Gastric acid and intrinsic factor are required for dietary vitamin B12 absorption.

Physiological and clinical evidence strongly supports the critical roles of gastric acid and intrinsic factor in the absorption of vitamin B12, demonstrating that gastric pathologies directly lead to deficiency.

Gastric physiology and B12 absorption

• Multistep absorption cascade: Dietary vitamin B12 (cobalamin) is bound to animal proteins. Gastric acid and pepsin denature these proteins to release the bound cobalamin, which then binds to salivary haptocorrin. In the duodenum, pancreatic proteases degrade haptocorrin, allowing freed B12 to bind to intrinsic factor (secreted by gastric parietal cells).
• Active absorption: The intrinsic factor-B12 complex travels to the terminal ileum, where it binds to cubilin-amnionless receptors to undergo calcium-dependent, receptor-mediated endocytosis.

Pathological mechanisms of deficiency

• Hypochlorhydria: A reduction in gastric acid directly impairs the cleavage of vitamin B12 from dietary proteins, causing food-cobalamin malabsorption. While crystalline B12 (found in supplements) is absorbed normally, dietary B12 absorption is severely compromised.
• Atrophic gastritis: Progressive autoimmune or environmental destruction of parietal cells causes a dual failure—profound hypochlorhydria and a critical loss of intrinsic factor. This dual disruption completely compromises both dietary B12 liberation and receptor-mediated absorption.

Clinical evidence and risks

• Prevalence and association: Patients with autoimmune gastritis exhibit a markedly elevated risk of vitamin B12 deficiency, with an odds ratio of 11.52 compared to controls. The prevalence of B12 deficiency in atrophic gastritis cohorts ranges from 13.3% in autoimmune subtypes to as high as 38% in older adult populations. Clinical guidelines recommend routine screening for cobalamin deficiency and megaloblastic anemia in these populations.

Bottom line

• Both gastric acid and intrinsic factor are biochemically necessary for dietary vitamin B12 absorption. Hypochlorhydria and atrophic gastritis are leading causes of B12 deficiency due to impaired food-bound cobalamin cleavage and loss of active transport mechanisms.