Describe the digestive processes required for vitamin B12 absorption.

Describe the digestive processes required for vitamin B12 absorption.

• A. B12 is released in the stomach and binds intrinsic factor in the stomach, then absorbed in the colon.
• B. Gastric acid releases B12 from dietary proteins; B12 then binds to R-proteins in the stomach and intrinsic factor, enabling absorption in the terminal ileum.
• C. B12 is absorbed in the duodenum independently of intrinsic factor.
• D. B12 absorption occurs in the jejunum via passive diffusion.

Answer: (B)

Vitamin B12 absorption depends on a coordinated sequence that starts in the stomach and ends in the terminal ileum, using intrinsic factor (IF) as the key carrier. Gastric acid and pepsin release B12 from dietary proteins in the stomach. In the stomach, B12 initially binds to R-proteins (haptocorrin) which shield it from acid. As the contents move into the small intestine, pancreatic proteases degrade these R-proteins, freeing B12 to bind intrinsic factor, which is secreted by the stomach. The IF–B12 complex then travels to the terminal ileum, where it is absorbed by receptor-mediated uptake into enterocytes. A small amount of B12 can be absorbed by passive diffusion if large oral doses are given, but this is not the primary route. Absorption in the colon or independent absorption in the jejunum/duodenum without IF does not describe the main physiological pathway.

Vitamin B12 absorption depends on a coordinated sequence that starts in the stomach and ends in the terminal ileum, using intrinsic factor (IF) as the key carrier. Gastric acid and pepsin release B12 from dietary proteins in the stomach. In the stomach, B12 initially binds to R-proteins (haptocorrin) which shield it from acid. As the contents move into the small intestine, pancreatic proteases degrade these R-proteins, freeing B12 to bind intrinsic factor, which is secreted by the stomach. The IF–B12 complex then travels to the terminal ileum, where it is absorbed by receptor-mediated uptake into enterocytes. A small amount of B12 can be absorbed by passive diffusion if large oral doses are given, but this is not the primary route. Absorption in the colon or independent absorption in the jejunum/duodenum without IF does not describe the main physiological pathway.