Why are cobalamin deficiencies common in ileal disease?

Why are cobalamin deficiencies common in ileal disease?

• A. The ileum is not involved in cobalamin absorption.
• B. Cobalamin absorption occurs in the stomach.
• C. The ileum is where cobalamin is absorbed; ileal disease reduces absorption causing deficiency.
• D. Cobalamin is synthesized by intestinal bacteria and does not need dietary intake.

Answer: (C)

Cobalamin (vitamin B12) absorption depends on the terminal ileum. The B12–intrinsic factor complex is made in the stomach and small intestine, but its absorption happens specifically in the terminal ileum through receptor-mediated uptake by ileal enterocytes. If the ileum is diseased or surgically removed, this absorption step is impaired, so B12 isn’t efficiently absorbed and stores eventually run low, leading to deficiency. The stomach is not the site of B12 absorption, and we don’t rely on intestinal bacteria to provide usable B12 for humans, so those ideas don’t explain why ileal disease causes deficiency.

Cobalamin (vitamin B12) absorption depends on the terminal ileum. The B12–intrinsic factor complex is made in the stomach and small intestine, but its absorption happens specifically in the terminal ileum through receptor-mediated uptake by ileal enterocytes. If the ileum is diseased or surgically removed, this absorption step is impaired, so B12 isn’t efficiently absorbed and stores eventually run low, leading to deficiency. The stomach is not the site of B12 absorption, and we don’t rely on intestinal bacteria to provide usable B12 for humans, so those ideas don’t explain why ileal disease causes deficiency.