Where are B12 and bile salts absorbed?

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

Answer: (B)

B12 and bile salts are absorbed mainly in the terminal part of the small intestine. For B12, it must first bind intrinsic factor in the stomach; the B12–intrinsic factor complex travels to the ileum, where specialized receptors uptake it into enterocytes and then release it into the bloodstream. Bile salts, after aiding fat digestion, are reclaimed in the ileum through the apical sodium-dependent bile acid transporter and then return to the liver via the portal circulation. The colon can reabsorb some bile acids, but the ileum is the primary site. Damage to the ileum, such as from Crohn’s disease, can lead to B12 deficiency and fat malabsorption due to impaired bile acid reabsorption.