The absorption of the B12-intrinsic factor complex occurs in which part of the small intestine?

• A. Duodenum
• B. Jejunum
• C. Distal ileum
• D. Ascending colon

Answer: (C)

Absorption of the B12-intrinsic factor complex occurs in the distal ileum. In this region, specialized enterocytes express receptors (the cubilin–amnion complex) that specifically bind the intrinsic factor–vitamin B12 complex and take it up by endocytosis in a calcium-dependent process. This site is the only physiologically significant location for IF-bound B12 uptake; the duodenum and jejunum handle many other nutrients, but not this complex, and the ascending colon is outside the small intestine. After uptake, B12 is released from intrinsic factor inside enterocytes and then bound to transcobalamin II in the blood for transport to tissues. (High oral B12 doses can be absorbed by passive diffusion, but that is not the normal, receptor-mediated pathway.)