Low vitamin B12 and cardiovascular risk: what the evidence actually says

The short version

Vitamin B12 deficiency is common, frequently missed, and easy to treat. Its connection to cardiovascular disease runs through homocysteine: B12 is required to convert homocysteine into methionine, and when B12 runs low, homocysteine rises. Raised homocysteine is consistently associated with stroke and coronary events in observational studies.

That association is real. The causal claim is weaker than most supplement marketing suggests: trials that lowered homocysteine with B vitamins did not consistently reduce cardiovascular events. This page explains both halves, because acting on only one of them leads people to either ignore a genuine deficiency or buy pills that promise more than the data supports.

Why B12 matters beyond anaemia

B12 is a cofactor for two reactions. In the cytosol, it supports methionine synthase, which recycles homocysteine into methionine and regenerates tetrahydrofolate for DNA synthesis. In mitochondria, it supports methylmalonyl-CoA mutase, which converts methylmalonyl-CoA to succinyl-CoA. When B12 is short, both reactions slow: homocysteine accumulates, and methylmalonic acid (MMA) accumulates.

The haematological consequence — macrocytic anaemia — is the one most people know. The neurological consequence is the one that matters more: peripheral neuropathy, loss of vibration and position sense, gait disturbance, and cognitive change can all occur without any anaemia at all. That is why "my blood count is normal" is not reassurance.

How deficiency gets missed

Three practical problems make under-diagnosis common.

First, serum B12 is an imperfect test. It measures what is circulating, not what is available to tissue. Values between roughly 200 and 300 pg/mL are a grey zone where deficiency is possible but not proven. MMA and homocysteine rise earlier and are more sensitive, especially MMA.

Second, the risk factors are ordinary. Metformin lowers B12 in a meaningful fraction of long-term users. Proton-pump inhibitors and H2 blockers reduce the acid needed to release B12 from food protein. Atrophic gastritis — often silent, and more common with age — removes intrinsic factor. Bariatric and ileal surgery remove absorptive surface. Plant-only diets supply little or no B12 unless foods are fortified.

Third, symptoms are non-specific. Fatigue, brain fog, tingling and breathlessness have a hundred causes, so nobody orders the test.

The homocysteine pathway

Homocysteine is an amino acid intermediate. At elevated concentrations it is measurably harmful to the endothelium in laboratory work: it increases oxidative stress, reduces nitric oxide bioavailability, promotes smooth-muscle proliferation, and shifts haemostasis toward a prothrombotic state.

Folate, B12 and B6 are all required to keep homocysteine low. This is why a B12 deficiency can produce a cardiovascular-looking risk profile even when classic vascular risk factors are absent — and why the deficiency is worth finding regardless of what you conclude about causation.

What the epidemiology shows

Observational cohorts consistently find a graded relationship: higher homocysteine, higher risk of stroke and ischaemic heart disease. The Homocysteine Studies Collaboration's pooled analysis of prospective studies reported that a 25% lower homocysteine level was associated with roughly an 11% lower risk of ischaemic heart disease and 19% lower risk of stroke. Mendelian randomisation work — using genetic variants that raise homocysteine — has been less consistent, which weakens the causal story.

What the trials showed

This is the part usually left out.

• HOPE-2 (2006): folic acid, B6 and B12 lowered homocysteine and reduced stroke, but did not significantly reduce the composite of death from cardiovascular causes, myocardial infarction and stroke. A secondary analysis suggested stroke benefit.
• VITATOPS (2010): B vitamins in patients with recent stroke or TIA did not reduce the primary composite outcome.
• SEARCH (2010): folic acid and B12 in survivors of myocardial infarction did not reduce major vascular events.
• Cochrane review (2017): concluded that homocysteine-lowering interventions do not reduce cardiovascular events, and that any benefit is at best uncertain.

The honest interpretation: homocysteine is a risk marker, and B12 status is a determinant of that marker. Correcting a deficiency is clinically sensible — for the nervous system and for haematological health. But B vitamins are not a cardiovascular intervention.

Who should be tested

Reasonable indications include: plant-only diets; long-term metformin; long-term proton-pump inhibitors or H2 blockers; atrophic gastritis, pernicious anaemia, coeliac disease or inflammatory bowel disease; gastric, ileal or bariatric surgery; age over 60 with unexplained fatigue, cognitive change or neuropathy; macrocytosis on a blood count; and unexplained anaemia.

What to ask for

1. Serum B12 — the first-line test, with the 200–300 pg/mL grey zone in mind.
2. Methylmalonic acid — the most specific confirmatory marker.
3. Homocysteine — sensitive, not specific; also the link to vascular risk.
4. Full blood count with MCV — supportive, not exclusionary.
5. If confirmed: intrinsic factor and parietal cell antibodies, plus folate and ferritin.

Where this leaves you

If your answers on our check put you in a higher band, the useful next step is a blood test — not a supplement. Test first, then treat, then re-test with MMA or homocysteine rather than serum B12 alone. If you have neurological symptoms, get seen promptly: damage from prolonged B12 deficiency can become permanent.

References

• Stabler SP. Vitamin B12 Deficiency. N Engl J Med. 2013;368:149–160.
• Langan RC, Goodbred AJ. Vitamin B12 Deficiency: Recognition and Management. Am Fam Physician. 2017;96(6):384–389.
• Wald DS, Law M, Morris JK. Homocysteine and cardiovascular disease: evidence on causality from a meta-analysis. BMJ. 2002;325:1202.
• The Homocysteine Studies Collaboration. Homocysteine and risk of ischemic heart disease and stroke: a meta-analysis. JAMA. 2002;288(16):2015–2022.
• Lonn E, et al. Homocysteine lowering with folic acid and B vitamins in vascular disease (HOPE-2). N Engl J Med. 2006;354:1567–1577.
• Martí-Carvajal AJ, et al. Homocysteine-lowering interventions for preventing cardiovascular events. Cochrane Database Syst Rev. 2017;8:CD006612.
• Spence JD. Homocysteine-lowering therapy: a role in stroke prevention? Lancet Neurol. 2007;6(9):830–838.

Educational material. Not medical advice, and not a substitute for a clinical assessment.