Three messengers for three jobs
A meal arrives in waves, and the gut answers in waves of peptide hormones. The oldest-known trio is worth memorizing because each one solves a clear problem. Gastrin, released by G cells in the stomach when protein arrives, tells the stomach to make acid and churn. Secretin, released by S cells of the duodenum when acidic chyme spills in, asks the pancreas and bile ducts for a flood of bicarbonate to neutralize that acid. Cholecystokinin (CCK), released by I cells when fat and protein arrive, calls for pancreatic enzymes and squeezes the gallbladder to deliver bile.
- Protein in the stomach → gastrin → more acid and stronger churning.
- Acidic chyme enters the duodenum → secretin → pancreatic bicarbonate neutralizes it.
- Fat and protein in the duodenum → CCK → pancreatic enzymes and gallbladder contraction.
Feedback keeps the system from overshooting
Each signal carries its own off-switch, an example of negative feedback you can feel in your own digestion. Secretin's bicarbonate raises the pH of chyme; once the duodenum is no longer acidic, secretin release falls. As the stomach empties and protein is buffered, gastrin slows. CCK also acts on the brain to produce satiety, which is part of why a fatty meal feels filling. The point is not just *that* these hormones turn on, but that they turn off when their job is done.
Between meals a different rhythm takes over. Motilin is released in bursts during fasting and drives the “housekeeping” waves that sweep leftover debris down the gut. It is a reminder that gut hormones manage not only the busy hour after eating but also the quiet hours that follow.