High doses of folic acid do not prevent precancerous colon polyps in people prone to them and may actually increase the risk of developing the growths, a new study finds.
It's the latest evidence that taking too many vitamins may be harmful. Last month, a study linked heavy vitamin use to fatal prostate cancer, and other research has shown beta-carotene pills can heighten smokers' risk of lung cancer.
The results surprised scientists. Previous studies showed diets low in folic acid led to a higher risk of colon cancer.
Now researchers speculate that some folic acid helps — as long as the colon is free of microscopic cancer cells. But once cancer starts, folic acid may feed its growth.
Some scientists who reviewed the new findings said folic acid fortification, now required in some U.S. foods, should not be increased and that other nations considering fortification should be cautious.
The new findings, appearing in Wednesday's Journal of the American Medical Association, are based on data from 987 adults with a history of precancerous colon polyps. Those who took folic acid developed more growths, or adenomas, several years later than the people who took dummy pills.
"You really should not take folic acid to prevent colorectal adenomas. It's ineffective for that purpose," said study co-author Bernard Cole of Dartmouth-Hitchcock Medical Center.
Folic acid is an artificial version of folate, a B vitamin found in leafy vegetables, citrus fruit and beans. It prevents birth defects and is needed for the production of red blood cells.
In the study, participants randomly were assigned to take either folic acid or a dummy pill. Researchers followed them for about six years.
Participants got screening colonoscopies a few years into the study and 44.1 percent of the folic-acid takers had precancerous polyps. That compared to 42.4 percent of the dummy-pill group.
The difference was not statistically significant, but the results of a second round of colonoscopies a few years later were more troubling. Among the folic-acid takers, 11.6 percent had advanced adenomas while 6.9 percent of the dummy-pill group did. And folic acid more than doubled the risk of having three or more precancerous polyps.
For those who got the real vitamin, the daily dose was 1 milligram, more than double the recommended daily allowance for folic acid. All participants consumed even more folic acid than the researchers had in mind because the Food and Drug Administration began requiring enriched grains to be fortified with folic acid in 1998, several years after the study began.
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'sFred 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.