Understanding Stomach Acid: Functions & Role in Digestion

Gastric acid (HCl, pH 1.5–3.5) is secreted by parietal cells (H⁺/K⁺-ATPase). Its functions extend far beyond protein digestion.

Nine Key Functions

1. Protein denaturation: Unfolds tertiary structure → exposes peptide bonds for pepsin.
2. Pepsinogen → pepsin activation: Acid cleaves peptide; pepsin autoactivates. Optimal pH 1.5–2.5.
3. Intrinsic factor secretion (parietal cells): Binds B₁₂ → ileal absorption. Acid not directly required but co-secreted.
4. Iron absorption: Reduces Fe³⁺ → Fe²⁺ (soluble); maintains solubility for duodenal uptake.
5. Calcium absorption: Ionizes calcium salts → soluble Ca²⁺.
6. Antimicrobial barrier: Kills most ingested pathogens (except acid-tolerant: H. pylori, C. difficile spores). Prevents SIBO.
7. Gastric motility regulation: Acid in antrum → gastrin inhibition (negative feedback) → slows emptying. Duodenal acid → secretin/CCK → slows emptying, stimulates pancreatic bicarbonate.
8. Hormone release: Gastrin (G cells), secretin (S cells), CCK (I cells) – all pH-modulated.
9. Water/electrolyte absorption: Minor; mainly in small intestine.

Clinical Disorders

• Hypochlorhydria/achlorhydria: Atrophic gastritis, PPIs, H. pylori, pernicious anemia → B₁₂/iron malabsorption, SIBO, enteric infections.
• Hyperchlorhydria / GERD / PUD: Excess acid, defective barrier, H. pylori → erosions, ulcer, Barrett's.

PPI Use

Effective but long-term → monitor B₁₂, iron, magnesium, fracture risk, C. diff, SIBO. Deprescribe when possible.