Which factor is necessary for the absorption of Vitamin B12 in the ileum?

The absorption of Vitamin B12 in the ileum is critically dependent on intrinsic factor, which is a glycoprotein produced by the parietal cells of the stomach. When Vitamin B12 is ingested from dietary sources, it initially binds to a protein called haptocorrin in the stomach. This complex is then degraded in the intestine, after which intrinsic factor, which binds to Vitamin B12, facilitates its absorption in the ileum.

Without intrinsic factor, the absorption of Vitamin B12 is significantly impaired, leading to conditions such as pernicious anemia. This underscores the necessity of intrinsic factor for effective Vitamin B12 absorption, particularly since the ileum itself has specific receptor sites that recognize the Vitamin B12-intrinsic factor complex.

Other options, such as extrinsic factors, absorption factors, or intestinal flora, do not play a direct role in the absorption of Vitamin B12 in the ileum. Extrinsic factors might refer to dietary components while absorption factors could pertain to general absorption processes that do not specifically influence Vitamin B12. Intestinal flora is important for overall gut health, but they are not essential for the direct absorption of Vitamin B12. Thus, intrinsic factor is uniquely crucial in this process.