Vitamin B12: an essential supplement, explained without hand-waving
B12 is made by bacteria, not by animals or plants. Here is why supplementation is the honest answer, what dose works, and how to test.
In short
- B12 is synthesised by microbes; farmed animals are routinely supplemented, so much of the world's B12 supply already comes from a factory.
- Reliable vegan sources are supplements and fortified foods — not seaweed, tempeh, mushrooms or unwashed produce.
- A common effective regimen is 50–250 µg daily or 2,000 µg weekly of cyanocobalamin.
- Deficiency develops slowly and can cause irreversible neurological damage, which is why testing beats guessing.
Key numbers
Where B12 actually comes from
Bacteria and archaea synthesise cobalamin. Ruminants obtain B12 from rumen microbes when cobalt supply is adequate; feed for pigs and poultry commonly contains added B12. Supplements and fortified foods provide reliable direct sources for vegans.
Sources that do not work
Spirulina mainly contains inactive analogues. Some seaweeds, mushrooms and fermented foods contain active B12, but amounts and bioavailability vary. They are not dependable substitutes for supplements or appropriately fortified foods.
- Reliable: cyanocobalamin or methylcobalamin tablets, B12-fortified plant milks, nutritional yeast labelled as fortified, fortified cereals.
- Unreliable: algae, fermented foods, unwashed vegetables, 'natural' B12 claims without a labelled microgram figure.
Dose, and why weekly works
Absorption through intrinsic factor saturates near 1.5–2 µg per dose, after which a small percentage crosses passively. That is why a 2,000 µg weekly tablet and a 50 µg daily tablet both maintain status: infrequent large doses lean on passive diffusion.
Testing without being misled
Serum B12 alone is a weak marker. Methylmalonic acid (MMA) and homocysteine reflect functional status better; ask for MMA if symptoms or borderline results appear. Folate supplementation can mask the anaemia while nerve damage continues, which is the reason for the neurological caution.
At a glance
Common regimens and what they suit
| Regimen | Form | Best for | Notes |
|---|---|---|---|
| 50–250 µg daily | Cyanocobalamin | Most adults | Cheapest, most robust |
| 2,000 µg weekly | Cyanocobalamin | People who forget daily pills | Relies on passive absorption |
| 3 × fortified foods daily | Fortified | Adults checking total intake | Provide at least 3 µg/day across servings |
| 1,000 µg daily | Methylcobalamin | Diagnosed deficiency, under supervision | Doctor-guided |
| Injections | Hydroxocobalamin | Absorption disorders | Prescription only |
Common objections
ClaimIf a diet needs supplements it must be unnatural.
What the evidence showsFarmed animals are supplemented too, and iodised salt, fortified flour and vitamin D are standard across omnivore diets. A tablet is a delivery method, not a verdict.
ClaimSpirulina covers B12.
What the evidence showsIt supplies inactive analogues that do not correct deficiency and can distort test results.
ClaimYou only need B12 if you feel tired.
What the evidence showsB12 deficiency can cause neurological problems without anaemia, and damage can be permanent. Reliable intake matters; symptoms need prompt medical assessment.
Questions people ask
- How much B12 should a vegan take?
- Healthy adults can use cyanocobalamin daily or 2,000 µg weekly as a preventive regimen. Individual needs differ; suspected deficiency or malabsorption requires medical advice.
- Can I get B12 from fortified foods alone?
- Yes, if fortified foods provide sufficient B12 in divided servings — the Vegan Society advises at least 3 µg daily. Check labels; supplements are another reliable option.
- How often should I test?
- Discuss testing with a clinician based on intake, symptoms and risk factors. Seek prompt advice for tingling, fatigue or cognitive changes; MMA can help clarify borderline B12 results.
- Is cyanocobalamin safe?
- Yes. The cyanide moiety is trivial in size and cyanocobalamin is the most stable, best-studied and cheapest form.
- Do children need B12 too?
- Yes — dosing is age-scaled, usually via fortified foods plus a paediatric supplement; ask a paediatric dietitian.
Sources
Figures are drawn from the sources listed at the end of this guide.
- NIH Office of Dietary Supplements — Vitamin B12
- EFSA — Dietary Reference Values for cobalamin
- Watanabe — Vitamin B12 sources and bioavailability
- British Dietetic Association — Vegan diets food fact sheet
- Academy of Nutrition and Dietetics — Position on Vegetarian Diets
Text on this page is free to reuse under CC BY 4.0 with attribution to Veg.ac.
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