March 10, 2009 -- There is mounting evidence that folic acid, taken in high doses, does not protect against cancer and may even promote certain cancers.
Nearly two years ago, a major study found that high doses of folic acid in supplement form failed to protect against colon cancer.
Now, another analysis of the same study's findings suggests a link between folic acid supplements and an increased risk for prostate cancer.
There were too few prostate cancers among the study participants to prove that folic acid promotes prostate cancer, Jane C. Figueiredo, PhD, of the University of Southern California, Los Angeles tells WebMD.
“What we can say is that a lot of folate is unlikely to be beneficial with regard to prostate cancer, and it just might be harmful,” she says.
Folic Acid and Prostate Cancer
The current study included 643 men originally recruited for a much larger study designed to determine if taking aspirin and a folic acid supplement could reduce the incidence of colon polyps.
The men were randomly assigned to placebo or supplements with daily low-dose aspirin and 1 milligram of folic acid daily -- two and a half times the recommended daily dose of the vitamin for men and for women who are not pregnant or nursing.
The analysis joins a growing number of studies suggesting that nutritional supplements are of no benefit for preventing cancer.
In an accompanying editorial, two cancer and nutrition experts concluded that “the prospects for cancer prevention through micronutrient supplementation have never looked worse.”
Large trials have shown no protective benefit in people taking multivitamins, selenium, beta-carotene, and vitamins E, C, D, B6, and B12.
Beta-carotene, taken in high doses, has even been shown to promote lung cancers in heavy smokers.
“The primary lesson from our experience in the nutritional prevention of cancer is that it is not simple,” write Alan R. Kristal, MD, of Seattle’s Fred Hutchinson Cancer Research Center and Scott Lippman, MD, of the University of Texas M.D. Anderson Cancer Center.
The study and editorial appear in the March 18 issue of the Journal of the National Cancer Institute.
Popping Pills Doesn’t Work
Kristal and Lippman write that it made sense to study single micronutrients early on because numerous studies had found that eating a healthy diet with plenty of micronutrient-rich fruits and vegetables can help protect against certain cancers.
But it is increasingly clear that if the foods we eat influence our cancer risk, the relationship is too complex to break down to single nutrients.
The newly published analysis is not the first to suggest that too much of a good thing -- in this case folic acid -- may be bad.
“The notion that some is good and therefore more is better has been proven wrong; it is more likely that for any given micronutrient, there is an optimal range of intake,” Kristal and Lippman write.
American Cancer Society epidemiologist Victoria Stevens, PhD, agrees.
“Instead of taking one multivitamin, some people will take two or three thinking that it will be two or three times better for them,” she says. “But studies like this one suggest that this approach does no good and may even be harmful.”
It is clear that taking a folic acid supplement is a good idea for women of childbearing years and those who are pregnant or nursing.
But everyone else can probably get enough folic acid without taking a supplement if they eat a balanced diet with plenty of fruits and vegetables, Steven says.
Cereals and breads are now fortified with folic acid, and folic acid is also found in green, leafy vegetables like spinach and lettuce, and in beans, peas, squash, and citrus fruits.