Which region is the site for absorption of the intrinsic factor–vitamin B12 complex?

Which region is the site for absorption of the intrinsic factor–vitamin B12 complex?

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

Answer: (A)

The key idea is that the intrinsic factor–vitamin B12 complex is absorbed in the terminal part of the small intestine. Intrinsic factor, produced by the stomach, binds vitamin B12 to form a stable complex. This complex travels to the ileum, where specialized enterocytes have receptors (a cubilin–amionless system) that recognize and bind the IF–B12 complex in a calcium-dependent process, allowing it to be taken up by endocytosis. Once inside the enterocyte, B12 is released and then binds to transcobalamin II in the blood for transport to tissues. So, while intrinsic factor is made in the stomach, the actual absorption of the complex occurs in the terminal ileum.

The key idea is that the intrinsic factor–vitamin B12 complex is absorbed in the terminal part of the small intestine. Intrinsic factor, produced by the stomach, binds vitamin B12 to form a stable complex. This complex travels to the ileum, where specialized enterocytes have receptors (a cubilin–amionless system) that recognize and bind the IF–B12 complex in a calcium-dependent process, allowing it to be taken up by endocytosis. Once inside the enterocyte, B12 is released and then binds to transcobalamin II in the blood for transport to tissues. So, while intrinsic factor is made in the stomach, the actual absorption of the complex occurs in the terminal ileum.