Vitamin B12 · Cobalamin

Evidence Fact Sheet

Cyanocobalamin · Methylcobalamin · Hydroxocobalamin · Adenosylcobalamin

Vitamin B12 (cobalamin) is an animal-source vitamin acting as a cofactor for methionine synthase and methylmalonyl-CoA mutase, supporting red blood cell formation, homocysteine metabolism and myelin maintenance. Human trials reliably correct deficiency markers; effects on cognition and cardiovascular events are mixed. RDA 2.4 μg/day; GRAS/EFSA/ANVISA authorized.

Also known as: Cobalamin · Cyanocobalamin · Methylcobalamin · Hydroxocobalamin

Overview

Vitamin B12 (cobalamin; forms include cyanocobalamin, methylcobalamin, hydroxocobalamin and adenosylcobalamin) is a water-soluble vitamin found naturally only in animal-source foods. Mechanistically it serves as a cofactor for methionine synthase (homocysteine to methionine remethylation) and methylmalonyl-CoA mutase, and supports myelin synthesis via methylation pathways. Typical research and label intakes range from the adult RDA of 2.4 μg/day up to high-dose oral replacement of 1000 μg/day or higher in malabsorption settings. It has a very wide safety margin, with no Tolerable Upper Intake Level established by NIH ODS. Regulatory status: GRAS and DSHEA dietary supplement in the US, EFSA-authorized health claims (Reg 432/2012), ANVISA-authorized functional claims in Brazil, and listed in China's Health-Food raw-material catalogue.

Mechanism of Action

Cofactor for methionine synthase (homocysteine → methionine remethylation) · Cofactor for methylmalonyl-CoA mutase (odd-chain fatty acid metabolism) · Supports myelin synthesis via methylation pathways

Body systems: Blood & Hematopoiesis · Neurological & Cognitive · METABOLISM

Evidence-Based Benefits

Each benefit below is anchored to a specific PubMed-indexed study. Effect sizes, sample sizes, and p-values are reported as published; no values are inferred. Honest negatives and null results are kept alongside the positive findings, and disease-research populations are described as such — Vitamin B12 · Cobalamin is not characterized as a treatment for any disease.

Deficiency Correction (Methylmalonic Acid)

RCT supported

0.13 μmol/lMMA lowered vs placebo
95%CI 0.06-0.19confidence interval

In a multicentre pragmatic RCT of adults with mild cobalamin deficiency, one month of 1000 μg/day oral cyanocobalamin lowered serum methylmalonic acid (a functional deficiency marker) significantly more than placebo, and serum B12 rose. However, homocysteine, haematocrit and mean corpuscular volume did not change significantly, and the card notes markers were not always sustained at 4-month follow-up.

Reported effect: MMA lowered by 0.13 μmol/l (95%CI 0.06-0.19) more than placebo after one month of 1000 μg/day oral B12

“One month of vitamin B12 treatment (N = 26) lowered serum MMA levels by 0.13 μmol/l (95%CI 0.06-0.19) more than the change observed in the placebo group. A significant change was observed for the B12 serum level, but not for the homocysteine level, hematocrit, or mean corpuscular volume.”

Source: PMID 21232119 · Favrat 2011 · BMC Fam Pract

Homocysteine Lowering & Cardiovascular Events

Null / no benefit Meta-analysis supported

RR 0.88 (0.82-0.95)stroke · significant
RR 0.98 (0.94-1.03)CVD · null

This meta-analysis of 19 RCTs reported that B-vitamin supplementation lowered blood homocysteine in all included trials and showed a significant protective effect on stroke, but found no significant effect on overall cardiovascular disease, myocardial infarction, coronary heart disease, cardiovascular death or all-cause mortality — an honest mixed signal where homocysteine lowering did not translate into broad cardiovascular benefit.

Reported effect: Stroke RR 0.88 (0.82-0.95); CVD RR 0.98 (0.94-1.03); all-cause mortality RR 0.99 (0.95-1.04)

“Blood Hcy levels were decreased in all included RCTs. ... B vitamin supplementation has a significant protective effect on stroke, but none on the risk of CVD, MI, CHD, cardiovascular death, or all-cause mortality.”

Source: PMID 22652362 · Huang 2012 · Clin Nutr

Stroke Prevention (Folic Acid, Often Co-Administered with B12)

Meta-analysis supported

RR 0.90 (0.83-0.98)stroke risk

A systematic review and meta-analysis of 21 RCTs reported that folic acid supplementation significantly reduced stroke risk by 10%. Folic acid is frequently co-administered with vitamin B12 in homocysteine-lowering regimens; this finding concerns the folic-acid arm and is included as adjacent context, not as a B12-specific effect.

Reported effect: Folic acid reduced stroke risk by 10% (RR 0.90, 95%CI 0.83 to 0.98)

“Folic acid supplementation significantly reduced the risk of stroke by 10% (RR 0.90, 95%CI 0.83 to 0.98).”

Source: PMID 38824900 · Zhang 2024 · Clin Nutr

Cognition in Alzheimer's Disease (B12 + Folic Acid)

Meta-analysis supported

SMD 0.21 (0.01-0.32)MMSE · p=0.04
SMD 0.06 (-0.22-0.33)ADAS-Cog · p=0.68 null

A meta-analysis of 5 RCTs found that 6 months of combined vitamin B12 and folic acid supplementation improved MMSE scores significantly more than placebo, but did not significantly change ADAS-Cog scores in Alzheimer's disease patients. The signal is modest and outcome-dependent, reported here as a research finding in a clinical population rather than a treatment claim.

