Beta-carotene, selenium and folic acid -- taken up to three times their recommended daily allowance, these supplements are probably harmless. But taken at much higher levels as some supplement manufacturers suggest, these three supplements have now been shown to increase the risk of developing a host of cancers.

"It's not that these nutrients are toxic -- they're essential and we need them, but we need them in a certain balance," says Tim Byers, MD, MPH, professor of epidemiology at the Colorado School of Public Health and associate director for prevention and control at the University of Colorado Cancer Center.

Byers is senior author of a commentary recently published in the Journal of the National Cancer Institute that discusses the clinical and policy implications of the increased cancer risk from high dose dietary supplements.

"We have a window into less than half of the biology of what these nutrients are doing," Byers says. "We say generalized things about them, calling them an antioxidant or an essential mineral, but true biology turns out to be more complex than that. The effects of these supplements are certainly not limited to the label we give them. And, as we've seen, sometimes the unintended effects include increased cancer risk."

Currently the FDA regulates dietary supplements as food, but, as Byers and colleagues suggest, supplements, especially at high doses, are more accurately described as inhabiting a mid-ground between food and drugs. Like drugs, supplement ingredients are biologically active -- sometimes for better and sometimes for worse

"We need to do a better job as a society in ensuring that the messages people get about value versus risk is accurate for nutritional supplements," Byers says. "My conclusion is that taking high doses of any particular nutrient is more likely to be a bad thing than a good thing."

This work was supported by the National Institutes of Health and the National Cancer Institute.