Washington: Supplements such as beta-carotene, selenium and folic acid, quaffed at much higher levels than their recommended daily doses, are most likely to elevate the risk of developing a host of cancers, a study says.

"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, study co-author and professor of epidemiology at the University of Colorado School of Public Health, U.S, the Journal of the National Cancer Institute reports.

"We have a window into less than half of the biology of what these nutrients are doing. We say generalised things about them, calling them an antioxidant or an essential mineral, but true biology turns out to be more complex than that," says Byers, according to a Colorado statement.

"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," adds Byers.

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.

"My conclusion is that taking high doses of any particular nutrient is more likely to be a bad thing than a good thing," Byers says.