Gastric inhibitory peptide (GIP) is a hormone secreted by K cells in the lining of the duodenum and upper small intestine when food — particularly fat and glucose — arrives from the stomach. Its classic action is to stimulate insulin release from the pancreas in a glucose-dependent manner, part of the “incretin effect” by which oral glucose provokes more insulin than the same glucose given intravenously. GIP was first named for its observed ability to inhibit gastric acid secretion and motility, though its insulin-stimulating role is now considered primary. It also promotes fat storage in adipose tissue. GIP pathways have become major drug targets: modern obesity and diabetes medicines exploit incretin biology, with dual GIP/GLP-1 agonists showing striking efficacy.