WASHINGTON — While Americans should have more vitamins C and E in their diets than now recommended, huge doses of these vitamins and other antioxidants have no proven benefit and may even be harmful, government researchers said Monday.

Antioxidants scavenge the body for roaming oxygen molecules known as “free radicals” suspected of triggering cancer and other disease. Many people routinely take high doses of vitamin C and other antioxidants in the belief that they will prevent or cure illnesses.

But not enough evidence exists to support claims that massive doses of antioxidants can improve health, said researchers from the Institute of Medicine. The institute, a private organization that advises the federal government, is reviewing the nation’s Recommended Daily Allowances, or RDAs, for nutrients.

For the first time the institute has set upper limits on the daily consumption of vitamins C and E to reduce the risk of harmful side effects.

Adults should keep their daily vitamin C intake from both food and supplements below 2,000 milligrams because anything higher may cause diarrhea.

The upper level for vitamin E, based only on vitamin supplements, is 1,000 milligrams.