Vitamin B12

Plant foods are not a reliable source of vitamin B12, so adults on an unsupplemented vegan diet have lower B12 status and more functional deficiency than omnivores, while supplement users show better biomarker levels. Long-term deficiency affects red blood cell production and the nervous system and is most serious in infants. In adults who are not deficient, randomised trials found no benefit of extra B12 for cognition or mood.

Energy
Mind
Vegan

Evidence retrieved up to 14 October 2026

What the research shows

Long-term low vitamin B12 intake can lead to deficiency that impairs red blood cell production and nerve function; some effects can be irreversible, and the consequences are most serious in infants.

Moderate certainty

Further research may change the size of the effect.

Take care: see the safety noteRisk

Who this is about

people with long-term low vitamin B12 intake, including infants of mothers with low B12 status

Practical note

Graded moderate: the sources here are reviews and a professional body page restating well-established physiology rather than a pooled analysis of outcomes.

Vegan note

Pregnant and breastfeeding vegans and their infants are the group in which delayed recognition matters most.

Safety note

Early signs listed by dietitians (fatigue, pale skin, memory problems) are non-specific; low B12 status is confirmed with blood tests by a health professional.

Sources

Supports “As vitamin B12 is essential for the synthesis of nucleic acids, erythrocytes and in the maintenance of myelin, deficiency may result in a variety of symptoms. Some of these symptoms may be severe while others may be irreversible.” (Abstract) Pawlak R, Lester SE, Babatunde T. 2014. European Journal of Clinical Nutrition. The prevalence of cobalamin deficiency among vegetarians assessed by serum vitamin B12: a review of literature. publisher copyright; metadata and short quotes only; Courtesy of the U.S. National Library of Medicine
Supports “The effects can be irreversible or even fatal, particularly in infants, if diagnosis or treatment is delayed” (1. Introduction) Benham AJ, Gallegos D, Hanna KL, Hannan-Jones MT. 2022. Nutrients. Vitamin B12 Supplementation Adequacy in Australian Vegan Study Participants. CC BY 4.0; credit Benham et al., Nutrients 2022
Supports “Deficiency of vitamin B12 impairs the production of healthy red blood cells, giving rise to anaemia.” (Deficiency) Dietitians Australia Vegetarian Interest Group. 2022. Dietitians Australia. Vitamin B12. Dietitians Australia, all rights reserved; short attributed quotes only; credit Dietitians Australia

Plant foods are not a reliable source of active vitamin B12, so fortified foods or B12 supplements are the practical ways for people following a vegan diet to get enough.

Moderate certainty

Further research may change the size of the effect.

No known concerns at usual amountsBenefit

Who this is about

people following a vegan diet

Practical note

Reference intakes cited in the literature are an RDA of 2.4 micrograms a day for adults (US Institute of Medicine) and an adequate intake of 4.0 micrograms a day (EFSA). A 2022 Australian survey noted that the share of B12 absorbed from the large doses in oral supplements is considerably lower than from food.

Vegan note

Which foods may be fortified with B12 varies by country; in Australia it is limited to a small number of products such as selected soy milks, yeast spread and vegetarian meat analogues.

Sources

Supports “Plant-derived foods do not naturally contain a reliable source of vitamin B12 and people on a vegan diet need to consume foods with added vitamin B12 (“fortified” foods) and/or take vitamin B12 supplements.” (1. Introduction) Benham AJ, Gallegos D, Hanna KL, Hannan-Jones MT. 2022. Nutrients. Vitamin B12 Supplementation Adequacy in Australian Vegan Study Participants. CC BY 4.0; credit Benham et al., Nutrients 2022
Supports “For someone following a vegan diet, consuming fortified foods or taking a vitamin B12 supplement are the only viable ways to obtain enough of this nutrient.” (Vegan diets) Dietitians Australia Vegetarian Interest Group. 2022. Dietitians Australia. Vitamin B12. Dietitians Australia, all rights reserved; short attributed quotes only; credit Dietitians Australia
Context “The European Food Safety Authority (EFSA) has set an adequate intake (AI) of 4.0 µg/day for adults” (Introduction) Norris J. VeganHealth.org (Vegan Outreach). Explanation of Vitamin B12 Recommendations. Vegan Health, all rights reserved; short attributed quotes only; credit VeganHealth.org

Adults following a vegan diet have lower serum B12 and higher homocysteine than omnivores, with markers indicating more functional B12 deficiency, and the risk is highest among vegans who do not use B12 supplements.

Moderate certainty

Further research may change the size of the effect.

No known concerns at usual amountsRisk

Who this is about

adults following a vegan diet

Practical note

Graded moderate: consistent observational evidence across a 2024 meta-analysis of 17 studies and a 2014 systematic review of 40 studies; downgraded from high for heterogeneity and the imprecision of serum B12 as a marker (the methylmalonic acid and holotranscobalamin differences did not reach significance).

Vegan note

Plant foods are not a reliable B12 source, so this applies to vegans who rely on unfortified foods.

