Participants were then followed for a minimum of six months. There were 2,471 strokes throughout the studies, all of which showed some benefit of taking vitamin B.
"Previous studies have conflicting findings regarding the use of vitamin B supplements and stroke or heart attack," said author Xu Yuming, with Zhengzhou University in Zhengzhou, China.
"Some studies have even suggested that the supplements may increase the risk of these events," Yuming said.
Vitamin B lowered the risk of stroke in the studies overall by seven per cent. However, taking supplements did not appear to affect the severity of strokes or risk of death from stroke.
Folic acid, a supplemental form of folate (vitamin B9), which is often found in fortified cereals, appeared to reduce the effect of vitamin B. Researchers did not find a reduction in stroke risk for vitamin B12.
"Based on our results, the ability of vitamin B to reduce stroke risk may be influenced by a number of other factors such as the body's absorption rate, the amount of folic acid or vitamin B12 concentration in the blood, and whether a person has kidney disease or high blood pressure," said Yuming.
The research was published in the journal Neurology, the medical journal of the American Academy of Neurology.


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