Taking extra folic acid and other B-vitamin supplements may not help protect against colon polyps, according to a U.S. study that contradicts observational studies suggesting people who get more of the vitamins are less likely to get colon cancer.

The study, led by Yiqing Song from Brigham and Women's Hospital in Boston and published in the Journal of the National Cancer Institute, followed 1,470 women with an average initial age of 62.

The women were randomly assigned to take daily folic acid and vitamins B6 and B12, or a vitamin-free placebo pill, and then followed them to see who developed colon polyps.

Women in the "active" treatment group took 2.5 milligrams of folic acid, 50 mg of vitamin B6 and 1 mg of vitamin B12 each day.

All participants had a colonoscopy or sigmoidoscopy to check for colon polyps, which can develop into cancer if not removed, sometime before mid-2007. According to their medical records, 355 had a confirmed polyp.

Polyp risk was not related to treatment group, with 24.3 percent of women taking the vitamins developing polyps compared to 24.0 percent of those on placebo pills. The lack of benefit remained after the researchers accounted for women's weight as well as smoking, alcohol and exercise.

"Where we really are is a kind of interesting impasse," said John Baron, an epidemiologist from the Geisel School of Medicine at Dartmouth University in Lebanon, New Hampshire, who was not involved in the study.

"The observational data continually show an inverse association between measures of folic acid, that is diet or blood level, and the risk of colorectal cancer. The clinical trial data such as we have... suggest no benefit overall."

One hypothesis is that folic acid can help ensure DNA replicates correctly, which in theory could decrease the risk of cancer. But there's also been some concern that high doses of folic acid can feed the growth of pre-cancerous polyps in people who already have them.

Baron, who wasn't involved in the study, told Reuters hHealth those worries are "largely theoretical," but they are a reason not to go overboard with folic acid nonetheless.

He added that the combined data don't support the protective effects of B vitamins, over and above what's in a normal diet, on the colon.

"Most people in the U.S. are reasonably well nourished, and with folic acid supplementation now there's not serious concern about folate deficiency," he said.

"For general health, a usual diet for most people is sufficient. Taking large doses (of folic acid) is, at a minimum, wasteful."