Sources

Supports “Higher deficiency prevalence was reported in vegans than in other vegetarians.” (Abstract) Pawlak R, Lester SE, Babatunde T. 2014. European Journal of Clinical Nutrition. The prevalence of cobalamin deficiency among vegetarians assessed by serum vitamin B12: a review of literature. publisher copyright; metadata and short quotes only; Courtesy of the U.S. National Library of Medicine
Supports “The dietary intake of vitamin B12 among unsupplemented vegans is notably lower compared to both vegetarians and omnivores.” (Abstract) Niklewicz A, Hannibal L, Warren M, Ahmadi KR. 2024. Nutrition Bulletin. A systematic review and meta-analysis of functional vitamin B12 status among adult vegans. publisher copyright; metadata and short quotes only; Courtesy of the U.S. National Library of Medicine
Context “Many studies have found a high prevalence of elevated homocysteine among vegans who don’t supplement with B12.” (Essential Information) Norris J. VeganHealth.org (Vegan Outreach). Vitamin B12. Vegan Health, all rights reserved; short attributed quotes only; credit VeganHealth.org

Adults with diabetes who take metformin have lower serum vitamin B12 and a higher rate of B12 deficiency than those who do not.

Low certainty

Further research is likely to change this.

Take care: see the safety noteRisk

Who this is about

adults with diabetes taking metformin

Practical note

Graded low: mostly observational studies with substantial heterogeneity (I2 64 percent for deficiency risk); the meta-analysis found no significant association with anaemia or neuropathy prevalence.

Vegan note

Vegans taking metformin combine two independent reasons for low B12 status.

Safety note

The meta-analysis authors recommend yearly B12 assessment for people with diabetes taking metformin; decisions about medicines belong with the prescribing clinician.

Sources

Supports “Metformin use led to significantly lowered vitamin B12 concentrations and significantly higher risk of vitamin B12 deficiency in diabetic patients.” (Abstract, Conclusions) Yang W, Cai X, Wu H, Ji L. 2019. Journal of Diabetes. Associations between metformin use and vitamin B12 levels, anemia, and neuropathy in patients with diabetes: a meta-analysis. publisher copyright; metadata and short quotes only; Courtesy of the U.S. National Library of Medicine

In adults without overt B12 deficiency or advanced neurological disorders, B12 supplementation alone or as a B complex did not improve cognitive function or depressive symptoms in randomised trials.

Moderate certainty

Further research may change the size of the effect.

No known concerns at usual amountsNo clear effect

Who this is about

adults without overt vitamin B12 deficiency

Practical note

Based on a 2021 meta-analysis of 16 randomised trials with 6276 participants; downgraded one level for heterogeneity of populations and regimens. Fatigue could not be analysed because only one trial reported it.

Sources

Supports “Vitamin B12 supplementation is likely ineffective for improving cognitive function and depressive symptoms in patients without advanced neurological disorders.” (Abstract) Markun S, Gravestock I, Jäger L, Rosemann T, Pichierri G, Burgstaller JM. 2021. Nutrients. Effects of Vitamin B12 Supplementation on Cognitive Function, Depressive Symptoms, and Fatigue: A Systematic Review, Meta-Analysis, and Meta-Regression. CC BY 4.0; credit Markun et al., Nutrients 2021

Among adults following a vegan diet, those who use B12 supplements have better B12 biomarker levels than those who do not, and oral B12 normalised serum B12 levels in trials of adults with low B12.

Moderate certainty

Further research may change the size of the effect.

No known concerns at usual amountsBenefit

Who this is about

adults following a vegan diet

Practical note

Graded moderate: the vegan evidence is observational subgroup comparison, and the randomised evidence (Cochrane 2018, 3 trials, 153 participants, low quality for serum B12) is in adults with low B12 rather than vegans specifically; the consistent direction and the directness of the biomarker outcome support moderate. One trial in the Cochrane review used 2000 micrograms a day orally.

Vegan note

Supplements and fortified foods are the reliable B12 sources on a vegan diet.

Sources

Supports “Low quality evidence shows oral and IM vitamin B12 having similar effects in terms of normalising serum vitamin B12 levels, but oral treatment costs less.” (Abstract, conclusions) Wang H, Li L, Qin LL, Song Y, Vidal-Alaball J, Liu TH. 2018. Cochrane Database of Systematic Reviews. Oral vitamin B12 versus intramuscular vitamin B12 for vitamin B12 deficiency. Copyright The Cochrane Collaboration, published by John Wiley and Sons; metadata and short quotes only
Supports “Subgroup analyses indicated that the use of vitamin B12 supplements among vegans contributes to significant improvements in all biomarker concentrations compared to their unsupplemented counterparts.” (Abstract) Niklewicz A, Hannibal L, Warren M, Ahmadi KR. 2024. Nutrition Bulletin. A systematic review and meta-analysis of functional vitamin B12 status among adult vegans. publisher copyright; metadata and short quotes only; Courtesy of the U.S. National Library of Medicine
Context “Vegans who do not ingest vitamin B12 supplements were found to be at especially high risk.” (Abstract) Pawlak R, Lester SE, Babatunde T. 2014. European Journal of Clinical Nutrition. The prevalence of cobalamin deficiency among vegetarians assessed by serum vitamin B12: a review of literature. publisher copyright; metadata and short quotes only; Courtesy of the U.S. National Library of Medicine