Vegan vs Vegetarian B12: Why Vegans Must Supplement

Why B12 is the vegan-specific problem

Every other nutrient on the vegan worry list (iron, zinc, calcium, omega-3, iodine, choline, vitamin D) has a clear plant-food path. B12 does not. The vitamin is produced exclusively by certain bacteria and archaea. Plants do not make it. Animals do not synthesise it directly either: ruminants get B12 from the bacteria in their rumen, and other animals (including humans) accumulate it from soil microbes, gut bacteria upstream of absorption, or from eating other animals. Modern hygiene has stripped the soil-and-water residual B12 that pre-industrial humans ingested incidentally, which is part of why even some omnivores in industrialised countries run low.

The biological consequence is straightforward. Once you remove every animal product from the diet, the only reliable B12 sources left are fortified foods (some plant milks, some breakfast cereals, some yeast extracts) and supplements. The NIH Office of Dietary Supplements fact sheet is unambiguous on this; so is the British Dietetic Association vegan factsheet; so is the Vegan Society B12 guidance.

For vegetarians who keep dairy and eggs in the diet, the picture is more comfortable but not relaxed. A typical UK egg supplies about 0.6 mcg of B12. A 250 ml glass of cow milk supplies about 1.1 mcg. A 30 g serving of cheddar supplies around 0.5 mcg. A vegetarian eating these foods most days clears the 2.4 mcg per day adult RDA. The complications are absorption efficiency, which declines with age, and medications such as metformin and proton pump inhibitors that interfere with B12 release from food protein.

The 2010 British Journal of Nutrition study by Gilsing and colleagues, working with the EPIC-Oxford cohort, found mean serum B12 was lowest in vegans (227 pmol/L), intermediate in vegetarians (262 pmol/L), and highest in meat-eaters (321 pmol/L). The proportion classed as deficient (below 118 pmol/L) was 52% in vegans, around 7% in vegetarians, and under 1% in meat-eaters. Those numbers were before fortified vegan products became as widespread as they are now, and they are why the modern vegan guidance is the high-dose weekly or moderate-dose daily supplement rather than relying on fortified food alone.

How much B12 do you actually need

The US Institute of Medicine and the UK Scientific Advisory Committee on Nutrition both set the adult RDA at 2.4 mcg per day, with 2.6 mcg in pregnancy and 2.8 mcg in lactation. The UK reference nutrient intake is slightly lower at 1.5 mcg per day, which reflects different methodology not different biology. The European Food Safety Authority sets an adequate intake of 4 mcg per day, which is higher because it accounts for a wider population variation. Whatever number you anchor to, the broad answer is that adults need a few micrograms a day and pregnant or breastfeeding women need a little more.

This is where the dosing of vegan supplements stops looking strange. A 10 mcg or 25 mcg daily B12 tablet sounds like a megadose against an RDA of 2.4 mcg, and it is. The reason is that absorption is biphasic. At low doses (under about 1.5 to 2 mcg per intake), absorption is via intrinsic factor in the ileum, with an efficiency around 50%. Above that, intrinsic factor saturates and additional absorption is via passive diffusion at roughly 1% efficiency. A 10 mcg tablet therefore yields around 1.5 mcg via intrinsic factor plus around 0.08 mcg via passive diffusion, for a net of around 1.6 mcg absorbed. A 2,000 mcg weekly dose yields about 1.5 mcg via intrinsic factor plus 20 mcg via passive diffusion, averaged across the week. Either path comfortably exceeds the RDA.

The Vegan Society pragmatic recommendation is therefore: either two or three servings of B12-fortified food per day (each containing at least 2 mcg), or a daily supplement of 10 mcg, or a weekly supplement of 2,000 mcg. Pick the one you will actually do. The most common failure is not the dose being too low but the supplement being abandoned after three months.

Which form of B12 is best

The four forms you will see on a supplement label are cyanocobalamin, methylcobalamin, hydroxocobalamin, and adenosylcobalamin. Cyanocobalamin is the cheapest, most stable, and best-studied form. It is converted in the body to the two active coenzyme forms (methylcobalamin and adenosylcobalamin) as needed. The cyanide moiety attached is a small (a 10 mcg dose contributes about 0.7 mcg cyanide, well below the food-supply background a meat-eater ingests daily), and it does not accumulate.

Methylcobalamin and adenosylcobalamin are the active coenzyme forms. They are more expensive and slightly less stable in tablet form. There is no compelling evidence they are absorbed or retained more efficiently than cyanocobalamin in people without specific metabolic disorders. Hydroxocobalamin is what your GP injects when treating diagnosed deficiency; it has a longer half-life in serum than cyanocobalamin but is rarely sold as an oral supplement.

The Vegan Society defaults to cyanocobalamin in its position guidance because it is the form with the longest safety record and the largest body of dose-response data. Methylcobalamin is reasonable if you prefer it, particularly if you have an MTHFR polymorphism (though clinical relevance is debated). What is not reasonable is paying a premium for any obscure form on the promise of superior bioavailability; the data for that claim do not exist.

