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June 5, 2007 -- Popular folic acid supplements fail to protect against colon cancer, but they may increase an adult's risk of other cancers.
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.
And that's not all folic acid is supposed to do. There's evidence -- but no proof -- that the supplement also may cut a person's risk of stroke and heart disease.
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.