Supplement Update

Its claim to fame: Many studies have found an association between vitamin C and a reduced risk for cancer and heart disease, although few established a true cause-and-effect relationship.

Robert A. Jacob, Ph.D., a member of the institute's panel and research chemist with the USDA Western Human Nutrition Research Center in Davis, California, says most of the studies on megadoses of vitamin C had mixed results or were neutral, including those that linked vitamin C to staving off the common cold or preventing heart disease and cancer.

"Studies didn't show harmful outcomes. But if they didn't show consistently positive results, you can't use that as a basis for an RDA," Jacob says.

Most of the research on vitamin C has never shown direct cause and effect. Studies have found only an "association" with health benefits. For example, research may show that people with a high vitamin C intake have lower blood pressure. However, there is no scientific proof that the vitamin is responsible because many other components of a healthy diet may lower blood pressure. "There is an association there, but it doesn't prove that vitamin C lowers blood pressure," Jacob says.

Old RDA: 60 milligrams for all adults.

New RDA: 75 mg per day for women, 90 mg per day for men. People who smoke should increase the RDA of vitamin C by 35 milligrams.

Recommended upper limit: 2,000 mg per day, for adults over age 19. The NIH report notes that the risks associated with exceeding this limit are very low, although exceeding the upper intake limit may cause gastrointestinal distress.

Best food sources: Citrus fruits and juices, kiwifruit, broccoli, strawberries, and red or green sweet peppers. It's easy to get enough vitamin C in your diet. One 6-ounce glass of orange juice, for example, has about 78 milligrams. Vitamin C-rich fruits and vegetables typically average about 40 milligrams per serving, Jacob says.