The FDA adopted the fortification policy to prevent birth defects, and some health advocates want even higher levels of folic acid in foods. But fortification may have unintended effects on people at risk of cancer, said Dr. Joel Mason, an expert on folate and cancer prevention at Tufts University in Boston who was not involved in the new study.

"Right now it would not be appropriate to blindly go forth and further increase the levels of folic acid without better understanding the potential risks," Mason said. "And whether we continue folic acid fortification should be an open debate over the next few years."

Mason said he believes folic acid someday may earn a role in cancer prevention, perhaps at smaller doses than given in the study.

Cancer patients should discuss taking vitamins with their doctors, and anyone over 50 who takes vitamins should have a colorectal screening test, said Cornelia Ulrich of Seattle's Fred Hutchinson Cancer Research Center, who co-wrote an accompanying editorial.

"Older individuals often have abnormalities in the colon and folic acid may promote their growth," Ulrich said.

Researchers did not see any real difference in rates of colon cancer, just in the precancerous colon growths.

Some of the participants also took aspirin as part of the study. The aspirin seemed to protect the colon against the harms of folic acid.

The study was not designed to look at prostate cancer, but more men who took folic acid developed prostate cancer than did the other men (7.3 percent vs. 2.8 percent).

Grants from the National Institutes of Health funded the study. Some of the researchers reported financial ties to drug companies, such as consulting work and research support.

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