Ghrelin

Ghrelin is a hormone that increases appetite by signaling hunger to the brain. Released mostly in the stomach, ghrelin is typically at an increased level when the body needs food and the levels decrease when a person is satiated (doesn't need food). One of two 'hunger hormones', the other is leptin which is an appetite suppressor. Functioning as a regulator of body weight, levels of ghrelin are higher before eating and these levels decrease after eating.

Research has shown that ghrelin is increased in patients with anorexia nervosa and decreased in obese patients. This is further evidence that ghrelin levels are involved in hunger: anorexia nervosa patients are in dire need of nutritive substances so their body is increasing ghrelin in an attempt to stimulate hunger while obese patients are satiated and the body doesn't need nutrition at the moment and this causes a low level of grehlin in the body.