Animal studies led researchers to think that folic acid might protect people against colon cancer. That belief got huge support from the Nurses Health Study, which found that women with the highest folic acid intake were least likely to get colon cancer.
For these reasons, many Americans have begun taking inexpensive folic acid supplements or multivitamins that contain folic acid.
To find out whether folic acid has colon-cancer-preventing powers, the National Institutes of Health funded a clinical trial that enrolled more than a thousand U.S. men and women who previously had polyps removed from their colons. Left untreated, these polyps can become cancerous.
Study participants were randomly assigned to take daily pills containing either 1 milligram of folic acid or an inactive placebo. Patients also took low-dose aspirin, regular-dose aspirin, or placebo.
What happened? People who took folic acid got just as many new colon polyps as those who took placebo pills, reports researcher Robert Sandler, MD, chief of the division of gastroenterology and hepatology at the University of North Carolina at Chapel Hill School of Medicine.
"We are disappointed and surprised that it didn't work. In fact, there was some evidence that folic acid increased cancer risk," Sandler tells WebMD.
That evidence of increased cancer risk isn't what researchers call "statistically significant" -- meaning it could be a chance finding. Still, the finding is disturbing. Most of the increased risk came from prostate cancer. Men who took folic acid had a 7.3% chance of getting prostate cancer -- more than the 2.8% risk seen in the placebo group.
"It could be that folate helps prostate cancer to grow," Sandler says. "Another study suggested this previously, but that finding did not reach statistical significance. Nevertheless, now we have two experiments that suggest that folic acid might increase the risk for prostate cancer."
Sandler, Dartmouth researcher Bernard F. Cole, PhD, and colleagues report the findings in the June 6 issue of The Journal of the American Medical Association.
Folic Acid Concern
Sandler says these small amounts of folic acid are not a concern. All of the people in the study continued to eat foods fortified with folic acid. The amount of folic acid people get in supplement pills is far larger than the amount they get from fortified foods.
How could something that is good for babies be bad for grown-ups? It's all a matter of timing, argues an editorial accompanying the folic acid study. Editorial co-author Cornelia M. Ulrich, PhD, of the Fred Hutchinson Cancer Research Center and the University of Washington in Seattle, has studied folic acid for more than a decade.
"What happens is that folate is beneficial when taken when we are young because it decreases genetic mutations," Ulrich tells WebMD. "But as we get older there are more and more abnormalities in the colon -- things like polyps. And at that stage, folic acid probably increases the growth of these abnormalities."
Ulrich notes that the study by Cole, Sandler, and colleagues looked only at people who already had colon polyps removed by colonoscopy before the study began. Such people clearly had abnormalities in the colon. Whether folic acid might prevent cancer in people with normal colons remains an open question, she says.
But who has a normal colon? Nearly all colon cancers occur in people over age 50. Fewer than one in three over-50 adults has had a screening colonoscopy -- still the main way to know for sure whether or not one has polyps.
An estimated 30% of people over age 60 have polyps. And these are the people who might be at risk from taking folic acid supplements.
"Once you are older and have a polyp, there seems no cancer benefit -- and possible harm -- from folic acid," Ulrich says. "People who have had polyps, based on this trial, should not take folic acid."
Sandler says no vitamin or supplement has been proven to prevent cancer.
"The take-home message from our study is people should be cautious about taking supplements to prevent diseases such as cancer because those treatments might be ineffective and could be harmful," he says.
Sandler notes that everyone should get regular screening tests for colon cancer beginning at age 50. Though colonoscopy may be the surest way to know whether you are at risk, different patients may prefer different colon cancer screening tests.