What is vitamin B12 and who is at risk for deficiency?
B12 is essential for red cell formation and neurologic function. Vegans, older adults, and long-term users of PPIs or metformin are at risk. Symptoms can precede serum-level abnormalities.
Full answer
Vitamin B12 (cobalamin) is essential for red blood cell formation, neurologic function, and DNA synthesis. It is absorbed only with intrinsic factor in the terminal ileum, making absorption a common failure point. B12 deficiency presents with macrocytic anemia, fatigue, cognitive decline, and peripheral neuropathy — often before serum B12 drops below the conventional 200 pg/mL threshold. Functional B12 deficiency can exist with serum levels of 200-400 pg/mL; methylmalonic acid is the confirmatory marker. At-risk populations include vegans, vegetarians, older adults, post-bariatric-surgery patients, and long-term users of PPIs, H2 blockers, or metformin. Provider-reviewed wellness uses an optimization band higher than the lab reference floor.
Educational. Not a substitute for individualized provider review.
- Vitamin B12 — Primary source · oMeds
- Reviewed by Dr. Laura Purdy, MD, MBA · Clinical Director, oMeds · Last reviewed 2026-05-29