Which statement best describes the absorption mechanism of vitamin B12 and the role of intrinsic factor?

Which statement best describes the absorption mechanism of vitamin B12 and the role of intrinsic factor?

• A. B12 is absorbed passively in the stomach without intrinsic factor.
• B. B12 binds intrinsic factor produced by parietal cells; the complex is absorbed in the duodenum via intrinsic factor–B12 receptors.
• C. B12 binds intrinsic factor produced by parietal cells; the complex is absorbed in the jejunum via intrinsic factor–B12 receptors.
• D. B12 binds intrinsic factor produced by parietal cells; the complex is absorbed in the ileum via intrinsic factor–B12 receptors.

Answer: (D)

The key idea is that vitamin B12 absorption depends on intrinsic factor produced by stomach parietal cells, and the B12–intrinsic factor complex is taken up mainly in the terminal ileum by specific receptors on ileal enterocytes. Once the complex binds these receptors, B12 is internalized and transported into the bloodstream bound to transcobalamin II. A small amount of B12 can be absorbed passively without intrinsic factor, but this is minimal compared to the receptor‑mediated pathway. The site of absorption being the ileum explains why conditions that damage the ileum or remove intrinsic factor (like pernicious anemia or ileal disease) cause B12 deficiency.

The key idea is that vitamin B12 absorption depends on intrinsic factor produced by stomach parietal cells, and the B12–intrinsic factor complex is taken up mainly in the terminal ileum by specific receptors on ileal enterocytes. Once the complex binds these receptors, B12 is internalized and transported into the bloodstream bound to transcobalamin II. A small amount of B12 can be absorbed passively without intrinsic factor, but this is minimal compared to the receptor‑mediated pathway. The site of absorption being the ileum explains why conditions that damage the ileum or remove intrinsic factor (like pernicious anemia or ileal disease) cause B12 deficiency.