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Folic acid - a vitamin that prevents birth defects in babies - appears to sharply reduce the risk of stroke in adults as well, according to new research.

The study, published in today's edition of The Lancet, found that people who took folic acid supplements saw their risk of stroke fall by about 18 per cent.

Those who took the vitamins daily for an extended period (three years or more) saw their risk fall even more substantially, by 29 per cent.

But researchers also cautioned that folic acid does not necessarily help prevent other cardiovascular conditions, as widely touted.

Xiaobin Wang, a researcher at the Children's Memorial Research Center of Northwestern University in Chicago, led a team that conducted a meta-analysis, combining and interpreting the results of eight previous studies on folic acid and stroke.

"Our meta-analysis provides coherent evidence that folic acid supplementation can significantly reduce the risk of stroke in primary prevention," she said.

But at the same time, Dr. Wang said a lot of questions remain unresolved, including the role of fortification of foods with folic acid (as is done in Canada and the United States), the optimal dose of folic acid, and the importance of other B vitamins in stroke prevention.

Folic acid works by lowering the concentration of homocysteine, an amino acid, in the blood. High amounts of homocysteine are thought to damage the lining of arteries and promote the formation of clots which, in turn, increase the risk of stroke, heart attack and deep-vein thrombosis.

In an editorial also published in The Lancet, Cynthia Carlsson, an assistant professor at the University of Wisconsin school of medicine and public health, said the new research helps clarify some issues, but many questions remain about the pros and cons of taking folic acid for preventing stroke and heart disease.

She said that while homocysteine levels are markedly higher in these conditions, it is not certain if the amino acid plays a role in these diseases or is merely an innocent bystander.

Dr. Carlsson also noted that in a number of studies folic acid, when taken with a number of other B vitamins, actually seemed to increase the risk of heart disease.

She said more research is needed "before we can conclude that the benefit of continued use of previously deemed 'safe' vitamin supplements outweighs the risk of other adverse outcomes."

Folate, or vitamin B9, is found in leafy green vegetables such as spinach, and in legumes like lentils and kidney beans. The synthetic version, folic acid, is added to commercially prepared grain products such as white flour, bread and pasta. The recommended daily intake is 0.4 milligrams, the amount commonly found in daily multivitamin supplements.

Folic acid (as well as prenatal multivitamins) has been shown to dramatically reduce the risk of birth defects such as spina bifida, in which the spine is severely deformed, and common childhood cancers like neuroblastoma, as well as prevent heart malformations and cleft palate.

It is recommended that all women of childbearing age take a daily folic acid supplement even if they are not planning to get pregnant.

This is because folic acid is essential in the first month after conception - when many women are not yet aware they are pregnant - as the neural tube, which later becomes the spinal cord, forms.