Incretin

Incretins are a group of metabolic hormones that stimulate a decrease in blood glucose levels. Incretins do so by causing an increase in the amount of insulin released from pancreatic beta cells of the islets of Langerhans after eating, before blood glucose levels become elevated. They also slow the rate of absorption of nutrients into the blood stream by reducing gastric emptying and may directly reduce food intake. They also inhibit glucagon release from the alpha cells of the islets of Langerhans. The two main candidate molecules that fulfill criteria for an incretin are the intestinal peptides glucagon-like peptide-1 (GLP-1) and gastric inhibitory peptide (also known as: glucose-dependent insulinotropic polypeptide or GIP). Both GLP-1 and GIP are rapidly inactivated by the enzyme dipeptidyl peptidase-4 (DPP-4); both GLP-1 and GIP are members of the glucagon peptide superfamily.

"Many factors stimulate insulin secretion, but the main one is blood glucose. Incretins, especially GIP and GLP-1 secreted, respectively, by K and L cells in the gut are also important", (Rang and Dale's Pharmacology (2015)).