A meta-analysis published in the most recent issue of The Lancet concluded that supplementing with the B vitamin folic acid can reduce stroke risk by at least 18 percent. Researchers analyzed eight randomized trials involving folic acid and stroke. Participants supplementing with folic acid lowered their risk of stroke by an average of 18 percent compared to those who did not use folic acid supplements. Trials involving folic acid supplementation longer than 3 years showed an even greater reduction in stroke risk (29%). Folic acid supplementation was also more significant in people with reduced homocysteine levels, those with no prior stroke risk, or those who lived in area without folic acid fortification of foods.
It is believed that folic acid's ability to reduce homocysteine, an amino acid found in the blood that is toxic in excess, may be the reason for the lower incidence of strokes.
Source: Efficacy of folic acid supplementation in stroke prevention: a meta-analysis, X. Wang, X. Qin, H. Demirtas, J. Li, G. Mao, Y. Huo, N. Sun, L. Liu, X. Xu, Lancet 2007 June 2;369(9576):1876-82
