(HealthDay News) -- Antioxidants are substances in food that may delay or prevent some types of cell damage.

Fruit and vegetables are prime sources of antioxidants, which include vitamin C, selenium and beta-carotene, the U.S. National Institutes of Health (NIH) says.

While antioxidants haven't been shown definitively to prevent disease, studies have shown that people who eat more fruit and veggies may be at lower risk of developing conditions such as cardiovascular disease and cancer, the NIH says.

Antioxidant supplements, on the other hand, actually may be harmful -- especially if they contain high doses, the agency says. Such supplements may conflict with certain medications and in some cases have been linked to conditions such as hemmorrhagic stroke and prostate cancer.

So eat lots of fruit and veggies, but don't take axtioxidant supplements as an alternative to a healthy diet, the agency advises.