Vitamin B12 deficiency is associated with heart disease, but supplements paint a more complex picture

Researchers have linked low vitamin B12 levels with cardiovascular disease, while also asking a broader question: how deficiency affects blood vessels, what clinical trials actually show about supplements, and which groups may benefit most.

Review: Vitamin B12 and cardiovascular health: mechanisms, clinical evidence, and future directions. Image Credit: CL STOCK / Shutterstock

Low vitamin B12 levels are associated with cardiovascular disease, but supplementation has not consistently prevented major cardiovascular events, according to a recent narrative review published in Nutrition Journal. The clearest evidence for a possible clinical benefit involves combined B-vitamin therapy for stroke prevention, especially in populations without mandatory folic acid fortification.

The review brings together laboratory findings, large population studies, and clinical trials to assess vitamin B12’s role in cardiovascular health and whether supplements have any cardioprotective effect.

Background

Vitamin B12 is found naturally only in animal-derived foods such as meat, fish, eggs, and dairy products. It must pass through several steps before the body can absorb it.

First, stomach acid and digestive enzymes release B12 from food. A protein called intrinsic factor binds to the vitamin and carries it to the ileum, where absorption occurs. In the bloodstream, only about 20% to 25% is in the biologically active form known as holotranscobalamin.

Vitamin B12 is essential for two major enzymes. One converts homocysteine into methionine, supporting DNA methylation and playing a major role in gene regulation. The other helps break down certain fatty acids and amino acids.

When B12 is deficient, homocysteine and methylmalonic acid (MMA) rise.

Possible cardiovascular effects of B12 deficiency

The authors outline several ways in which B12 deficiency may harm cardiovascular health, including oxidative stress, endothelial dysfunction, elevated homocysteine, and impaired methylation. These processes are all linked to atherosclerosis and vascular injury.

High homocysteine, or hyperhomocysteinemia, is associated with endothelial dysfunction and may contribute to vascular damage and plaque buildup. Animal studies have shown greater plaque accumulation when homocysteine is elevated. In postmenopausal women, hyperhomocysteinemia has been tied to poorer endothelial function, as shown by weaker flow-mediated dilation (FMD), a measure of how well an artery expands in response to increased blood flow.

Another mechanism is the methyl-folate trap. Without B12, methionine synthase cannot use 5-methyl-folate, leaving folate trapped in a form that cannot easily support other folate-dependent reactions. This disrupts DNA synthesis and cell replication, while reduced availability of S-adenosylmethionine (SAM), the body’s main methyl donor, may alter methylation and gene regulation in blood vessels.

The clinical importance of these proposed vascular effects is still uncertain. A further possible pathway involves endothelial dysfunction through reduced nitric oxide sensitivity.

Low vitamin B12 is also associated with signs of electrical instability in the heart, including electrocardiographic markers that may indicate vulnerability to arrhythmias. However, these findings do not prove that clinical arrhythmias occur more often in people with low B12.

A case report involving a child with cobalamin C deficiency described severe myocardial dysfunction. Because this inherited disorder is very different from dietary B12 deficiency, the finding has limited relevance for most patients.

What the evidence shows

Earlier observational studies have linked low vitamin B12 levels with stroke, heart failure, markers of arrhythmia susceptibility, atherosclerosis, and death. In one study of patients with heart failure, elevated MMA predicted a higher risk of death from any cause.

Randomized trials have been less persuasive. For example, NORVIT and HOPE-2 tested combinations of folic acid, B12, and in some groups B6. NORVIT showed a substantial drop in homocysteine, around 27%, but the interventions did not consistently reduce heart attacks, cardiovascular death, or overall vascular events. HOPE-2 found fewer strokes among people receiving combined B-vitamin therapy, while NORVIT also suggested possible harm from the three-vitamin combination.

A Cochrane review covering more than 71,000 participants found no meaningful benefit for coronary outcomes.

Across pooled trials, homocysteine-lowering B-vitamin regimens were linked to about a 10% relative reduction in stroke risk. Most studies used vitamin combinations, so the result cannot be attributed to B12 alone. The strongest benefit appeared in regions without folic acid fortification, where subgroup analysis showed a 30% to 35% relative reduction in stroke incidence, and among people who started with low folate levels.

The authors suggest that stroke may be more responsive to homocysteine lowering because small brain vessels depend heavily on endothelial health and nitric oxide balance. Heart attacks, by contrast, often occur when coronary plaques rupture and trigger clot formation, a process that homocysteine lowering has not been shown to prevent.

