In older adults, vitamin B12 absorption is affected primarily by which condition?

• A. Increased stomach acid production.
• B. Adequate intrinsic factor production.
• C. Decreased intrinsic factor production due to atrophic gastritis.
• D. Enhanced absorption in the small intestine.

Answer: (C)

Vitamin B12 absorption relies on intrinsic factor produced by the stomach’s parietal cells. The B12 that binds intrinsic factor can then be absorbed in the ileum via specific receptors. In older adults, atrophic gastritis reduces the number of parietal cells and lowers intrinsic factor production, so most vitamin B12 cannot be effectively absorbed. While some B12 can be taken up by slow, passive diffusion without intrinsic factor, this route is minimal and not enough to meet needs. So the primary issue in aging is diminished intrinsic factor from atrophic gastritis.

Other options don’t fit as well because increasing stomach acid would mainly affect release of B12 from food, and in many older individuals acid production is actually reduced; having adequate intrinsic factor would permit normal absorption but isn’t the case here; and enhanced absorption in the small intestine isn’t the problem—it's the lack of intrinsic factor that blocks the main absorption pathway.