Why we measure it

Elevated homocysteine damages the endothelium — the thin cellular lining of blood vessel walls that regulates vascular tone, inflammation, and clotting. The mechanisms include oxidative stress, impaired nitric oxide bioavailability, promotion of smooth muscle cell proliferation, and direct toxic effects on endothelial cells. These processes accelerate atherosclerosis and increase thrombotic risk. A landmark meta-analysis by Boushey et al. in JAMA (1995), synthesising 27 studies involving over 4,000 patients, established that elevated plasma homocysteine is an independent risk factor for cardiovascular disease — comparable in magnitude to the risk conferred by smoking or hypercholesterolaemia. Subsequent large prospective analyses confirmed the association with coronary artery disease, stroke, and peripheral arterial disease. The European Concerted Action Project demonstrated that a plasma homocysteine above 12 µmol/L significantly increases cardiovascular risk, independent of other established risk factors. Homocysteine also has clinical significance beyond cardiovascular disease. Elevated levels are associated with cognitive decline and dementia — several large cohort studies have linked hyperhomocysteinaemia to accelerated hippocampal atrophy and increased Alzheimer's risk. The VITACOG trial (Smith et al., 2010) demonstrated that B-vitamin supplementation to lower homocysteine in individuals with mild cognitive impairment slowed brain atrophy by nearly 30% over two years on MRI. The drivers of elevated homocysteine are also clinically informative: deficiencies in folate, vitamin B12, or vitamin B6 are the most common causes, making homocysteine an indirect functional marker of B-vitamin status. The MTHFR genetic variant — measured in the Foundation layer — impairs the folate-dependent remethylation pathway and is one of the most common genetic contributors to elevated homocysteine.

Why every 3 months

Homocysteine levels respond to nutritional status and supplementation on a timescale of weeks to months — B12 and folate supplementation typically produces measurable reductions within 4 to 8 weeks. This responsiveness makes quarterly tracking appropriate for monitoring whether a dietary change or supplementation protocol is achieving the intended metabolic effect. Annual testing cannot resolve this; the signal is too slow relative to the intervention timescale. Quarterly measurement provides the resolution to see whether homocysteine is moving in response to what is being done, and whether a reduction is being maintained.

What movement means

The European Concerted Action Project defined a plasma homocysteine above 12 µmol/L as associated with significantly elevated cardiovascular risk. Below 10 µmol/L is generally considered the desirable range in the cardiovascular research literature. The precise threshold for 'elevated' varies across studies and guidelines — the ranges below reflect classifications used in major epidemiological analyses, not a single authoritative clinical guideline. As with HOMA-IR, these should be read as directional reference values rather than absolute diagnostic cutoffs.

In the protocol