Since the oxidative stress of free radicals can interact and harm cells, it's thought that antioxidants in food can react with free radicals to limit the damage.
While it's difficult to overdose on antioxidants in fruits and vegetables, it's easier when taking a pill with high concentrations, said Prof. Jim Kehrer of the pharmacy department at the University of Alberta in Edmonton.
Advertisers have put forth the idea that a little is good, more is better and a lot is great but that isn't really correct, said Kehrer, who has been researching the effects of free radicals since the 1970s.
"You get to a point, and sometimes early, that the high doses become hazardous," Kehrer said in an interview with CBC News.
"The good thing about antioxidants is that you have to take absolutely massive doses to get overt toxicity. But that doesn't mean that there can't be subtle effects building up over many periods of years if you take more than you should be getting."
Taking antioxidants in excess can overwhelm free radical and oxidative stress that cells use to talk to one another and function properly, which Kehrer said is probably part of the reason why taking too many isn't as good as taking the right amount.
Eating a mixture of fruits and vegetables has a safety factor built in to prevent exposure to massive doses while getting beta carotene from carrots, lycopene from tomatoes and polyphenols from grapes, for example, said Venket Rao, a professor emeritus in the nutritional sciences department at the University of Toronto.
Studies in Canada, the U.S., Australia, Italy, Finland and elsewhere have shown that giving megadoses of antioxidants like vitamin E to people with lung cancer and cardiovascular diseases actually promotes oxidation instead of countering it, Rao said.
"That's the paradox," Rao said. "We have evidence that antioxidants are good for you, but we also have well-controlled clinical studies showing that at mega doses they could be harmful to you. So what do you believe in?"
For example, a 2009 review of randomized trials on antioxidants published by the Cochrane Library concluded that "Antioxidant supplements need to be considered medicinal products and should undergo sufﬁcient evaluation before marketing."
Kehrer suggested that people need a better grasp of good nutritional intake and not try to overdo selected chemicals.
At the produce section of a grocery store in Toronto, shopper Bora Skenderi called the marketing a fad.
"You hear about all the super foods," Skenderi said. "If you lead a healthy lifestyle, you get all the antioxidants."