Folic acid supplements, widely used by women to prevent birth defects, also may fight hypertension in women, perhaps because they relax blood vessels, researchers said.

Folic acid occurs naturally in substances such as orange juice and leafy green vegetables and is added to some products, but only high-dose supplements seem to affect blood pressure, the report said.

"This is the first major study to show an association between folate intake and prevention of high blood pressure," said John Forman, a physician and researcher at Brigham and Women's Hospital in Boston who led the study.

"While the findings are encouraging, we are not yet ready to recommend that women start increasing their folate intake until more research is conducted," he added.

The study, published in this week's Journal of the American Medical Association, was based on data from thousands of nurses whose health histories were tracked for years.

Among 93,803 women aged 27 to 44, those who consumed at least 1,000 micrograms a day of total folate -- in foods and supplements -- had a 46 percent decreased risk of hypertension compared with those who consumed less than 200 micrograms a day, the study found.

In a second group of 62,260 women aged 43 to 70, those with a high total folate intake had an 18 percent reduced risk of hypertension, it added.

None of those studied had a history of high blood pressure when the study started. It also was not known if the supplement would help men.

For women in the study who did not use any folic acid supplements, getting the substance from foods alone failed to lower the risk of high blood pressure, the study found.

Folic acid supplements of at least 400 micrograms per day are recommended for women before and during pregnancy to prevent neural tube defects, severe fetal growth problems afflicting the brain and spinal cord that often result in stillbirths.

Forman said the substance may help women reduce their risk of developing high blood pressure because of it relaxes blood vessels and eases blood flow.

"The prospect that folate has the potential to protect against high blood pressure is exciting given that the vitamin is both readily available and safe," he said. "The next step will be a randomized controlled trial comparing women given either folic acid supplements or placebo."