Homocysteine

What is Homocysteine?

Homocysteine is a sulphur-containing amino acid formed as an intermediate in the breakdown of methionine, an essential amino acid from protein-rich food. The body disposes of homocysteine via two routes: remethylation back to methionine (requiring folate as 5-methyltetrahydrofolate and vitamin B12 as a cofactor) and transsulphuration to cysteine (requiring vitamin B6 as a cofactor). When these processes are disrupted by a deficiency of folate, B12, or B6, or by genetic polymorphisms in the MTHFR or CBS genes, the homocysteine concentration in the blood rises. Fasting reference values for adults are typically below 15 µmol/L; most preventive guidelines regard below 10–12 µmol/L as optimal. Mildly elevated (15–30 µmol/L) is the most common finding with nutritional deficiencies; moderately elevated (30–100 µmol/L) is rarer and can occur with more severe deficiencies or the MTHFR 677TT genotype; markedly elevated (> 100 µmol/L) is classic for homocystinuria, a rare inherited enzyme deficiency. The MTHFR C677T polymorphism is homozygous in 10–15% of White populations and gives a milder enzyme inactivation of roughly 50%, leading to mildly elevated homocysteine when folate intake is insufficient. Homocysteine is measured fasting, ideally in the morning. Non-fasting measurements are 5–10% higher due to post-prandial methionine uptake. The measurement is stable and not substantially affected by acute stress or brief exercise.

Why is Homocysteine relevant?

Homocysteine is a functional marker of B-vitamin status that simultaneously provides information about cardiovascular risk — two dimensions in one test. When folate, B12, or B6 functionally falls short, homocysteine rises before the vitamin concentrations themselves become clearly abnormal. It is therefore an early signal of subclinical B-vitamin deficiency, which is particularly valuable for groups at elevated risk: vegans and vegetarians (lower B12 supply), older adults (reduced gastric acid and intrinsic-factor secretion), people on long-term proton pump inhibitors (reduced B12 absorption), and those with IBD or bariatric surgery. Elevated homocysteine is associated with cardiovascular disease: endothelial damage, increased thrombogenicity, and accelerated atherosclerosis are mechanistically plausible. Randomised trials on B-vitamin supplementation to lower homocysteine showed mixed results on hard cardiovascular endpoints — the relationship is probably not directly causal but more a marker of an unfavourable B-vitamin profile. Nevertheless, B-vitamin supplementation demonstrably lowers homocysteine, and in stroke patients this has shown a modest but statistically significant protective effect. For neurological health, the association with cognitive decline and dementia is an active research area: higher homocysteine values are associated with faster hippocampal atrophy and increased Alzheimer's risk, although causality has not been proven here either. The biological plausibility via DNA methylation and neurotoxicity is, however, well supported.

Homocysteine high or low — what it means

Homocysteine is almost always interpreted together with vitamin B12 and folate — only then can you determine whether an elevation is likely nutrition-related or has another cause. When MMA (methylmalonic acid) is also available, it adds another layer: elevated MMA with elevated homocysteine points specifically to a B12 deficiency at the cellular level, while normal MMA with elevated homocysteine fits more with a folate or B6 deficiency, or an MTHFR variant with insufficient dietary folate. Lifestyle and other factors meaningfully influence homocysteine: smoking raises it through oxidative stress and B6 consumption; high coffee intake has a mildly elevating effect; metformin raises homocysteine by reducing B12 absorption via intrinsic factor; methotrexate and phenytoin inhibit folate metabolism. Kidney function is a critical confounder: with declining eGFR, homocysteine rises independently of B-vitamin status because the kidney plays a key role in its excretion. With an elevated value, the first step is supplementation with folate (0.5–5 mg/day) and vitamin B12, and in smokers also B6. Repeating the test after 8–12 weeks shows whether the value responds. With values that remain high despite supplementation, MTHFR genotyping, kidney function review, and medication evaluation are the next steps. MTHFR genotyping on its own is generally not clinically actionable without an elevated homocysteine.

Educational information only — not medical advice. Consult a healthcare professional for clinical decisions.