Groups at higher risk

Older adults face a greater risk of B12 deficiency because stomach acid production and absorption tend to decline with age. Trials of B12 supplementation in this group have not shown clear cardiovascular benefit.

Very high B12 levels have also been associated with increased mortality, likely because they reflect underlying illness such as liver or blood disorders rather than causing harm directly.

Vegetarians and vegans are at increased risk of B12 deficiency and elevated homocysteine, both of which can be corrected with supplementation.

People who have undergone bariatric surgery may have reduced B12 absorption. Because liver stores can delay diagnosis for years, long-term monitoring is needed. UK NICE guidance recommends considering injections when deficiency is caused by some forms of postoperative malabsorption. If oral treatment is used, the recommended dose is at least 1 mg daily.

People with chronic kidney disease on dialysis often have high homocysteine levels, which can be lowered with B12-folate supplementation. However, trials have not shown fewer cardiovascular events. Monitoring B12 status and treating confirmed deficiency remain appropriate, including when anemia is caused by low B12.

During pregnancy, low B12 has been associated with preeclampsia, gestational diabetes, lower birth weight, and possible later metabolic problems in children. Adequate intake is recommended. Maternal supplementation can improve B12 status, but evidence for better infant neurodevelopment remains limited and inconsistent.

Testing and treatment

At present, there are no recommendations for routine B12 screening across the general population to prevent cardiovascular disease. Testing is aimed at people with symptoms of deficiency or those at higher risk.

A normal blood B12 result does not always mean the body has enough usable B12, because total B12 includes forms that cells cannot readily use. When levels are low or borderline, roughly 150 to 300 pg/mL, further testing of MMA or holotranscobalamin is often used to confirm functional deficiency. Since homocysteine can also rise with folate or vitamin B6 deficiency, elevated MMA is a more specific marker of B12 deficiency. Kidney disease can also raise MMA, so results must be interpreted alongside clinical findings and renal function.

For treatment, a network meta-analysis found that oral, intramuscular, and sublingual B12 all increased B12 levels, with no statistically significant differences between methods. The study also found no significant differences in homocysteine outcomes. Oral doses of 1,000 to 2,000 µg daily are a reasonable first option for deficiency related to diet or age. Intramuscular replacement is recommended for pernicious anemia and some causes of irreversible malabsorption, while high-dose oral treatment may still be suitable for certain other patients depending on the cause and response.

Supplements are generally safe, but stronger evidence is still needed before high-dose B12 can be recommended as a way to protect the heart.

Lifestyle factors

Factors that can lower B12 status include heavy alcohol use, smoking, infection with Helicobacter pylori, and long-term use of acid-reducing medicines such as proton pump inhibitors and metformin. Improving B12 status involves addressing modifiable risk factors and monitoring people taking medications associated with deficiency. A balanced diet with enough B6 and folate, along with exercise and weight control, also supports cardiovascular health.

The possible value of B12 food fortification remains debated, and where it exists, implementation is generally limited and voluntary.

Genetics

People who carry the MTHFR C677T variant may have higher homocysteine, especially when folate is low. The TCN2 gene also influences B12 transport.

Current guidelines do not recommend routine genetic testing for cardiovascular prevention, since genotype-guided treatment has not been shown to improve outcomes. The authors say more research is needed to define optimal B12 ranges, because both low and very high levels have been linked to higher mortality.

Conclusion

Low vitamin B12 is associated with greater cardiovascular risk, but supplements have not reliably prevented heart disease in clinical trials. Treating B12 deficiency has clear blood-related and neurological benefits, while the strongest signal for stroke prevention comes from combined B-vitamin therapy in selected populations. These findings do not prove cardiovascular protection from B12 alone.

Holotranscobalamin and MMA can improve detection of functional B12 deficiency when total serum B12 results are unclear, but their ability to predict cardiac events remains uncertain. Future studies should focus on better biomarkers and precision approaches that combine blood tests, nutritional status, and possibly genetics.

Regarding combined B-vitamin therapy, the authors wrote:

“Its greatest utility lies in correcting deficiencies and reducing stroke risk in nutritionally vulnerable populations or in those with established cerebrovascular disease. In well-nourished, folate-fortified populations, the incremental cardiovascular benefit is negligible.”

Source:

Journal reference:

• Awashra, A., AbuBaha, M., AbuBaha, B., et al. Vitamin B12 and cardiovascular health: mechanisms, clinical evidence, and future directions. Nutrition Journal.