Symptoms, testing, and what to do if your level is borderline

B12 deficiency is sneaky because the body stores 2 to 5 mg of B12 in the liver, which is roughly two to four years of supply at standard turnover. A new vegan with prior good stores can feel fine for years before deficiency emerges. The early symptoms are non-specific: fatigue, low mood, brain fog, glossitis (smooth red tongue), tingling in hands or feet, and balance problems. The diagnostic blood picture is macrocytic megaloblastic anaemia with enlarged red blood cells and hypersegmented neutrophils on a peripheral smear.

The clinically important point is that the neurological symptoms can become permanent. Subacute combined degeneration of the spinal cord is the classical late picture: progressive sensory loss in the legs, loss of position sense, and eventually motor weakness. This is why the UK National Institute for Health and Care Excellence (NICE) guidance treats unexplained B12 deficiency in any patient as a "treat now, investigate cause in parallel" condition. For a vegan or vegetarian the cause is almost always dietary, but folate-masking of B12 deficiency in a high-folate plant-based diet is a real diagnostic pitfall.

The first-line lab test is serum total B12. A result below 148 pmol/L (200 pg/mL) is consistent with deficiency. Between 148 and 220 pmol/L is the grey zone where the more sensitive markers (methylmalonic acid, MMA, and homocysteine) become useful. Elevated MMA above ~280 nmol/L with normal renal function plus elevated homocysteine indicates functional B12 deficiency even when serum total B12 looks borderline. Holo-transcobalamin (active B12) is the most specific marker if your lab offers it. A reasonable annual or biennial check for any committed vegan is serum B12 plus MMA.

If you are borderline or deficient, the standard NICE-aligned reply for dietary cause is a course of oral cyanocobalamin 1,000 mcg per day for 2 to 4 weeks to refill stores, then a maintenance dose. If neurological symptoms are present, intramuscular hydroxocobalamin under medical supervision is the safer choice. Self-treating documented deficiency with low-dose tablets is unwise; document with bloods first, then move to maintenance once stores are full.

Foods you can build a B12 strategy around

The practical take. A vegetarian eating a fortified breakfast cereal, a cup of dairy, an egg or yogurt at lunch, and a slice of cheese at dinner is comfortably above the RDA. A vegan needs a deliberate plan: either three fortified servings spread across the day, or a single daily 10 mcg tablet, or a 2,000 mcg weekly tablet on a fixed weekday so it is easy to remember. Mixing strategies is fine. The biggest risk factor is forgetting to start once you transition, then noticing nothing for two years because of liver stores, then having a problem.

Special situations

Pregnancy and breastfeeding. The RDA rises to 2.6 mcg in pregnancy and 2.8 mcg in lactation. Maternal B12 status determines infant B12, and breast milk B12 reflects maternal serum B12 within a couple of weeks. There are documented case reports of severe neurological damage in exclusively breastfed infants of vegan mothers who were unsupplemented; the literature on this is unfortunately consistent. A pregnant or breastfeeding vegan should be supplementing without exception, and infants should receive paediatric review with B12 status checked at the standard immunisation visits. See vegan vs vegetarian pregnancy and vegan and vegetarian breastfeeding for the full guidance reading.

Over 60s. Parietal cell function and intrinsic factor production both decline with age, so even omnivores commonly run low. The Institute of Medicine recommends that everyone over 50 should obtain most of their B12 from fortified foods or supplements (not from natural food protein binding) because the protein-cleavage step in the stomach becomes inefficient. A vegan or vegetarian over 60 should default to a daily 25 to 50 mcg tablet, not the 10 mcg used by younger vegans.

Medications. Metformin (for type 2 diabetes) reduces ileal B12 absorption; long-term users should test annually and may need higher doses. Proton pump inhibitors (omeprazole, lansoprazole) reduce stomach acid, which impairs release of B12 from dietary protein but does not affect supplement absorption; vegans on PPIs do not need to worry, vegetarians eating animal-source B12 do. H2-blockers have a similar but milder effect.

Related nutrient pages

Keep reading

• Iron: heme vs non-heme bioavailability
• Vitamin D: D2 vs D3 and the lanolin question
• Omega-3: ALA vs EPA, DHA, and algae oil
• Calcium: vegan fracture risk in EPIC-Oxford
• Older adults: B12 absorption decline
• The full vegan vs vegetarian health page

Frequently asked questions about B12

Sources cited. NIH Office of Dietary Supplements Vitamin B12 fact sheet for health professionals; The Vegan Society B12 guidance; British Dietetic Association vegan factsheet; Gilsing AMJ et al., Serum concentrations of vitamin B12 and folate in British male omnivores, vegetarians and vegans, Br J Nutr 2010; 103: 1733-1736; Academy of Nutrition and Dietetics position paper on vegetarian diets, J Acad Nutr Diet 2016; 116: 1970-1980; USDA FoodData Central for per-food B12 figures; NICE Clinical Knowledge Summary on Anaemia (B12 and folate deficiency). All values as of May 2026.