Vitamin B12

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. The RDA is 2.4 mcg/day for adults (slightly higher in pregnancy and lactation). There is no established upper limit because excess is excreted; oral doses far above the RDA are used 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-diet); older adults often lose the ability to release B12 from food due to reduced stomach acid and may need fortified foods or supplements (see senior-training); and people on long-term acid-suppressing or metformin therapy, or with absorption disorders, are also at risk. Because the liver stores several years' worth, deficiency from a new vegan diet may take time to appear — but should still be prevented proactively.

• 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 for adults; no upper limit (excess excreted).
• Vegans/strict vegetarians must supplement or use fortified foods.
• Older adults, acid-suppressant/metformin users, malabsorption are at risk.

• plant-based-diet — the key completeness gap for vegans.
• vitamins-overview — a water-soluble B-vitamin.
• senior-training — reduced absorption with age.
• iron — both relevant to anemia and restrictive diets.