Which absorption process is specific to the ileum for bile acids and vitamin B12?

Which absorption process is specific to the ileum for bile acids and vitamin B12?

• A. Ileal reabsorption of bile acids via the apical ASBT transporter and B12 absorption with intrinsic factor-mediated uptake
• B. Iron absorption occurs in the duodenum
• C. Fat digestion occurs in the stomach
• D. Water absorption occurs in the colon

Answer: (A)

The ileum is specialized for two tightly linked absorption processes: reabsorption of bile acids and uptake of vitamin B12 bound to intrinsic factor. Bile acids are reclaimed in the terminal ileum by the apical ASBT transporter on ileal enterocytes, feeding back into the enterohepatic circulation. Vitamin B12, after binding intrinsic factor in the stomach, is absorbed in the ileum via receptor-mediated endocytosis of the IF–B12 complex through the cubilin/amnionless receptor. This combination of bile acid reclamation and IF-bound B12 uptake is specific to the ileum, which is why this scenario best matches the question. Other statements refer to processes in other parts of the gut or general digestion, such as iron absorption mainly in the duodenum, fat digestion starting in the small intestine rather than the stomach, and water absorption occurring prominently in the colon.

The ileum is specialized for two tightly linked absorption processes: reabsorption of bile acids and uptake of vitamin B12 bound to intrinsic factor. Bile acids are reclaimed in the terminal ileum by the apical ASBT transporter on ileal enterocytes, feeding back into the enterohepatic circulation. Vitamin B12, after binding intrinsic factor in the stomach, is absorbed in the ileum via receptor-mediated endocytosis of the IF–B12 complex through the cubilin/amnionless receptor. This combination of bile acid reclamation and IF-bound B12 uptake is specific to the ileum, which is why this scenario best matches the question. Other statements refer to processes in other parts of the gut or general digestion, such as iron absorption mainly in the duodenum, fat digestion starting in the small intestine rather than the stomach, and water absorption occurring prominently in the colon.