Vitamin B12
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B12 (cobalamin) is needed for red blood cells and nerves, comes almost only from animal foods, and must be supplemented by vegans; the adult RDA is 2.4 mcg/day.
Detail
Vitamin B12 (cobalamin) is a water-soluble vitamin essential for red blood cell formation, DNA synthesis, and nervous system function. Deficiency causes megaloblastic anemia and, if prolonged, irreversible nerve damage (numbness, balance problems, cognitive changes).
B12 is found almost exclusively in animal foods — meat, fish, eggs, dairy — plus fortified foods (some plant milks, nutritional yeast, breakfast cereals). Plants are not a reliable source. Under the Dietary Reference Intakes (1998, still current) the RDA is 2.4 mcg/day for adults, 2.6 mcg in pregnancy and 2.8 mcg in lactation. No Tolerable Upper Intake Level was set, because suitable data on adverse effects were lacking (no UL is not the same as proven safe at any dose); oral doses far above the RDA are used clinically to treat deficiency.
At-risk groups are well defined: vegans and strict vegetarians must obtain B12 from fortified foods or a supplement (see Plant-Based, Vegetarian, and Vegan Diets); older adults: because 10–30% of older people may be unable to absorb food-bound B12 (reduced stomach acid, atrophic gastritis), the DRIs advise people over 50 to meet the RDA mainly from B12-fortified foods or a supplement (see Senior Training (Redirect → Older Adults)). The 2025–2030 Dietary Guidelines likewise list B12 among nutrients older adults need in equal or greater amounts, and tell vegetarians and vegans to monitor B12 status; the Academy of Nutrition and Dietetics (2025) notes that B12 supplements may be the easiest way for vegetarians, especially vegans, to stay adequate; and people on long-term acid-suppressing or metformin therapy, or with absorption disorders, are also at risk. Body stores are large relative to daily losses (the DRI report works with body pools of roughly 1,000–3,000 mcg turning over at about 0.1% a day), so deficiency from a new vegan diet can take a long time to appear. It should still be prevented proactively.
Key facts
- Water-soluble; needed for red cells, DNA synthesis, nerve function.
- Found almost only in animal foods + fortified products; plants unreliable.
- RDA 2.4 mcg/day adults; 2.6 pregnancy; 2.8 lactation (DRI 1998, current). No UL set (data lacking).
- Vegans/strict vegetarians must supplement or use fortified foods.
- Over 50: meet the RDA mainly from fortified foods or a supplement (10–30% of older people malabsorb food-bound B12).
- Acid-suppressant/metformin users and people with malabsorption are also at risk.
Connections
- Plant-Based, Vegetarian, and Vegan Diets — the key completeness gap for vegans.
- Vitamins — Fat-Soluble vs Water-Soluble Overview — a water-soluble B-vitamin.
- Senior Training (Redirect → Older Adults) — reduced absorption with age.
- Iron — both relevant to anemia and restrictive diets.
Sources
- GuidelineInstitute of Medicine. Dietary Reference Intakes for Thiamin, Riboflavin, Niacin, Vitamin B6, Folate, Vitamin B12, Pantothenic Acid, Biotin, and Choline. Washington, DC: The National Academies Press (chapter 9, Vitamin B12) Published 1998 · DOI 10.17226/6015 · Accessed 2026-10-13
- GuidelineNational Academies of Sciences, Engineering, and Medicine. Dietary Reference Intakes for Sodium and Potassium, Appendix J: Dietary Reference Intakes Summary Tables Published 2019 · DOI 10.17226/25353 · Accessed 2026-10-13
- GuidelineDietary Guidelines for Americans, 2025–2030 — accessible (Section 508) version (USDA/HHS) Published 2026-01-16 · Accessed 2026-10-13
- Position standRaj S et al. Vegetarian Dietary Patterns for Adults: A Position Paper of the Academy of Nutrition and Dietetics. J Acad Nutr Diet 2025;125(6):831–846 Published 2025 · DOI 10.1016/j.jand.2025.02.002 · Accessed 2026-10-13
- SourceNIH Office of Dietary Supplements. Vitamin B12 — Fact Sheet for Health Professionals (updated July 2, 2025) Published 2025-07-02 · Accessed 2026-10-13
Corrections (2)
- 2026-10-13 — Reconciled against the 1998 B-vitamin DRI report (still the current B12 DRI) and the 2019 NASEM DRI summary tables: adult RDA 2.4 mcg/day, pregnancy 2.6, lactation 2.8 confirmed. Corrected the reason for 'no upper limit': the DRIs set no UL for B12 because suitable data on adverse effects were lacking, not simply because excess is excreted. Added the DRI advice that people over 50 meet the RDA mainly from fortified foods or supplements (10–30% of older people may malabsorb food-bound B12). 'Liver stores several years' worth' restated from the DRI report's turnover figures. NIH ODS fact sheet citation removed (not retrievable for re-verification on 2026-10-13); added the 2025–2030 Dietary Guidelines and the 2025 Academy of Nutrition and Dietetics vegetarian position.
- 2026-10-13 — Verified against the NIH ODS Vitamin B12 health-professional fact sheet (updated 2025-07-02, read in a browser): gastric acid inhibitors (proton pump inhibitors such as omeprazole, H2-receptor antagonists such as cimetidine) and metformin can reduce B12 absorption and serum B12 -- the medication risk line stands; ODS re-added as a source.
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Educational content only — not medical advice. Always consult a qualified professional for individualized guidance, especially around injury, pregnancy, or medical conditions.