Vitamin B12
Vitamin B12 (cobalamin) is the essential water-soluble vitamin required for DNA synthesis, red blood cell formation, and neurological function. The IOM RDA is 2.4 μg/day for adults. It is produced only by bacteria and is essentially absent from plant foods — deficiency is common in strict vegans, older adults, and anyone with atrophic gastritis or on long-term proton pump inhibitors.
What B12 does
Vitamin B12 (cobalamin) is a cobalt-containing tetrapyrrole cofactor for two mammalian enzymes: methionine synthase (recycling homocysteine to methionine, feeding the one-carbon pool that methylates DNA) and methylmalonyl-CoA mutase (a step in odd-chain-fatty-acid oxidation). Long-standing deficiency causes megaloblastic anaemia and a specific spinal-cord syndrome (subacute combined degeneration) — both of which can precede any drop in serum B12 if functional markers are not tracked.
Intake references
- IOM RDA: 2.4 μg/day for adults; 2.6 μg/day pregnancy; 2.8 μg/day lactation.
- EFSA AI: 4.0 μg/day for adults.
- UL: not established; B12 has extremely low toxicity even at pharmacologic doses.
Sources
| Food | B12 (μg) |
|---|---|
| Clams (3 oz, cooked) | 84 |
| Beef liver (3 oz) | 71 |
| Salmon (3 oz) | 4.8 |
| Beef (3 oz, ground) | 2.4 |
| Cottage cheese (1 cup) | 1.4 |
| Milk (1 cup) | 1.3 |
| Egg (1 large) | 0.5 |
| Fortified nutritional yeast (2 tbsp) | ~8 |
| Fortified plant milk (1 cup) | ~1.2 |
| Unfortified plant foods | Trace (bioavailable form absent) |
Populations at risk
Strict vegans and long-term vegetarians (Pawlak, Nutrition Reviews 2013;71:110–117 found 52% and 7% deficiency, respectively, without supplementation). Adults over 50, roughly 10–30% of whom have atrophic gastritis with impaired protein-bound-cobalamin release. Adults on chronic metformin (long-term use lowers serum B12 by ~20%). Adults on long-term proton pump inhibitors or H2 blockers. People with pernicious anaemia, ileal resection, or bariatric surgery.
How NakedCalorie treats B12
B12 is shown per serving in μg on the nutrient panel where declared. B12-fortified breakfast cereals and plant milks are one of the few instances where a Group 4 formulation choice delivers a genuinely important nutrient that is otherwise hard to obtain without animal foods. The score still reflects the NOVA group and additive profile — the fortification is surfaced separately in the nutrient panel. See iron for the sister nutrient concentrated in animal foods.
Frequently asked questions
- How much B12 do adults need?
- The IOM RDA is 2.4 μg/day for adults, rising to 2.6 μg/day in pregnancy and 2.8 μg/day during lactation. EFSA uses 4.0 μg/day as the adequate intake for adults — a difference driven by more conservative assumptions about absorption efficiency.
- Where does B12 come from?
- It is synthesised only by bacteria and archaea. Animal foods concentrate it because animals absorb it from their gut microbiota; strict plant foods contain effectively none. Fortified nutritional yeast, fortified plant milks, and fortified cereals are reliable vegan-friendly sources.
- Why are older adults at higher risk?
- Stomach acid liberates dietary B12 from its bound protein form. Atrophic gastritis, common past age 60, reduces acid production; long-term proton pump inhibitors do the same. The IOM specifically recommends that adults over 50 get most of their B12 from fortified foods or supplements because the free form absorbs more reliably.
- How is deficiency diagnosed?
- Serum B12 below 200 pg/mL is deficient. In the 200–350 pg/mL 'grey zone' the tie-breakers are elevated methylmalonic acid (MMA) or homocysteine — functional markers that rise before serum B12 falls (Herrmann & Obeid, Clinical Chemistry and Laboratory Medicine 2013;51:1387–1395).
- Do I need injections?
- Oral high-dose supplementation (1,000–2,000 μg/day) works for most causes of deficiency, including atrophic gastritis, because passive diffusion absorbs about 1% of the dose independent of intrinsic factor (Butler, Family Practice 2006;23:279–285). Injections are reserved for severe malabsorption, neurological presentations, or documented pernicious anaemia.
Written by NakedCalorie editorial team.
Medically reviewed by Pending registered dietitian review — last reviewed July 15, 2026.
Educational information only, based on peer-reviewed literature and regulatory sources. Not medical advice.
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