# Vitamin B12 / Folate

Vitamin B12 (cobalamin) and folate (vitamin B9) are essential helper molecules. They work in 'one-carbon metabolism'. B12 is needed for two enzymes. One is methionine synthase. It remakes methionine from homocysteine. The other is methylmalonyl-CoA mutase. Folate supplies methyl groups through the folate cycle. Running low on either causes megaloblastic anemia, by stalling DNA synthesis. But B12 deficiency does extra harm. It damages nerves. That includes a spinal-cord problem called subacute combined degeneration. Folate cannot fully cover those routes. Low B12 is common in older adults, strict vegans, and people on metformin or acid-blockers (PPIs). Total serum B12 can look falsely normal. So your true status shows up better in two other markers. Those are high methylmalonic acid, and active-B12 (holotranscobalamin). Both vitamins are checked when investigating high homocysteine and its heart and cognitive risks.

## Sources

- Scott JM. (1999). Folate and vitamin B12. Proceedings of the Nutrition Society. https://doi.org/10.1017/s0029665199000580
- Stabler SP. (2013). Vitamin B-12 deficiency. New England Journal of Medicine. https://doi.org/10.1056/NEJMcp1113996
- Martí-Carvajal AJ, Solà I, Lathyris D. (2015). Homocysteine-lowering interventions for preventing cardiovascular events (Cochrane review). Cochrane Database of Systematic Reviews. https://doi.org/10.1002/14651858.CD006612.pub4

---

_Canonical: https://longevity-switzerland.com/en/glossary/vitamin-b12-folate · Part of Longevity Cities · Updated 2026-06-22_