cholecystokinin (CCK)

CCK is the gut’s “bring out the digestive tools, and tell me when I’ve had enough” hormone. When fatty or protein-rich food reaches the small intestine, CCK orders the gallbladder to squeeze out bile and the pancreas to release enzymes — and it sends a fullness signal to the brain.

Secreted by I-cells in the duodenum and jejunum in response to fats and proteins, cholecystokinin literally means “move the gallbladder.” It triggers gallbladder contraction to deliver bile, which emulsifies fat, and stimulates the pancreas to secrete digestive enzymes. It also relaxes a muscular valve (the sphincter of Oddi) so bile and enzymes can flow into the intestine, and slows gastric emptying so digestion can keep pace.

Beyond digestion, CCK is one of the body’s satiety signals: it acts on vagus nerve endings and the brain to curb appetite during a meal. This dual role — digestion plus satiety — makes it a recurring target in obesity research, though by itself it has proven hard to harness into a durable weight-loss therapy.