Vitamin B12 on a vegan diet

by Dr Leila Dehghan

Vitamin B12 is a water-soluble vitamin that is required for energy production, DNA synthesis, red blood cell formation and the development of the nervous system. It is one of the most widely discussed nutrients on a vegan diet because it cannot be found in plants. However, it should be pointed out that vitamin B12 is produced by micro-organisms in soil, not by animals. Animal products contain vitamin B12 because it is routinely added as a supplement to their feeds. Humans can obtain their recommended dose either by taking a supplement or via foods fortified with B12.

Several studies have demonstrated that vegans who do not supplement regularly are at risk of B12 deficiency (1, 2, 3). Therefore, supplementation is essential.

Causes of deficiency

The most common cause of vitamin B12 deficiency is inadequate intake. Furthermore, the absorption of B12 can be reduced by certain medications such as antacids (e.g. omeprazole, ranitidine), metformin (used to treat diabetes), anti-inflammatory drugs (e.g. ibuprofen) and oral contraceptives. Certain groups are at increased risk of B12 deficiency and should take extra precaution to prevent deficiency: older adults, vegans, vegetarians, infants of vegan mothers, individuals with gastrointestinal disorders. 

Symptoms of vitamin B12 deficiency

Our body has the capacity to store a substantial amount of vitamin B12, the majority of which is found in the liver. If we stop consuming B12, it will take 3 to 5 years before these stores are depleted and any symptoms of deficiency manifest. Symptoms include anaemia causing weakness, tiredness, pallor and shortness of breath. Severe cases cause nerve damage resulting in tingling or numbness in the hands and feet, muscle weakness and even dementia.

How much B12 should a vegan take?

In the UK, the recommended daily intake (RDI) for adults aged 19-64 is 1.5mcg (4), however, the body can only absorb a fraction of the consumed B12 at any time. That means the less frequently you consume B12, the more you need. Therefore, vegans are recommended to take either a 25-100mcg daily or 2000mcg weekly supplement to ensure adequate intake. Pregnant and lactating vegan women need slightly more. Those over the age of 65 years – vegan as well as non-vegans – are at a high risk of poor B12 intake. Some experts recommend a 500mcg vitamin B12 dosage for seniors to normalise their poor B12 status (5). 

Vegans who prefer to obtain their recommended daily B12 via fortified foods need to consume at least two servings per day and aim for a daily intake of 3mcg or more. Plant-based B12 sources include nutritional yeast enriched with B12, yeast extract (Marmite), B12 fortified plant milks, yogurts, cereals, cheeses and meat-alternatives. However, it can be quite challenging to determine the amount one obtains from fortified foods. For that reason it’s recommended that vegans who take their B12 via fortified foods also supplement on occasions to avoid low B12 levels.


Due to lack of supplementation poor vitamin B12 status is common in vegans. Since the symptoms of B12 deficiency manifest late, vegans are advised to monitor their levels with annual blood test and take a regular supplement.


  1. Crane MG, Sample C, Patchett S, Register UD. “Vitamin B12 studies in total vegetarians (vegans). Journal of Nutritional Medicine.1994;4:419-430
  2. Gilsing AM, Crowe FL, Lloyd-Wright Z, Sanders TA, Appleby PN, Allen NE, Key TJ. Serum concentrations of vitamin B12 and folate in British male omnivores, vegetarians and vegans: results from a cross-sectional analysis of the EPIC-Oxford cohort study. Eur J Clin Nutr. 2010 Sep;64(9):933-9.
  3. Hokin BD, Butler T. Cyanocobalamin (vitamin B-12) status in Seventh-day Adventist ministers in Australia. Am J Clin Nutr. 1999 Sep;70(3 Suppl):576S-578S.
  4. NHS
  5. Park S, Johnson MA. What is an adequate dose of oral vitamin B12 in older people with poor vitamin B12 status? Nutr Rev. 2006 Aug;64(8):373-8.