Vitamin B12 Absorption Mechanism

Anatomical Site of Absorption

• The terminal ileum is the exclusive site of physiological vitamin B12 absorption in humans 1

Mechanism of Intestinal Absorption

• Vitamin B12 is released from dietary proteins by gastric acid and pepsin in the stomach, then binds to intrinsic factor, a glycoprotein secreted by gastric parietal cells, which is necessary for its absorption in the terminal ileum 1, 3

Clinical Implications of Ileal Dysfunction

• Resection of more than 20 cm of distal ileum mandates prophylactic vitamin B12 supplementation (1000 mcg intramuscularly monthly for life) 1, 2
• Resection of more than 30 cm of distal ileum significantly increases the risk of B12 deficiency 1
• Ileal Crohn's disease involving more than 30-60 cm of ileum puts patients at risk for B12 deficiency even without resection 1, 2
• Patients with ileal involvement should be screened yearly for B12 deficiency 1, 2