Reported effect: MMSE SMD = 0.21 (95%CI 0.01 to 0.32, p = 0.04); ADAS-Cog SMD = 0.06 (95%CI -0.22 to 0.33, p = 0.68)

“After a 6-month treatment, administration of vitamin B12 and folic acid improved the MMSE scores more than placebo did (SMD = 0.21, 95% CI = 0.01 to 0.32, p = 0.04), but did not significantly affect ADAS-Cog scores (SMD = 0.06, 95% CI = -0.22 to 0.33, p = 0.68).”

Source: PMID 38700503 · Lee 2024 · Aging (Albany NY)

Cognitive Function, Depression & Fatigue (General Populations)

Null / no benefit Meta-analysis supported

This systematic review and meta-analysis of 16 RCTs found no evidence that B12 alone or B-complex supplementation improved any subdomain of cognitive function, and no overall effect on measures of depression, in patients without advanced neurological disorders. Fatigue could not be analysed as only one study reported it. The abstract reports direction and conclusion but no pooled numeric effect size, so no effect-size figure is extracted.

Effect size: this study reports the direction of the finding but does not state a specific numeric effect size, so none is given here rather than estimated.

“we found no evidence for an effect of B12 alone or B complex supplementation on any subdomain of cognitive function outcomes. ... We also found no overall effect of vitamin supplementation on measures of depression. ... Vitamin B12 supplementation is likely ineffective for improving cognitive function and depressive symptoms in patients without advanced neurological disorders.”

Source: PMID 33809274 · Markun 2021 · Nutrients

Dosage (research context · not a recommendation)

RDA 2.4 μg/day (adults); high-dose oral replacement 1000 μg/day can be effective in food-cobalamin malabsorption, but is not always sufficient at 4 months follow-up (Favrat 2011 PMID 21232119 BMC Family Practice pragmatic RCT n = 50, 1000 μg/day x 4 weeks · MMA partially normalised vs placebo but markers not sustained at 4 months); higher doses (5000 μg/day) may be required for sustained correction

Regulatory Status · 4 Markets

US · FDA: GRAS · DSHEA dietary supplement · multiple structure/function claims permitted; DV 6 μg (current label)
EU · EFSA: Authorized claims Reg 432/2012 cover red blood cell formation, energy metabolism, fatigue, nervous system, homocysteine metabolism, psychological function, immune system, cell division; NRV 2.5 μg
CN · China: Listed in the Health-Food Raw-Material Catalogue (Nutrient Supplements) 2020 edition (vitamin category, filing/notification route) and a permitted food nutrition fortifier under GB 14880-2012. Notified products may claim 'supplements vitamin B12' only; any function claim (e.g. homocysteine reduction) would require the registration route. SAMR recognized.
BR · ANVISA: RDC 243/2018 dietary supplement · IN 28/2018 Anexo V alegações funcionais verbatim: "A vitamina B12 auxilia na formação de células vermelhas do sangue." + additional energy / nervous system claims available when nutrient threshold met

Authorized Claims

EFSA — “Vitamin B12 contributes to normal red blood cell formation.” (Reg 432/2012)

EFSA — “Vitamin B12 contributes to the reduction of tiredness and fatigue.” (Reg 432/2012)

EFSA — “Vitamin B12 contributes to normal energy-yielding metabolism.” (Reg 432/2012)

EFSA — “Vitamin B12 contributes to normal functioning of the nervous system.” (Reg 432/2012)

ANVISA — “A vitamina B12 auxilia na formação de células vermelhas do sangue.” (IN 28/2018 Anexo V alegação funcional)

Safety

Very wide safety margin in healthy adults — no Tolerable Upper Intake Level established by NIH ODS; rare allergic reactions to cyanocobalamin reported; methylcobalamin or hydroxocobalamin alternatives available for cyanide-sensitive individuals

Goals: cognitive-support · longevity-stack

Lifestyles: plant-based · senior-60-plus

References

PubMed-indexed citations anchoring the benefit findings above. Effect sizes are reported as published.

PMID 21232119 · Favrat 2011 · BMC Fam Pract — Deficiency Correction (Methylmalonic Acid)
PMID 22652362 · Huang 2012 · Clin Nutr — Homocysteine Lowering & Cardiovascular Events
PMID 38824900 · Zhang 2024 · Clin Nutr — Stroke Prevention (Folic Acid, Often Co-Administered with B12)
PMID 38700503 · Lee 2024 · Aging (Albany NY) — Cognition in Alzheimer's Disease (B12 + Folic Acid)
PMID 33809274 · Markun 2021 · Nutrients — Cognitive Function, Depression & Fatigue (General Populations)

Frequently Asked Questions

1. What does vitamin B12 actually do in the body?

B12 (cobalamin) is a cofactor for methionine synthase, which remethylates homocysteine to methionine, and for methylmalonyl-CoA mutase in fatty-acid metabolism, and it supports myelin synthesis via methylation pathways. These mechanisms underpin its roles in red blood cell formation, homocysteine metabolism and nerve maintenance. It occurs naturally only in animal-source foods.

2. How is B12 regulated, and is it safe?

B12 has a very wide safety margin in healthy adults, with no Tolerable Upper Intake Level established by NIH ODS; rare allergic reactions to cyanocobalamin have been reported, and methylcobalamin or hydroxocobalamin are alternatives. It is GRAS and a DSHEA dietary supplement in the US, EFSA-authorized for several claims (Reg 432/2012), ANVISA-authorized in Brazil, and listed in China's Health-Food raw-material catalogue. The adult RDA is 2.4 μg/day.

Last evidence review: 2026-06-04

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