Which acid is naturally found in the human stomach, aiding digestion?

Hydrochloric acid is naturally found in the human stomach, where it aids digestion. It is the acidic component of gastric juice and is produced mainly by parietal cells in glands lining the stomach.

This acid creates a highly acidic environment, usually with a stomach pH of about 1 to 3. It helps unfold or denature proteins and activates pepsinogen, an inactive enzyme precursor, into pepsin, which breaks proteins into smaller fragments. Gastric acid also helps destroy or inhibit many microorganisms swallowed with food.

A common mix-up is confusing gastric acid with the entire contents of gastric juice. Gastric juice also includes mucus, enzymes such as pepsinogen and gastric lipase, water, and electrolytes. The stomach lining is protected by a mucus-and-bicarbonate barrier, which helps prevent the acid from damaging its own cells.

In the small intestine, pancreatic bicarbonate neutralizes much of the acid arriving from the stomach. The role of stomach acid in digestion was famously investigated in the 1820s and 1830s by physician William Beaumont, who studied digestion through observations of Alexis St. Martin's stomach fistula.