Homocysteine: The Overlooked Cardiovascular and Cognitive Risk Marker

The idea that homocysteine harms arteries goes back to a 1969 pathology report on the vascular damage seen in homocysteinemia, the very high levels caused by rare inherited enzyme defects.^1,8 The questions since then have been whether the much smaller rises common in the general population matter, and whether lowering them helps.

How Homocysteine Is Made and Cleared

Two pathways account for virtually all methionine metabolism: the methionine cycle, which every tissue has, and transsulfuration, which occurs only in the liver, kidney, small intestine and pancreas and is how homocysteine is broken down.⁹ Recycling homocysteine back to methionine requires folate and vitamin B12, while transsulfuration requires pyridoxal-5'-phosphate, the active form of vitamin B6.¹

Homocysteine therefore rises when folate or vitamin B12 runs low, and folic acid lowers it most in people with low folate or high homocysteine to begin with.³ Some medications affect vitamin B12: in the Diabetes Prevention Program, low or borderline-low B12 was more common with long-term metformin (20.3% versus 15.6% after 13 years),¹⁰ and two or more years of proton pump inhibitors was associated with higher odds of B12 deficiency (odds ratio 1.65).¹¹ Two copies of the common MTHFR C677T variant also raise homocysteine (see below).¹²

Heart Disease and Stroke: The Evidence

In a meta-analysis of individual data from 30 studies with 5,073 heart disease events and 1,113 strokes, a 25% lower usual homocysteine (about 3 µmol/L) was associated with 11% lower risk of ischemic heart disease and 19% lower risk of stroke after adjustment for other risk factors. The authors concluded that homocysteine is at most a modest independent predictor in healthy populations.²

Trials that lowered homocysteine tell a more sobering story. In HOPE-2, 5,522 people aged 55 or older with vascular disease or diabetes took folic acid 2.5 mg, vitamin B6 50 mg and vitamin B12 1 mg, or placebo, for an average of five years. Major cardiovascular events were not reduced (18.8% versus 19.8%, relative risk 0.95), although strokes were fewer (relative risk 0.75) and hospitalizations for unstable angina more common (relative risk 1.24).⁴ In NORVIT, among 3,749 people who had just had a heart attack, folic acid plus vitamin B12 lowered homocysteine by 27% with no effect on further events, and the combination with vitamin B6 showed a trend toward harm (relative risk 1.22).⁵ In VITATOPS, B vitamins given to 8,164 people with a recent stroke or transient ischemic attack did not clearly reduce major vascular events (15% versus 17%, relative risk 0.91).¹³

Stroke is the exception that keeps appearing. In a trial in Chinese adults with high blood pressure, adding folic acid to the blood pressure drug enalapril cut first strokes from 3.4% to 2.7% over a median of 4.5 years (hazard ratio 0.79), with no effect on heart attacks.¹⁴ A 2017 Cochrane review of the trials found no effect of B-vitamin homocysteine lowering on heart attacks, deaths or adverse events, and a small effect favoring it for stroke.⁶

Homocysteine and the Brain

In the Framingham Study, 1,092 adults without dementia (mean age 76) were followed for a median of eight years. Each standard-deviation rise in homocysteine raised the risk of Alzheimer's disease 1.8-fold, and a level above 14 µmol/L nearly doubled it.¹⁵ In an Oxford case-control study, homocysteine in the top third (14 µmol/L or more) was associated with 4.5 times the odds of confirmed Alzheimer's disease, and over three years patients with higher levels showed more radiological progression.¹⁶

The VITACOG trial tested whether lowering homocysteine changes that course. In 271 people over 70 with mild cognitive impairment, folic acid 0.8 mg, vitamin B12 0.5 mg and vitamin B6 20 mg a day slowed whole-brain atrophy among the 168 who completed the MRI scans, to 0.76% a year against 1.08% on placebo. In participants whose homocysteine was above 13 µmol/L, the rate of atrophy was 53% lower.⁷ The trial measured brain shrinkage rather than dementia, and its authors called for trials of whether the same treatment slows progression to Alzheimer's disease.⁷

MTHFR and the Folic Acid Question

MTHFR (methylenetetrahydrofolate reductase) makes 5-MTHF, the form of folate used to remethylate homocysteine. People with two copies of the common C677T variant have a heat-sensitive enzyme with reduced activity and higher homocysteine.¹² They still respond to folic acid: in a randomized trial, 677TT carriers responded to folate-rich foods or a 400 µg folic acid supplement, though they needed more folate than other genotypes to reach the same homocysteine level.¹⁷ In healthy women, methylfolate and folic acid lowered homocysteine equally.¹⁸

Testing and What to Do

An expert review sets out how homocysteine should be measured, its biological determinants and reference intervals, and the conditions in which it helps with diagnosis, including folate and vitamin B12 deficiency, kidney failure and the inherited disorder homocystinuria.¹⁹ No trial has shown that pushing homocysteine below a particular number improves outcomes, so this page gives no target beyond your laboratory's reference range.

If your result is raised, the usual causes are worth checking with your clinician: low folate or vitamin B12, medications such as metformin and acid suppressants, and kidney function.^10,11,19 Folic acid with vitamin B12 typically lowers homocysteine by about a quarter to a third.³ Whether that prevents heart disease is doubtful on the trial evidence; it may modestly reduce stroke risk.⁶

Derek Giordano

Founder & Editor, IQ Healthspan

Derek Giordano is the founder and editor of IQ Healthspan. A father of four with a lifelong interest in athletics, fitness and the supplement industry, he built the site to show what the research actually supports. Derek is not a physician: articles cite peer-reviewed studies with numbered references you can check, and corrections are logged publicly. Articles are researched, drafted and fact-checked with the help of AI tools, and every claim is checked against the studies it cites. IQ Healthspan has no supplement brand partnerships, affiliate relationships or financial conflicts of interest.

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Sources Listed With Numbered Citations

19 references at the end of this article, checked against PubMed; studies link to their PubMed record