In short
Homocysteine is an amino acid produced as an intermediate in methionine metabolism; elevated blood levels (hyperhomocysteinemia) are associated with increased cardiovascular disease risk and, separately, cognitive decline.
Homocysteine is normally recycled back into methionine or converted into cysteine through pathways that depend on adequate folate, vitamin B12, and vitamin B6 as cofactors. When these vitamins are deficient, or when a person carries certain common gene variants (such as MTHFR variants that reduce enzyme efficiency), homocysteine can accumulate in the blood.
Elevated homocysteine is associated in observational studies with increased risk of cardiovascular disease, stroke, and — in a body of research including the Oxford Healthy Aging Project and other cohort studies — accelerated brain atrophy and cognitive decline in older adults. This led to substantial interest in whether B-vitamin supplementation to lower homocysteine would reduce these risks.
That causal question has largely been answered in the negative for cardiovascular outcomes: multiple large randomized trials (including the HOPE-2 and NORVIT trials) showed that B-vitamin supplementation reliably lowers homocysteine but does not reduce cardiovascular events. The picture for cognitive outcomes is more nuanced — the VITACOG trial (2010) found that B-vitamin supplementation slowed brain atrophy specifically in older adults who had both elevated homocysteine and mild cognitive impairment, suggesting a possible benefit in that specific subgroup rather than a general population effect. Homocysteine should therefore be understood as a validated risk marker whose modification has not translated into a proven general cardiovascular benefit, with the cognitive-decline evidence still an active and more targeted area of study.