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April 20, 1999
Web posted at: 4:00 p.m. EDT (2000 GMT)

(CNN) -- Researchers at the National Institutes of Health are recommending doubling the adult daily allowance of vitamin C.

Close to 40 percent of Americans consume less than the current recommended daily allowance (RDA) of 60 milligrams of vitamin C. In this week's Journal of the American Medical Association (JAMA), the NIH said the RDA should be boosted to between 100 and 200 milligrams.

The recommendation was last revised in 1989, and according to NIH's Dr. Mark Levine, "There has been a wealth of new data" since then.

Studies show people who eat at least five servings a day of fruits and vegetables -- foods high in vitamin C -- are at lower risk for certain cancers.

"Cancer is a decades-long process, and nutrition is one way of being able to thwart each step of the cancer process," said Dr. Mitch Gaynor of Strang Cancer Prevention Center.

NIH researchers said vitamin C should come from food rather than supplements.

A grapefruit and a cup of strawberries contain almost 200 milligrams of the antioxidant. It also can be found in papayas, kiwis, oranges and cantaloupe.

Vegetables rich in vitamin C are peppers, broccoli and kale, but they should be microwaved or steamed -- boiling can destroy up to 80 percent of the vitamin.

The NIH researchers said only those who cannot eat produce should consider taking a 200 milligram supplement of vitamin C. Taking 1000 or more milligrams can cause nausea and kidney stones and will not prevent colds, they said.

"We believe the evidence now, all of it, suggests the only people who benefit from vitamin C when they have colds are those who were deficient in the first place," Levine said.

With this new research in hand, a government recommendation for increased vitamin C intake is expected within the year.