Which segment of the small intestine is responsible for absorption of bile acids and B12 with intrinsic factor?

• A. Duodenum
• B. Cecum
• C. Ileum
• D. Jejunum

Answer: (C)

The ileum handles these two specialized absorption tasks. Bile acids are reabsorbed mainly in the terminal ileum as part of enterohepatic circulation, using the apical sodium-dependent bile acid transporter to return them to the liver for reuse. Vitamin B12, after pairing with intrinsic factor produced in the stomach, is taken up by ileal enterocytes through a receptor system (cubilin with its cofactor amnionless) that recognizes the B12–intrinsic factor complex, allowing endocytosis. Once inside the enterocytes, B12 is released and travels in the blood bound to transcobalamin II.

Other segments aren’t the primary sites for these processes. The duodenum and jejunum do most general nutrient absorption, but not the B12–IF complex, and bile acids reabsorption occurs mainly in the ileum rather than the cecum, which is part of the large intestine. The ileum’s role in both bile acid reclamation and B12 absorption is why it’s the correct site.