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Worldwide, obesity and related metabolic disorders like diabetes, heart disease, hypertension and sleep apnea are increasing. In some industrialized countries, the problem of obesity has reached almost epidemic proportions.This also leads to the rise in the treatment-related costs, with millions of dollars being spent to address these lifestyle-associated problems.
Scientific research revealed that the problem is rather complex, and there is no single simple way of addressing it. Body weight control involves many physiological mechanisms.
Regulation of appetite is a new target in the obesity research
In recent years it is becoming clear that the so-called hunger hormone, ghrelin, which increases appetite plays an important role in developing the obesity and its complications.
Regulation of body weight is a very complex process. In our body there is a certain balance between energy intake and energy expenditure. In obese people, there is a positive energy balance which results in increased body fat and weight gain. Researchers everywhere are trying to understand the relation between appetite control and food intake. With the discovery of ghrelin a new field of research has begun resulting in newer modalities of medical and surgical options for obesity treatment.
Hunger hormone production and action in the body
The hunger hormone ghrelin is produced mainly in the stomach, though smaller amounts are also produced in pancreas, small intestine and colon. The concentration of ghrelin decreases from duodenum to colon. A very small amount of ghrelin is also secreted by hypothalamus and pituitary in brain.
The main action of ghrelin is to increase appetite. The mechanism behind its action is rather complex and involves biochemical pathways responsible for regulation of multiple other hormones in the body, growth hormone in particular. Ghrelin binds to the growth hormone secretion-promoting receptors in the hypothalamus and pituitary in brain. In the arcuate nucleus of hypothalamus, ghrelin activates the appetite stimulating nerve cells whereas in pituitary gland, ghrelin promotes the release of growth hormone by increasing calcium levels inside the cells. This results in increased production and release of growth hormone which promotes overall growth by controlling the body metabolism.
The hypothalamus area of brain strongly controls the eating habits and appetite of people. The ghrelin containing nerve cells are found in two areas of hypothalamus - the arcuate nucleus and paraventricular nucleus. The former is the primary site of ghrelin action. In the arcuate nucleus, ghrelin stimulates the release of certain compounds responsible for increasing the appetite. It also suppresses the release of other compounds by other neurons which suppress appetite. The net effect is an increase in appetite caused by hunger hormone. In the paraventricular nucleus, ghrelin stimulates corticotrophin releasing hormone-related neurons which in turn release adrenocorticotrophic hormone (ACTH) and cortisol. This results in an increase in blood sugar levels.
Studies have also shown that ghrelin in stomach transmits signals through vagus nerve. Vagus nerve supplies fibers to the stomach and promotes stomach contractions. Vagus nerve arises from the brain, reaches stomach and provides nerve supply to many other organs. To study the role of vagus nerve, the experiments in rats were done where the nerve fibers to stomach were blocked. Ghrelin in these rats failed to induce appetite, growth hormone secretion and activation of appetite-causing compounds in nerve cells. This experimental study showed that vagus nerve is mainly responsible for transmission of ghrelin induced signals from stomach to brain.
Ghrelin also increases acid secretion in stomach and stimulates stomach contractions. These stomach contractions are felt as hunger pangs.