Homocysteine, a normal breakdown product of the essential amino acid methionine, is believed to exert several toxic effects.

A growing body of evidence suggests that an elevated homocysteine level is a risk factor for heart disease, independent of other known risk factors, such as elevated serum cholesterol and hypertension.^{1, 2} The evidence is not all one-sided, however. In some research the link has appeared only in women,³ and a few scientists still have doubts about the importance of elevations in homocysteine for anyone.⁴ The clear association between elevated homocysteine levels and heart disease reported in most studies⁵ does not conclusively prove that homocysteine causes heart disease. It might only be a marker for something else that is the real culprit.⁶ Nonetheless, many cardiologists take seriously the association between elevations in homocysteine and increased risk of heart disease.

Anger and hostility correlate with the risk of heart disease.^{7, 8} A preliminary study found a link between high homocysteine levels and hostility and repressed anger.⁹ While anger, hostility, high homocysteine, and heart disease all appear to be tied together, which of these is cause and which is effect remains somewhat unclear.

Increased homocysteine levels may also be a risk factor for the development of many other conditions, including stroke,¹⁰ thromboembolism¹¹ (blood clots that can dislodge and cause stroke, heart attack, and other complications), osteoporosis,¹²Crohn’s disease, ulcerative colitis,¹³Alzheimer’s disease,¹⁴ death from diabetes,¹⁵ miscarriage,^{16, 17, 18, 19, 20} other complications of pregnancy,^{21, 22, 23, 24, 25} and hypothyroidism.²⁶

Scientists have yet to prove that elevated homocysteine levels cause any of these diseases. However, most doctors believe that high homocysteine increases the risk of at least heart disease. Fortunately, homocysteine levels can easily be reduced with safe and inexpensive B vitamin supplementation.