Gastrointestinal hormones, commonly referred to as gut hormones, are a group of hormones secreted by specific cells located primarily in the stomach and small intestine. The hormones control a number of different functions of the digestive organs. Cells that secrete the hormones, called enteroendocrine cells or endocrinocytes, are scattered throughout the digestive system. There have been over 24 types of gastrointestinal hormones identified.
All of the gastrointestinal hormones are peptides, which are chains of amino acids and very similar to proteins. These hormones function in several different ways. They travel through the blood stream to affect the digestive system, including the digestive tube, liver, pancreas, and the brain. They also affect the cells that produce them by interacting with those cell receptor sites. Gastrointestinal hormones can also be secreted into the tissue surrounding them or into nearby cells to produce a reaction.
There are six gastrointestinal hormones that are generally recognized as the primary hormones. They include hormones from the secretin, Gastrin-cholecystokinin, and motilin families. Hormones are grouped together based on their chemical structure, and often perform similar functions.
Secretin and gastric inhibitory peptide (GIP) are part of the secretin family, and is produced as a result of acidic pH in the small intestine. It stimulates secretion of water and bicarbonate in the pancreas and the bile ducts. GIP responds to elevated blood glucose levels in the small intestine, and it inhibits the motility of the small intestine. It also stimulates the beta cells in the pancreas to release insulin.
Ghrelin and motilin are part of the motilin family. It is unclear what stimulates them to be secreted, but they are associated with certain physiological states. Ghrelin seems to be a stimulant of appetite and feeding, as its secretion peaks right before feeding and drops once there is gastric filling. It also strongly stimulates growth hormone secretion. Motilin seems to be associated with fasting, and it also seems to help keep motility of the stomach and small intestine.
The gastrin-cholecystokinin family includes gastrin and cholecystokinin. Gastrin is secreted in the presence of amino acids and peptides in the gastric cavity, stimulates the growth of the epithelium in the gastric tube, and also stimulates gastric acid secretion. Cholecystokinin responds to fatty acids and peptides in the small intestine, and it stimulates the gallbladder to contract and empty in response. It also stimulates the pancreas to secrete enzymes that break down the peptides and fatty acids.
