Gut Hormones and Appetite
Over two dozen hormones have been identified in various parts of the gastrointestinal system. All of them are peptides and many of them are also found in other tissues, especially the brain.
The following eight hormones are the ones most people associate with digestion and appetite:
Gastrin is a mixture of several peptides. It is secreted by cells in the stomach and duodenum. Gastrin stimulates the exocrine cells of the stomach to secrete gastric juice; a mixture of hydrochloric acid and pepsin.
Secretin is secreted by cells in the duodenum when they are exposed to the acidic contents of the emptying stomach. It stimulates the exocrine portion of the pancreas to secrete bicarbonate into the pancreatic fluid (thus neutralising the acidity of the intestinal contents).
CCK is also a mixture of peptides which are secreted by cells in the duodenum when they are exposed to food. It acts on the gall bladder, stimulating it to contract and force its contents of bile into the intestine. CCK also acts on neurons leading back to the medulla oblongata which give a satiety signal: in other words this hormone makes you feel full after eating.
Is important to mention as it inhibits the release of gastrin. It acts on the duodenum where it inhibits the release of secretin and cholecystokinin . Also, acts on the pancreas where it inhibits the release of glucagon.
Taken together, all of these actions lead to a reduction in the rate at which nutrients are absorbed from the contents of the intestine. Somatostatin is also secreted by the hypothalamus and the pancreas.
Ghrelin is the hunger hormone. It’s made in the stomach, and it acts on the hypothalamus to stimulate feeding i.e it generates a hunger feeling. Ghrelin doesn’t cause obesity, but scientists think taming it might help us control over-eating eating habits. It is secreted by endocrine cells in the stomach. This action counteracts the inhibition of feeding by leptin and PYY3-36 .
Neuropeptide Y (NPY)
Neuropeptide Y (which is also secreted by neurons in the hypothalamus) is a potent feeding stimulant and causes increased storage of ingested food as fat. Neuropeptide Y also blocks the transmission of pain signals to the brain.
Peptide YY3-36 contains 34 amino acids, many of them in the same positions as those in neuropeptide Y. However, the action of PYY3-36 is just the reverse of that of NPY, being a potent feeding inhibitor. It is released by cells in the intestine after meals. The amount secreted increases with the number of calories that were ingested. PYY3-36 acts on the hypothalamus to suppress appetite. It also acts on the pancreas to increase its exocrine secretion of digestive juices and the gall bladder to stimulate the release of bile. The appetite suppression mediated by PYY3-36 works more slowly than that of cholecystokinin and more rapidly than that of leptin.
Leptin is manufactured in fat cells (adipose tissue), and the level of circulating leptin is directly proportional to the total amount of fat in the body. Leptin acts on receptors in the hypothalamus of the brain where it counteracts the effects of neuropeptide Y: the result being inhibition of food intake.
This inhibition is long-term, in contrast to the rapid inhibition of eating by cholecystokinin (CCK) and the slower suppression of hunger between meals mediated by PPY3-36
The absence of this functional hormone (or its receptor) leads to uncontrolled food intake and results in obesity. Leptin also acts on hypothalamic neurons responsible for stimulating the secretion of gonadotropin-releasing hormone (GnRH). You may be interested to know that women who are very thin from limited food intake or intense physical training may cease to menstruate because of their lack of leptin-secreting fat cells.
In addition to its effect on the hypothalamus, leptin acts directly on the cells of the liver and skeletal muscle where it stimulates the oxidation of fatty acids in the mitochondria. This reduces the storage of fat in those tissues (but not in adipose tissue).