What is homocysteine and why does it matter?

Homocysteine is an amino acid your body makes as a byproduct of processing methionine, which comes from protein in your diet. Normally it's quickly converted into other compounds — a process that depends on B vitamins.

How the body clears it

• Folate and vitamin B12 help convert homocysteine back into methionine, a key methylation step.
• Vitamin B6 helps convert it into cysteine.

When any of these nutrients run low, homocysteine can build up. That's why elevated homocysteine is a useful clue for vitamin B12 or folate deficiency. The NIH Office of Dietary Supplements notes that homocysteine and methylmalonic acid levels rise when B12 is insufficient.

Why it matters

Observational studies link higher homocysteine to cardiovascular disease and stroke. That led to a logical idea: lower homocysteine with B vitamins, and cardiovascular risk should fall too.

A Cochrane review of randomized trials tested this. B-vitamin supplementation lowered homocysteine but did not reduce heart attacks or deaths. It was associated with a small reduction in stroke. So homocysteine appears to be more of a marker than a simple lever.

Who might care most

• Older adults and people taking metformin or acid-reducing drugs, who are at higher risk of low B12.
• People following a vegan diet without B12 supplementation.
• People with kidney disease.
• Those with MTHFR variants and low folate intake — see our MTHFR guide.

Practical steps

Eat folate-rich greens and legumes, get enough B12 from food or supplements if you're plant-based, and ask your clinician whether testing makes sense for you. Homocysteine is a useful signal, not a standalone diagnosis.