Cholecystokinin is a hormone released by cells in the upper small intestine when fat and protein arrive from the stomach. It signals the gallbladder to contract and release bile for fat digestion, stimulates pancreatic enzyme secretion, and slows gastric emptying. It also acts on the brain as a satiety signal, contributing to the fullness felt after a rich meal. Researchers have studied it in appetite regulation and obesity, though therapeutic use remains experimental. Its discovery explained the coordination of digestion: the gut senses nutrients and orchestrates the organs needed to process them, with dietary fat the strongest trigger.