Vitamin B12 absorption occurs primarily in which section?

• A. Duodenum
• B. Proximal jejunum
• C. Distal ileum
• D. Colon

Answer: (C)

Vitamin B12 absorption depends on forming a complex with intrinsic factor produced in the stomach, and this complex is taken up mainly in the distal part of the small intestine. The terminal ileum contains specialized receptors (cubilin—with associated proteins) that bind the intrinsic factor–vitamin B12 complex and mediate its endocytic uptake. This region is uniquely equipped for B12-IF absorption, whereas the duodenum and proximal jejunum handle other nutrients, and the colon does not actively absorb B12 via this mechanism. A small amount of B12 can be absorbed passively at very high oral doses, but physiologic absorption occurs primarily in the distal ileum.