Vitamin B12: The One Non-Negotiable

Vitamin B12 is the one nutrient that every vegan must supplement. No plant food reliably contains B12. This is non-negotiable — B12 deficiency causes irreversible neurological damage.

⚠️ Do not skip B12 supplementation

B12 deficiency can take years to develop (the liver stores B12) but the neurological damage it causes can be permanent. Every vegan, regardless of how healthy their diet is, should supplement B12.

What is B12 and why do we need it?

Vitamin B12 (cobalamin) is a water-soluble vitamin essential for:

Red blood cell formation (deficiency causes megaloblastic anaemia)
DNA synthesis
Neurological function (myelin sheath maintenance)
Homocysteine metabolism (high homocysteine is a cardiovascular risk factor)

Why don't plant foods contain B12?

B12 is produced by bacteria — not by animals or plants. Animals accumulate B12 from bacteria in their gut or in the soil and water they consume. In modern farming, livestock are often given B12 supplements directly. It's somewhat ironic: vegans are told to get B12 from animals, when those animals are getting it from supplements.

Some plant foods (certain algae, fermented foods) contain B12 analogues, but these are not reliably bioavailable to humans and cannot substitute for supplementation. Spirulina does not contain active B12.

How much do you need?

2.4μg

recommended daily intake (adults)

NIH

1,000μg

weekly dose (cyanocobalamin)

50μg

daily dose if taking daily

2–5 years

B12 body stores can last

Because B12 absorption is complex (it requires intrinsic factor from the stomach and absorption improves at higher doses), the RDA is not a simple "take 2.4μg daily" recommendation in practice. Higher supplementation doses are needed to ensure adequate absorption:

Daily supplementation: 50–100μg of cyanocobalamin or methylcobalamin
Weekly supplementation: 1,000–2,000μg two to three times per week
Fortified foods: Eating fortified foods 2–3 times daily (plant milks, nutritional yeast, cereals) can work but supplementation is more reliable.

Which form of B12?

Cyanocobalamin

Dose: 1,000–2,000μg

Frequency: 2–3× per week

Form: Tablet / sublingual

The most studied form. Stable and cheap. The body converts it to active forms.

Methylcobalamin

Dose: 1,000μg

Frequency: Daily

Form: Sublingual

An active form. May be preferable for those with MTHFR gene variants. Slightly less evidence at high doses.

💡 Which to choose

Cyanocobalamin is cheaper, more stable, and better studied at high doses. It is the form recommended by most nutritionists for vegans. Methylcobalamin is a good alternative. Both work.

Testing your levels

A standard serum B12 test can be misleading — it measures total B12, including inactive analogues. More accurate markers include:

Holotranscobalamin (HoloTC) — measures active B12
Methylmalonic acid (MMA) — elevated if B12 is functionally deficient
Homocysteine — elevated in B12 deficiency

If you're not sure whether you're absorbing B12 properly, ask your GP to test homocysteine and MMA rather than just serum B12.

The bottom line

B12 is the easiest nutrition concern for vegans to address: buy a bottle of B12 supplements (cyanocobalamin, 1,000μg) and take one or two per week. It costs pennies and removes the only genuine nutritional risk of a vegan diet.