Vitamin B12 absorption occurs primarily in which part of the small intestine?

• A. Duodenum
• B. Jejunum
• C. Ileum
• D. Cecum

Answer: (C)

Vitamin B12 absorption hinges on intrinsic factor and happens in the distal part of the small intestine. Intrinsic factor, produced in the stomach, binds B12 and forms a complex that travels to the ileum, where specific receptors on enterocytes recognize and internalize it. Once inside the cell, B12 is released and then bound to transcobalamin II for transport in the blood. If the terminal ileum is damaged or removed, or intrinsic factor is deficient, B12 absorption drops, leading to deficiency. The duodenum and jejunum handle other nutrients, while the cecum is part of the large intestine, where B12 absorption does not occur under normal conditions. Thus, the ileum is the primary site of B12 absorption.