Does autoimmune gastritis cause vitamin B12 malabsorption and pernicious anemia?

Pathophysiology of parietal cell loss

• Autoimmune gastritis is driven by an organ-specific, CD4+ T-cell-mediated autoimmune destruction of gastric parietal cells within the oxyntic mucosa, specifically targeting the H+/K+-ATPase proton pump.
• The progressive destruction and subsequent atrophy of these cells result in a dual secretory failure: a severe reduction in hydrochloric gastric acid (leading to achlorhydria) and a critical deficiency in intrinsic factor (IF) synthesis.

Malabsorption and hematologic impact

• Gastric acid is necessary to liberate dietary vitamin B12 (cobalamin) from food proteins. Without sufficient acid, initial cobalamin release is severely impaired.
• More significantly, the absence of intrinsic factor prevents the formation of the IF-B12 complex. Because this complex is required for receptor-mediated endocytosis in the terminal ileum via cubilin receptors, its absence causes profound vitamin B12 malabsorption.
• Chronic depletion of vitamin B12 stores impairs folate metabolism and DNA synthesis during erythropoiesis, leading to megaloblastic marrow changes and the clinical manifestation of pernicious anemia.

Secondary systemic and neoplastic pathways

• The loss of gastric acid removes the negative feedback loop on G-cells, causing compensatory hypergastrinemia.
• Elevated circulating gastrin exerts a strong trophic effect on enterochromaffin-like (ECL) cells in the gastric mucosa. Chronic stimulation drives ECL cell hyperplasia, which can progress to dysplasia and type 1 gastric neuroendocrine tumors.

Bottom line

Autoimmune gastritis initiates a cascade where T-cell-mediated destruction of parietal cells causes both acid and intrinsic factor deficiencies, directly driving chronic vitamin B12 malabsorption and pernicious anemia. Over time, the resulting hypergastrinemia also increases the clinical risk of ECL cell hyperplasia and type 1 gastric neuroendocrine tumors, highlighting the need for systemic surveillance beyond simple B12 replacement therapy.