Gastrin is the hormone that commands the stomach to produce acid. Secreted by G cells in the stomach lining (and upper small intestine) when food — especially protein — arrives, it travels in the blood to the gastric glands, stimulating parietal cells to release hydrochloric acid and prompting pepsinogen secretion and stomach churning. It is the centrepiece of the gastric phase of digestion, and its release is switched off as acidity rises, in a neat feedback loop. Tumours called gastrinomas can flood the body with gastrin, causing the severe ulceration of Zollinger-Ellison syndrome. Drugs that suppress acid, from antacids to proton pump inhibitors, ultimately work by countering the effects of this hormonal drive.