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Folic acid is one of the more essential B vitamins, especially for women of childbearing age, and a little bit—100 micrograms a day—goes a long way toward preventing spina bifida and other birth defects. Now a major study published in the Journal of the American Medical Association has confirmed what smaller studies had only hinted at: that women who consume large amounts of folate (in the region of 1,000 micrograms a day) have a lower risk of developing hypertension—for younger women, it's significantly lower.

The report, part of the ongoing Nurses' Health Study, tracked the folate intake of nearly 94,000 women ages 27 to 44 over eight years and found that those who consumed at least 1,000 micrograms a day had a 46% lower risk of high blood pressure than women who took less than 200 a day. A parallel study of 62,000 women aged 43 to 70 found an 18% reduction.

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It's not yet clear how folic acid does this, although the supplement is known to reduce levels of homocysteine, a blood component that can damage blood vessels. Women who got their folate eating foods naturally high in the vitamin—such as nuts, leafy greens and berries—didn't derive the same benefit as those who took folic acid supplements, perhaps because it's hard to eat enough vegetables in one day.

Men haven't yet been studied as thoroughly, but there's no reason to think they wouldn't get the same benefit from high doses of folate. Dr. John Forman of Brigham and Women's Hospital in Boston, the principal investigator, isn't ready to say that everybody should take 1,000 micrograms of folate a day. He wants to see his findings confirmed in a large, randomized trial in which half the subjects take the supplement and half take a placebo. Other scientists caution that early studies suggesting that large doses of vitamin E protect against heart disease haven't panned out.

But since millions of people in Asia suffer from high blood pressure, and there isn't any known downside to taking folate, it might not be a bad idea for all adults to take the current recommended dose of 400 micrograms a day.

• Sanjay Gupta
• Megadoses of the vitamin lower blood pressure in women—and maybe in men