Intrinsic factor is required for absorption of vitamin B12 in which part of the GI tract?

• A. Duodenum
• B. Terminal ileum
• C. Jejunum
• D. Cecum

Answer: (B)

The essential idea is that vitamin B12 absorption requires a specific carrier from the stomach, intrinsic factor, and this complex is taken up only in a particular part of the small intestine. Intrinsic factor is a glycoprotein secreted by gastric parietal cells. After B12 is freed from dietary proteins, intrinsic factor binds it to form a B12–intrinsic factor complex. This complex travels to the distal part of the small intestine, where specialized receptor-bearing enterocytes in the terminal ileum recognize and internalize the complex through receptor-mediated endocytosis. The terminal ileum has the necessary receptors (cubilin with amnionless) to absorb the B12–intrinsic factor complex efficiently.

The other segments you might think of don’t provide this specific uptake mechanism. The duodenum and jejunum lack the receptors needed for the intrinsic factor–B12 complex, so they’re not the primary sites of absorption. The cecum is involved in other intestinal processes and isn’t equipped for this specialized uptake. Therefore, absorption of vitamin B12, with the help of intrinsic factor, occurs in the terminal ileum.