Which acid is found in the human stomach?

The stomach lining secretes hydrochloric acid, strong enough to bring the contents down to a pH of roughly 1.5 to 3. That acidity does several jobs at once. It kills most bacteria arriving with food, it unfolds proteins so digestive enzymes can attack them, and it converts the inactive precursor pepsinogen into pepsin, the enzyme that actually breaks proteins into fragments. Making the enzyme in an inactive form and switching it on only inside the stomach cavity is a safety measure. The stomach survives its own acid thanks to a thick layer of mucus and bicarbonate secreted by the lining, plus rapid replacement of surface cells. When that defence fails, often because of infection with Helicobacter pylori or long-term use of certain anti-inflammatory drugs, ulcers form. Heartburn is acid escaping upward into the oesophagus, which has no such protection, and antacids work by neutralising it.

The discovery that a bacterium causes most stomach ulcers overturned decades of advice blaming stress and spicy food.