Homocysteine

Homocysteine is an amino acid byproduct of methionine metabolism, and elevated blood levels have long been associated with cardiovascular disease, stroke, and cognitive decline in observational studies. The claim originally proposed by Kilmer McCully in the 1960s and popularized through the 1990s: homocysteine damages blood vessel walls directly, and lowering it with B vitamins (folate, B6, B12, which the body needs to metabolize homocysteine) should reduce cardiovascular events. This hypothesis drove a wave of B-vitamin supplementation trials.

The results were disappointing for the direct-causation hypothesis. The HOPE-2 trial, a large randomized trial of folic acid plus B6 and B12 in patients with vascular disease, found that the vitamin combination lowered homocysteine levels substantially but did not significantly reduce the composite outcome of cardiovascular death, myocardial infarction, or stroke over five years, despite a statistically significant reduction in stroke alone as a secondary outcome [1]. This result, published in the New England Journal of Medicine, was one of several large B-vitamin trials (including NORVIT and VISP) that collectively failed to confirm that lowering homocysteine through vitamin supplementation prevents heart attacks, even though it reliably lowers the biomarker itself [6]. Earlier prospective cohort data had shown a graded association between elevated homocysteine and cardiovascular risk, which is what motivated the trials in the first place [3].

How to measure it: a standard blood draw measures total plasma homocysteine via immunoassay or chromatography, usually fasting to reduce variability from recent methionine-rich meals (meat, eggs, dairy). Levels are affected by kidney function, vitamin B12/folate status, and genetic variants in the MTHFR gene that impair homocysteine metabolism [4].

How to intervene on it: folic acid, vitamin B6, and vitamin B12 supplementation reliably lower homocysteine by roughly 20-30% in most people, especially those with baseline vitamin insufficiency [5], but this fails to translate into reduced cardiovascular events in the major trials, suggesting homocysteine may be a marker of vascular damage or poor nutritional status rather than a primary causal driver [2].

Critics argue the homocysteine-lowering trials may have been conducted too late (in patients with established vascular disease) to show benefit, since the theorized damage from years of elevated homocysteine may already be irreversible by the time a trial intervenes, and note that folate fortification of the food supply in many countries had already lowered population homocysteine levels by the time some later trials ran, potentially diluting any detectable effect.

The plain takeaway: elevated homocysteine correlates with cardiovascular and cognitive risk in cohort studies, but multiple large randomized trials lowering it with B vitamins have not shown the expected reduction in heart attacks or strokes, so homocysteine functions better as a risk marker worth investigating (particularly for B12/folate deficiency or MTHFR-related metabolism issues) than as a proven, independently treatable cause of cardiovascular disease.