misuma/iStock/GettyImages
The stomach is a muscular, sack‑like organ situated in the left upper abdomen. It can accommodate up to 2 liters (approximately ½ gallon) of food and liquid. Once food enters, the stomach’s powerful muscular walls churn it, mixing the contents with gastric juice to form a semi‑liquid mixture called chyme.
Gastric juice contains mucus, hydrochloric acid, and the hormone gastrin. Gastrin stimulates the secretion of pepsinogen, the inactive precursor of the digestive enzyme pepsin.
When the stomach’s gastric glands are stimulated—by tasting, smelling, or even thinking of food—they release gastric juice. The hydrochloric acid in this juice lowers the pH to between 1 and 3, the optimal range for pepsin activity. At this acidic pH, pepsinogen is cleaved to produce pepsin by removing a small peptide segment. The stomach’s typical pH of 1.5 to 3.5 provides the environment needed for this conversion.
The acidic environment causes protein molecules to unfold—a process known as denaturation. Denaturation exposes peptide bonds, allowing pepsin to cleave them into smaller fragments called peptides or polypeptides. This proteolytic action can continue for several hours while the partially digested food slowly moves toward the small intestine.
Once chyme enters the small intestine, pancreatic enzymes and brush‑border peptidases further break down the peptides into individual amino acids. These amino acids are readily absorbed into the bloodstream, making them available for cellular use.
Mucus lining the stomach protects the epithelium from the corrosive effects of hydrochloric acid and pepsin. However, when the protective mucus barrier is compromised—often by infection with Helicobacter pylori—pepsin can erode the stomach lining, leading to ulcer formation. Antacids raise gastric pH, which inactivates pepsin, thereby reducing mucosal damage.
While antacids can alleviate ulcer pain and protect the gastric lining, prolonged inhibition of pepsin may impair protein digestion. Incomplete protein absorption can trigger allergic reactions or other gastrointestinal disturbances. Therefore, long‑term antacid use should be monitored by a healthcare professional.
