When findings from eight previously reported studies were combined, researchers found the benefits were greatest among people who took folic acid supplements the longest.
"We only looked at stroke as an outcome, and we saw a clear benefit for supplementation in people who had not had previous strokes," researcher Xiaobin Wang, MD, PhD, tells WebMD.
Folic Acid Targets Homocysteine
Folic acid lowers blood levels of homocysteine, an amino acid widely believed to play a role in stroke and heart disease. Some studies have shown benefits from homocysteine lowering with folic acid, while others have not.
In research published last year, folic acid supplements were not found to reduce the risk of heart attacks or death in high-risk people but did appear to lower stroke risk.
A widely reported study, also published last year, suggested that folic acid supplements and two other B vitamins may actually increase heart risk in high-risk people.
In an effort to better understand the research, Wang and colleagues from Chicago's Northwestern University Feinberg School of Medicine limited their analysis to studies which assessed the impact of folic acid supplements on stroke risk.
The largest improvements in stroke risk were seen in people who:
- Took folic acid supplements for more than three years (29% lower risk).
- Achieved the highest reductions in blood homocysteine levels (23% risk reduction).
- Had no prior history of stroke (25% reduction).
Taking folic acid supplements was also found to lower stroke risk by 25% among people living in areas where grains have not been fortified with folic acid.
Fortification of breads, cereals, and other grains began in the U.S. in 1998 in an attempt to reduce the risk of neural tube birth defects, a birth defect affecting the spinal cord and brain. Spinach and other leafy green vegetables and dried beans and peas are also good food sources of the B vitamin.
She says more research is needed to determine if folic acid supplements are beneficial for lowering stroke risk in these people.
University of Wisconsin professor of medicine Cynthia M. Carlsson, MD, tells WebMD that many questions must be answered before folic acid supplements can be recommended to lower stroke risk.
"While there may be certain subgroups of people who may benefit from homocysteine-lowering therapy, it is premature to recommend folic acid for primary stroke prevention," she says.