In Europe there are around 650,000 stroke deaths each year. Smoking, obesity, high cholesterol and physical inactivity are all controllable risk factors for the event, which is more common at younger ages in men.
There is currently some evidence that folic acid and other related B-vitamins, B6 and B12, can prevent the accumulation of high blood levels of the amino acid homocysteine, thought to be a risk factor for heart disease and strokes.
The new study, carried out over 14 years by researchers in Illinois, US, found that men with the highest intake of folic acid were almost 30 per cent less likely to develop an ischaemic stroke than men with the lowest folic acid intake (less than 260 micrograms). Ischaemic stroke accounts for almost 80 per cent of all strokes and is caused by a blocked artery leading to the brain.
Study author Dr Ka He of Northwestern University in Illinois said the results appear conclusive and consistent enough to suggest that men change their behavior to protect their health.
He and colleagues enrolled 43 732 men, aged 40 to 75 years, who were free of cardiovascular diseases and diabetes at baseline in 1986. Dietary information was assessed every four years using a food frequency questionnaire. A total of 725 strokes, including 455 ischaemic, 125 haemorrhagic, and 145 unknown types of stroke, were documented during the 14-year follow-up, report the researchers in this month's issue of Stroke.
After adjustment for major lifestyle and dietary factors, the relative risk of ischaemic stroke was 0.71 for men in the highest quintile of intake compared with those who in the lowest quintile. Intake of vitamin B12, but not B6, was also inversely associated with risk of ischaemic stroke, said the researchers.
Last year an Australian team investigating the link between folate and B-12 concentrations in the blood and death from coronary heart disease or cardiovascular disease found no significant result.
The new findings appear to depend on the connection between certain B vitamins and the role played by homocysteine. There was no significant assocation between B vitamins and haemorrhagic stroke in the study, caused in a different way to ischaemic stroke.
The recommended dietary allowance for folic acid in adults is 400 micrograms daily. Europe's Scientific Committee for Food has advised an upper safe level of 1000 micrograms daily although it has not yet set limits for vitamin B12.
While the US, Canada and Chile have mandatory folic acid fortification of grains, Europe appears more hesitant to implement such a programme. Recent research by the UK's food safety organisation found that folic acid could mask deficiency of B12 in the elderly.
Folic acid has also been shown to significantly reduce risk of birth defects.