Updated June 08, 2015.

A high blood homocysteine level is a risk factor for stroke. This is in spite of the fact that homocysteine is a common nutrient utilized by our bodies to make proteins. But early studies found that high homocysteine levels increase a person's risk for heart disease, and subsequent studies also demonstrated that they can also increase the risk of stroke. It is not clear how homocysteine does this, but one likely possibility is through the promotion of atherosclerosis.

_Source:

I. M. Graham et. al Journal of the American Medical Association Vol. 277 No. 22, June 11, 1997 Plasma homocysteine as a risk factor for vascular disease. The European Concerted Action Project