Ghrelin is a hormone mainly produced and released by the stomach, with smaller amounts released by the small intestine, pancreas and brain, cells known as as P/D1 cells and epsilon cells. Ghrelin is the main hormone responsible for stimulating hunger. Its levels increase and decrease before and after meals and are also dependent upon the amount of fat in the body. The hypothalamus is the region of the brain that is responsible for the release of Ghrelin into the bloodstream.
In animal studies, the removal of the hypothalamus and the ability to produce Ghrelin, resulted in significant decrease in appetite, a decrease in caloric intake, and weight loss. Since Ghrelin helps to induce hunger, some weight loss procedures aim at reducing Ghrelin levels in the body to induce the feeling of satiation, even with a small meal.
Leptin is another hormone produced by the body with a function generally the opposite of Ghrelin. This hormone is produced by fat and adipose tissues in the body. Leptin helps to establish a feeling of satiation or fullness and leads to decreased feeling of hunger.
Functions of Ghrelin
Ghrelin plays an important role in hunger, weight gain, and a variety of other functions. It also plays an important role in a process known as neurotrophy, which is the psychological process of adapting to new environments and learning new processes. Ghrelin has also been shown to act on regions of the brain involved in reward processing such as the amygdala.
Studies suggest that learning is most effective throughout the day and when an individual is hungry during periods of elevated Ghrelin levels. In addition, Ghrelin affects the brain and the learning process through the hippocampus where it modifies nerve connections and cells.
It has has been called the “hunger hormone” because it stimulates appetite, resulting in increased food consumption which can lead to fat storage. Studies have demonstrated that when ghrelin is administered to humans, food intake increases by up to 30%. Ghrelin also stimulates the release of growth hormone, which breaks down fat tissue and builds muscle.
Ghrelin also has also been shown to have a protective effect on the cardiovascular system, as well as playing a role in the release of insulin.
Effects of Ghrelin
A study conducted by Kirchner et al, suggests that Ghrelin is a lipid nutrient sensor rather than a hunger-inducing factor in response to fasting. This study indicates that Ghrelin informs the central nervous system of the availability or unavailability of energy. In addition, the study suggests that gastric distension and water intake does not inhibit the production of the hormone. However, the study indicates that glucose administration helps to reduce Ghrelin levels in the bloodstream.
Eating reduces concentrations of ghrelin, with different nutrients slowing down ghrelin release to varying degrees. Carbs and proteins restrict the production and release of ghrelin to a greater extent than fats.
Somatostatin also restricts ghrelin release, as well as many other hormones released from the digestive tract.
Cardiovascular Effects of Ghrelin
Studies suggest a negative association between Ghrelin and insulin secretion. However, the exact mechanism by which insulin affects Ghrelin levels are unclear. Studies indicate that Ghrelin levels may be under cholinergic control because cholinergic agonists and cholinergic antagonists raise and reduce Ghrelin levels respectively.
Ghrelin and Weight
Ghrelin is a hormone that promotes hunger. Obese people tend to have higher Ghrelin levels. There is a syndrome called Prader-Willi that is a genetic disease causing severe obesity, extreme hunger and learning impairment. Patients with this disease have high levels of circulating ghrelin identified before the onset of obesity. This potentially indicates ghrelin may play be a factor in their increased appetite and body weight.
Dieting increases Ghrelin levels, which may explain why maintaining diet-induced weight loss can be difficult. Oddly enough, ghrelin levels are usually lower in people with higher body fat level as compared with lean people. This may indicate that some people are more sensitive to the hormone, or more resistant.
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