Homocysteine is a sulfur amino acid that rises when B12, folate or B6 run short. High levels correlate with cardiovascular disease and cognitive decline, though lowering them with vitamins has disappointed in trials.
Standard range versus optimal range
| Framework | Homocysteine value | Source |
|---|---|---|
| Desirable | below 10 µmol/L | Common practice target |
| Mildly elevated | 15-30 µmol/L | Common classification |
| Cardiovascular association | each 5 µmol/L rise, about 20-30 % higher risk | Wald et al, BMJ 2002 |
When it runs high
High homocysteine usually signals low B12, folate or B6, sometimes a common MTHFR gene variant, sometimes kidney disease. Genetic studies link lifelong higher levels to more heart attacks, strokes and clots.
- Folate deficiency
- B12 deficiency, with a high MMA
- Vitamin B6 deficiency
- Kidney disease
- MTHFR gene variants
- Hypothyroidism and certain medications
When it runs low
Low homocysteine is considered benign and often simply reflects good B vitamin status.
- Good B vitamin status, the usual case
The metabolite with a cautionary history
In the 1990s homocysteine looked like the next cholesterol, high levels tracked tightly with heart attacks and strokes, and a landmark 2002 meta-analysis estimated that each 5 µmol/L rise carried about a 20 to 30 percent increase in ischemic heart disease risk. Then came the disappointment, large trials lowered homocysteine beautifully with B vitamins and barely moved heart outcomes. The molecule turned out to be more marker than maker of disease, though the stroke signal and the genetic evidence keep the debate alive.
Where it still earns its keep
Homocysteine shines in the diagnostic lab rather than the cardiology clinic. It rises in both B12 and folate deficiency, and paired with methylmalonic acid it separates the two, the pattern high homocysteine with normal MMA means folate, both high means B12. It also flags the rare but serious genetic disease homocystinuria, where levels run five to ten times normal.
Practical reading
If your homocysteine comes back above 15, the first response is cheap and safe, check B12, folate and kidney function, and fix the diet. Whether driving the number down adds years is unproven, but the deficiencies behind it are worth correcting on their own merits, especially before pregnancy and in older adults protecting cognition.
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