Which statement describes B12 absorption?

Which statement describes B12 absorption?

• A. B12 absorption occurs with intrinsic factor-mediated uptake in the ileum
• B. B12 is absorbed in the colon via bacterial synthesis
• C. B12 is absorbed in the stomach by intrinsic factor
• D. B12 does not require intrinsic factor

Answer: (A)

B12 absorption hinges on intrinsic factor and takes place in the terminal ileum. In the stomach, intrinsic factor is secreted by parietal cells and binds dietary B12 to form a complex that survives digestion. This complex then travels to the terminal ileum, where specific receptors on ileal enterocytes recognize the B12–intrinsic factor complex and enable its uptake into the cells. Once inside, B12 is released and binds to transcobalamin II for transport in the bloodstream to tissues. This is why the concept tested is about the necessity of intrinsic factor and the ileal site of absorption. The other statements don’t fit the physiological pathway. B12 absorbed in the colon via bacterial synthesis isn’t how humans take up the vitamin—although gut bacteria can produce B12, humans don’t efficiently absorb it there. While intrinsic factor is essential, absorption does not occur in the stomach itself; intrinsic factor is produced in the stomach, but the actual uptake happens in the ileum. Finally, B12 largely requires intrinsic factor for absorption; only a small amount can be absorbed passively at very high doses, so it’s not accurate to say intrinsic factor isn’t required.

B12 absorption hinges on intrinsic factor and takes place in the terminal ileum. In the stomach, intrinsic factor is secreted by parietal cells and binds dietary B12 to form a complex that survives digestion. This complex then travels to the terminal ileum, where specific receptors on ileal enterocytes recognize the B12–intrinsic factor complex and enable its uptake into the cells. Once inside, B12 is released and binds to transcobalamin II for transport in the bloodstream to tissues. This is why the concept tested is about the necessity of intrinsic factor and the ileal site of absorption.

The other statements don’t fit the physiological pathway. B12 absorbed in the colon via bacterial synthesis isn’t how humans take up the vitamin—although gut bacteria can produce B12, humans don’t efficiently absorb it there. While intrinsic factor is essential, absorption does not occur in the stomach itself; intrinsic factor is produced in the stomach, but the actual uptake happens in the ileum. Finally, B12 largely requires intrinsic factor for absorption; only a small amount can be absorbed passively at very high doses, so it’s not accurate to say intrinsic factor isn’t required.