The hunger-inducing hormone ghrelin, together with the hormones leptin and cortisol, regulates the sensation of hunger and satiety in animals and humans. In addition, it exerts an influence on numerous processes in the body, such as sleep behavior, stress reduction and blood circulation. Research is still needed regarding the exact relationships.
What is ghrelin?
Schematic diagram showing the anatomy and structure of the endocrine (hormone) system. Click to enlarge. Ghrelin is a hormone produced in the gastric mucosa and pancreas. It was discovered in 1999. Its name comes from English and is an abbreviation for Growth Hormone Release Inducing. It is a fat-insoluble hormone with a protein structure consisting of 28 amino acids. Its main function is to regulate the feeling of hunger and satiety. When no food is consumed for a long time, the level of ghrelin in the blood rises and increases the feeling of hunger. After eating, the level decreases again. In addition, ghrelin regulates the formation of the growth hormone somatropin, which is produced in the pituitary gland and ensures normal physical growth.
Production, formation, and manufacture
Glands in the mucosa of the stomach floor are primarily responsible for the production of ghrelin. In addition, the hormone is also produced by cells of the pancreas. A precursor of ghrelin is also produced in the brain, namely in the hypothalamus and the pituitary gland. This hormone precursor is converted into the active form by cleavage of some amino acids. Apparently, not only hunger leads to an increased release of ghrelin, but also shorter and poor sleep, as well as other stress factors.
Function, effect and properties
Ghrelin regulates food intake by increasing hunger. It also slows metabolism and restricts the body’s ability to burn fat. In addition, leptin and cortisol are also involved in controlling feelings of hunger and satiety. Cortisol is a stress hormone that causes appetite to increase. Leptin sends a message to the brain to decrease appetite and burn more calories. Ghrelin has numerous properties in addition to its function in food metabolism. It acts on a receptor in the pituitary gland that controls the release of growth hormone (somatropin). This means that growth hormone is released when we are hungry. Somatropin is important for normal growth. If somatropin production is reduced during adolescence, or if cells do not respond adequately to it, physical growth stops prematurely. In adults, somatropin regulates, among other things, the proportion of body fat and muscle mass as well as bone mineral density. In addition, ghrelin in the hippocampus in the brain is thought to influence memory performance and learning ability. In this context, a low level of ghrelin ensures better memory performance. This mechanism is probably responsible for the fact that learning is easier during the day than at night, since ghrelin secretion is increased during the night. Ghrelin also has an influence on sleep behavior and deep sleep phases. It is therefore suspected that people who sleep poorly or too little are more prone to obesity. Ghrelin could also contribute to alleviating depression. Animal studies have confirmed the hormone’s anxiety-reducing effect, which is also responsible for reducing stress. The influences of the hormone on sleep behavior, stress reduction and blood circulation are complex and have not yet been conclusively researched. Research is also still needed regarding the interaction with other hormones such as leptin and cortisol.
Diseases, ailments, and disorders
Presumably, ghrelin plays a role in the development of obesity because blood levels of ghrelin rise when people are hungry. However, contrary to expectations, overweight people have been found to produce too little ghrelin, not too much. It is possible that higher body weight leads to increased sensitivity to ghrelin, so that only small amounts are needed to trigger a feeling of hunger. However, more research is needed to conclusively clarify this question. Since lack of sleep provides for increased ghrelin release, poor sleep likely contributes to the development of obesity. Stress also leads to increased ghrelin levels, thus forming another factor in the development of obesity.It has also been found that stress-induced elevated ghrelin levels make the brain more sensitive to traumatic experiences, which may be associated with the development of post-traumatic stress disorder. In addition, ghrelin has been shown to be one of the factors contributing to the development of alcohol dependence. Animal experiments showed that mice injected with ghrelin drank more alcohol than other mice. In the rarely occurring Prader-Willi syndrome, ghrelin levels are sometimes greatly elevated. This disease is associated with a lack of a feeling of fullness. The cause is a gene peculiarity that leads to a dysfunction in the diencephalon. The excessive feeling of hunger in these patients often leads to severe obesity and secondary diseases such as diabetes mellitus. As a result, these individuals have a shortened life expectancy. Elevated values are also found in anorexia. In this case, the high ghrelin level does not lead to an increased feeling of hunger, but the patients are apparently resistant to the hormone’s hunger-inducing effect.
