Consuming megadoses of dietary antioxidants does not prevent chronic diseases, concluded the experts on the Institute of Medicine's (IOM) Food and Nutrition Board. According to a new report on Dietary Reference Intakes, high doses of the nutrients vitamin C, vitamin E, selenium and beta-carotene do not protect the body from a variety of illnesses, such as cardiovascular disease, diabetes, and various forms of cancer, nor do they prevent basic nutritional deficiencies. In fact, the opposite may be true. Extremely high doses of antioxidants may lead to health problems, including diarrhea, bleeding, and the risk of toxic reactions.
In recent years, researchers have focused on the possible role that dietary antioxidants play in promoting and maintaining health. Antioxidants help neutralize potentially damaging molecules--byproducts of the body's metabolism--that may damage critical cellular components, including genes. Antioxidants can help prevent the damage.
But after a comprehensive review of the scientific evidence, the IOM's Food and Nutrition Board, which is part of the National Academy of Sciences, concluded that although a large number of population studies reveal a link between a diet rich in foods containing antioxidants, such as fruits and vegetables, and a lower incidence of certain chronic diseases, they cannot be certain that antioxidants are the reason.
Since 1941, the Food and Nutrition Board has set Recommended Dietary Allowances (RDAs) for the types and quantities of nutrients that are needed for healthy diets. The board has updated and expanded the system over the past several years for determining these values--now called Dietary Reference Intakes, or DRIs. The 2000 report, "Dietary Reference Intakes for Vitamin C, Vitamin E, Selenium, and Carotenoids," is the latest in a series on DRIs.
"These IOM reports are a major source of scientific information for nutrition labeling," says Elizabeth Yetley, lead scientist in FDA's Center for Food Safety and Applied Nutrition. "They are an authoritative source, independent of the agency, and not influenced by regulatory applications."
To determine how much of a specific nutrient one needs on a daily basis, the board considers two types of numbers: the RDA--a daily intake goal for healthy individuals, and the "tolerable upper intake level"--the largest amount of a nutrient that healthy individuals can take each day without risking adverse health effects.
Although this is the first time the science board has raised the daily recommended levels for the nutrients vitamin C and vitamin E, scientists continue to struggle with whether beta-carotene and other carotenoids are true dietary antioxidants. Laboratory tests show only that carotenoids have antioxidant properties, but the results of human trials searching for health benefits have been inconsistent. The only clear role of carotenoids, according to the board, is in the formation of vitamin A.
Norman I. Krinsky, Professor of Biochemistry at Tufts University School of Medicine, says that most North American adults get enough vitamin C, vitamin E, and selenium from their normal diets to meet current recommendations. Those who don't, he adds, could get enough of these nutrients by simply improving their diets.