Which vitamin is primarily absorbed in the terminal ileum?

Which vitamin is primarily absorbed in the terminal ileum?

• A. Vitamin B12
• B. Vitamin C
• C. Vitamin D
• D. Thiamine

Answer: (A)

The main concept here is that vitamin B12 absorption depends on a specialized process that occurs in the terminal ileum. B12 from the diet first binds intrinsic factor produced by the stomach; this IF–B12 complex travels to the terminal ileum, where it binds to receptors on ileal enterocytes (cubilin/amnionless) and is absorbed. Once inside the cells, B12 is handed off to transport proteins to reach circulation. This site-specific, receptor-mediated uptake is why the terminal ileum is essential for B12 absorption. Other vitamins don’t share this exact requirement or site. Vitamin C is absorbed mainly in the jejunum through specific transporters, not in the terminal ileum. Vitamin D is fat-soluble and absorbed along the small intestine with lipids, not confined to the terminal ileum. Thiamine is absorbed primarily in the duodenum and jejunum via distinct transport systems.

The main concept here is that vitamin B12 absorption depends on a specialized process that occurs in the terminal ileum. B12 from the diet first binds intrinsic factor produced by the stomach; this IF–B12 complex travels to the terminal ileum, where it binds to receptors on ileal enterocytes (cubilin/amnionless) and is absorbed. Once inside the cells, B12 is handed off to transport proteins to reach circulation. This site-specific, receptor-mediated uptake is why the terminal ileum is essential for B12 absorption.

Other vitamins don’t share this exact requirement or site. Vitamin C is absorbed mainly in the jejunum through specific transporters, not in the terminal ileum. Vitamin D is fat-soluble and absorbed along the small intestine with lipids, not confined to the terminal ileum. Thiamine is absorbed primarily in the duodenum and jejunum via distinct transport systems.