requirements. The presence of diseases that can disrupt these connections is a potential cause of loss of appetite. The most striking example of such a pathology is a violation of appetite in patients with anorexia nervosa.
Victims of anorexia refuse to eat until the body is completely exhausted. Studies aimed at studying the causes of this disease have revealed that with anorexia, almost all parts of the mechanism that regulates appetite are affected. That is why anorexia nervosa is difficult to treat.
There are other types of eating disorders that are also associated with dysregulation of appetite https://en.wikipedia.org/wiki/Appetite (bulimia, psychogenic overeating, psychogenic vomiting).