Last reviewed: July 2026

B12: why it is the one supplement most vegans genuinely need

Adults need about 4 micrograms of vitamin B12 a day, and it is found in quantity almost exclusively in animal foods. This is not a marketing claim from a supplement brand; it is basic biochemistry. B12 is manufactured by bacteria, not plants or animals, and it reaches humans through meat, fish, eggs and dairy or through deliberately fortified foods. A fully plant-based diet has no dependable natural source, which is why every major dietetic body advises vegans to take a supplement or eat fortified foods. It is the clearest food-first exception there is.

Why plants do not deliver it

B12 is synthesised by microbes in soil and in the guts of animals. Herbivores absorb what their own gut bacteria produce; humans do not, so we depend on eating animal tissues that have accumulated it, or on fortification. The internet's favourite plant sources do not survive scrutiny: algae and spirulina largely contain inactive B12 analogues that can even interfere with the real thing, mushrooms carry only traces, and the argument that unwashed vegetables supply B12 amounts to eating soil bacteria, which is neither reliable nor advisable. Nutritional yeast is a genuine plant-friendly option, but only the fortified kind: the B12 is added, not grown, so you have to read the label.

B12 values are typical figures from food composition references (NIH Office of Dietary Supplements); links go to real Dietly product labels for the macros.

Why the shortfall hides for years

Unlike most water-soluble vitamins, B12 is stored in the liver in amounts that can last several years. That is a blessing and a trap. The blessing is that a short lapse does no harm. The trap is that a new vegan can feel completely fine for a long time while stores quietly drain, then meet deficiency abruptly once the reserve runs out. Because the early signs, tiredness and pins-and-needles, are vague and easy to attribute elsewhere, and because some nerve damage from prolonged deficiency can become permanent, the sensible strategy is to supplement from the start rather than wait for symptoms.

Who is at risk beyond vegans

• Older adults. Stomach acid production declines with age, and acid is needed to release B12 from food, so absorption falls even on an omnivore diet.
• People on long-term metformin or acid-suppressing drugs. Both reduce B12 absorption over time, so periodic checks are wise.
• People with pernicious anaemia or gut conditions. Loss of intrinsic factor or damage to the ileum blocks uptake regardless of intake, and may need injections rather than tablets.
• Breastfed infants of vegan mothers. A genuine clinical concern; maternal supplementation is important.

How to cover it, simply

For most vegans and vegetarians, the answer is undramatic: a daily or twice-weekly B12 supplement, or a habit of fortified foods such as fortified plant milk, cereal or nutritional yeast. Because absorption per dose is limited, supplement tablets often contain far more than 4 micrograms, and the excess is harmless and simply excreted. If you want certainty, a blood test measuring B12 alongside methylmalonic acid or homocysteine detects a functional shortfall earlier than the standard B12 level alone. If you are auditing a plant-based diet more broadly, pair this with the iron explainer and check your staples through Dietly search.

Bottom line

B12 is the one nutrient where the vegan supplement advice is not ideology, it is chemistry. Animal foods and fortified products are the only reliable sources, algae and mushrooms are not, and liver stores mean a deficiency can hide for years before it surfaces. Take a supplement or eat fortified foods, and if you eat animal products, you almost certainly already have this covered. For the wider vitamin picture, see what are micronutrients.