March 10 (HealthDay News) -- A 10-year study has found that men who took folic acid supplements faced more than twice the risk of prostate cancer as those who didn't take the supplements.
But the incidence of prostate cancer in the study was slightly lower in men who simply got adequate amounts of folate in their diet, according to a report in the March 10 online issue of the Journal of the National Cancer Institute.
"What we think is that perhaps too much folate is not necessarily beneficial, whereas adequate levels may be," said study leader Jan Figueiredo, an assistant professor of preventive medicine at the University of Southern California.
Folic acid is a synthetic version of folate, a basic nutrient found in green, leafy vegetables. In the study, which followed 643 men for slightly more than a decade, the estimated prostate cancer risk was 9.7 percent for the men who took the daily 1-milligram supplements, and 3.3 percent for men who took a placebo.
"Folate plays an important role in cell growth and division, and cancer cells often uprate their folate receptors," Figueiredo noted. "Folic acid, the synthetic version, has more bioavailability, meaning that the effective dose in the cell is higher than what you get from natural sources."
Dietary sources of folic acid in the United States now include cereals and other grain products. The U.S. Food and Drug Administration has required folic acid enrichment of those foods since 1996, in part to reduce the incidence of birth defects affecting development of the central nervous system.
"Since we fortified, the amount of folate we consume from fortified foods is probably more than sufficient," Figueiredo said.
The newly reported results resemble those of a study done several years ago by Victoria Stevens, strategic director of laboratory services at the American Cancer Society, who specializes in research on folate metabolism.
That study of folate intake and prostate cancer "found that it really didn't have much effect," Stevens said. "Our study actually suggested that folate might be protective for men with advanced prostate cancer, a group that wasn't included in this study."
Overall, "it's a pretty complicated picture," Stevens said. "Previous epidemiological evidence suggests that not having enough folate can be bad, but having an excess might not be good. You need to have adequate folate nutrition. But it doesn't get better if you have more, and it may get worse."
The study is the latest to throw cold water on the hope that supplements can reduce the risk of cancer. Two studies reported late last year that supplements containing selenium, vitamin E and vitamin C had no effect on the incidence of prostate cancer.
One of those studies included more than 35,000 men aged 50 and over who were followed for more than five years, and the other included almost 15,000 male physicians aged 50 and over who were followed for an average of eight years.
"It is safe to conclude that cancer prevention is not going to be as simple as recommending high-dose micronutrient supplements for middle-aged and older adults," said an editorial accompanying the latest report.
Detailed studies to understand how diet and supplements affect biological mechanisms of cancer in humans are needed, as well as large-scale epidemiological studies looking for ways in which diet can reduce risk, according to the editorial by Alan Kristal of the Fred Hutchinson Cancer Research Center in Seattle and Dr. Scott Lippman of the M.D. Anderson Cancer Center in Houston.
"There is no evidence that supplements of any type reduce cancer risk, and increasing evidence that they may increase the risk for some cancers in some people," said Kristal, who is a professor of epidemiology and associate director of the cancer prevention program at Fred Hutchinson. "The only exception is calcium for recurrence of colorectal polyps, where there is solid evidence that calcium can reduce risk."
SOURCES: Jane Figueiredo, Ph.D., assistant professor, preventive medicine, University of Southern California, Los Angeles; Victoria Stevens, Ph.D., strategic director, laboratory services, American Cancer Society, Atlanta; Alan Kristal, Dr.PH, professor, epidemiology, and associate director, cancer prevention program, Fred Hutchinson Cancer Research Center, Seattle; March 10, 2009, Journal of the National Cancer Institute, online