The ileum is the primary site for absorption of vitamin B12 and bile salts.

• A. True
• B. False; it's the duodenum
• C. False; it's the jejunum
• D. False; it's the colon

Answer: (A)

The terminal ileum is specialized for reabsorption of vitamin B12 and bile salts, making this statement true. Vitamin B12 absorption requires intrinsic factor produced in the stomach; the B12–intrinsic factor complex is absorbed in the ileum via receptor-mediated uptake on ileal enterocytes (cubilin/megalIN system) and then binds to transcobalamin II in the blood for transport. This process occurs predominantly in the terminal portion of the ileum, so damage or loss of this segment markedly impairs B12 absorption.

Bile acids, after aiding fat digestion, are reabsorbed mainly in the ileum through the apical sodium-dependent bile acid transporter (ASBT) and then returned to the liver via the enterohepatic circulation. The ileum’s transport systems and tight coupling to enterohepatic circulation are why B12 and bile acids are primarily absorbed there, not in the duodenum, jejunum, or colon.