Homocysteine is an amino acid produced as an intermediate in the metabolism of methionine, an essential amino acid found primarily in animal protein. In normal metabolism, homocysteine is either recycled back to methionine or converted to cysteine. Both of these conversion pathways require B vitamins: folate and B12 for the recycling pathway, B6 for the conversion pathway. When any of these vitamins are deficient, homocysteine accumulates in the blood. Elevated homocysteine damages the endothelium, promotes oxidative stress, and accelerates atherosclerosis through several well-characterised mechanisms.
What the Research Establishes
The Homocysteine Studies Collaboration meta-analysis, published in JAMA in 2002, pooled data from 30 prospective studies covering over 5,000 cardiovascular events and found that a 25 percent lower homocysteine level was associated with an 11 percent lower risk of ischaemic heart disease and a 19 percent lower risk of stroke. These associations held after adjusting for conventional risk factors. The association between elevated homocysteine and cognitive decline is similarly robust, with multiple cohort studies showing that adults with high homocysteine have significantly faster hippocampal atrophy and cognitive decline rates.
Optimal Levels and Treatment
- Optimal homocysteine: below 10 micromol/L; ideally below 7 micromol/L for cognitive protection
- Borderline elevated: 10 to 15 micromol/L; intervention appropriate
- Elevated: above 15 micromol/L; high cardiovascular and cognitive risk
- Treatment: methylfolate 400 to 1000 mcg plus methylcobalamin (B12) 500 to 1000 mcg plus pyridoxal-5-phosphate (B6) 25 mg daily; most adults with elevated homocysteine respond to this regimen
- Testing: a simple blood test, not routinely offered in standard NHS panels; requestable from private services or by specific GP request
Sources
- Homocysteine Studies Collaboration. (2002). Homocysteine and risk of ischemic heart disease and stroke. JAMA, 288(16), 2015-2022.
- Smith, A.D., et al. (2010). Homocysteine-lowering by B vitamins slows the rate of accelerated brain atrophy in mild cognitive impairment. PLOS ONE, 5(9), e12244.
- Refsum, H., et al. (2004). The Hordaland Homocysteine Study: a community-based study of homocysteine, its determinants, and associations with disease. Journal of Nutrition, 136(6 Suppl), 1731S-1740S.