Pregnant women are advised to take extra folic acid
Folic acid may help keep blood pressure in check, US researchers believe.
The study, in this week's Journal of the American Medical Association, adds to growing evidence of folate's cardiovascular benefits.
The Harvard team looked at data on about 156,000 nurses and found those with the lowest intakes of folate were at greater risk of hypertension.
Last week, researchers said folic acid - found in green leafy vegetables - might benefit people at risk of stroke.
Pregnant women and women trying to get pregnant are advised to take extra folic acid (0.4mg per day, rather than 0.2mg per day) to protect their baby against neural tube defects such as spina bifida.
Some campaigners in the UK have pushed for folic acid to be added to foods such as flour, as is already done in other countries, including the US.
The Food Standards Agency does not recommend this.
Future trials should examine folic acid supplementation as a means of lowering blood pressure and preventing hypertension.
The study authors
The study by Harvard Medical School and the Brigham and Women's Hospital in Boston is the first to look at the effect of folic acid on high blood pressure in a large number of people.
The nurses involved had taken part in two hypertension studies that looked at diet and health over eight years.
Overall, 19,720 of the women, aged between 27 and 70, had high blood pressure.
After looking at other risk factors for hypertension, such as physical activity and family history, they found an apparent protective effect of increased folic acid against high blood pressure, particularly among the younger women.
Young women who consumed at least 1mg per day had a 46% decreased risk of hypertension compared with those who consumed less than 0.2mg per day.
Older women with higher intakes reduced their risk of high blood pressure by 18%.
The researchers believe the benefits may be down to improved blood vessel function.
They said: "Future trials should examine folic acid supplementation as a means of lowering blood pressure and preventing hypertension."
Dr Howard Robson, a cardiologist at Cumberland Infirmary in Carlisle who has been investigating whether giving folic acid to heart patients could help prevent heart attacks, said the findings were interesting.
"We need more research. In the meantime, it is best to follow a healthy diet with plenty of fruit and vegetables.
"We would not recommend that everyone take supplements yet."
Andrew Russell from the Association for Spina Bifida and Hydrocephalus said the research added weight to the argument that food should be fortified with folic acid.
"A deficiency of folic acid is widespread in the population. In fact, it's almost universal."
He said it was extremely safe and would have multiple health benefits.
Professor Jeremy Pearson of the British Heart Foundation said: "It has long been recognised that folate is particularly important for the health of pregnant women and their babies, and that extra folate in the diet is safe.
"This paper from Harvard shows that those with increased folate intake, particularly younger women, had substantially lower risk of developing high blood pressure.
"Boosting folate also reduces the levels of homocysteine in the blood, a risk factor for heart disease.
"Together, these results strongly support the need for a large study to test directly whether folate supplementation protects against clinical incidents relating to heart disease."
Recently, researchers have warned of a potential link with maternal breast cancer when taken late into pregnancy.
However, the finding in no way questions the benefit of taking the supplements before and during the first months of pregnancy, they said.