Antioxidants' role in chronic disease prevention

INSUFFICIENT EVIDENCE exists to support claims that taking megadoses of dietary antioxidants, such as selenium and vitamins C and E, or carotenoids, including beta-carotene, can prevent chronic diseases, says the latest report on Dietary Reference Intakes (DRIs) from the Institute of Medicine of the National Academies.

In fact, extremely large doses may lead to health problems rather than confer benefits. But the report does call for increases in daily intakes of vitamins C and E to exploit their role in maintaining good health, and recommends an even larger amount of vitamin C for smokers.

For the first time, the report also sets a ceiling on daily consumption of selenium and vitamins C and E to reduce the risk of adverse side effects from overuse.

Dietary antioxidants are nutrients that help protect cells from a normal - but damaging - physiological process known as ``oxidative stress.'' Such nutrients are a part of the natural makeup of many types of food, particularly fruits and vegetables. They also have been added to some foods and are available in the form of dietary supplements.

For years, researchers have sought to understand the role of these antioxidants in reducing the risk of chronic diseases such as cancer, cardiovascular disease, eye diseases, and neurodegenerative diseases, which include Alzheimer's and Parkinson's. But many unanswered questions remain.

``A direct connection between the intake of antioxidants and the prevention of chronic disease has yet to be adequately established,'' said Norman I Krinsky, chair of the study's Panel on Dietary Antioxidants and Related Compounds, and a professor of biochemistry, Tufts University School of Medicine, Boston. ``We do know, however that dietary antioxidants can in some cases prevent or counteract cell damage that stems from exposure to oxidants, which are agents that affect a cell's molecular composition. But much more research is needed to determine whether dietary antioxidants can actually stave off chronic disease.''

The report - the third in a series on dietary recommendations for healthy Americans and Canadians - expands on the Recommended Dietary Allowances (RDAs) that the National Academy of Sciences has set periodically since 1941, and on Canada's Recommended Nutrient Intakes. DRIs contain four categories of reference intakes - including RDAs, which are a set of values intended to help people maintain their health; and another set of values to help them avoid taking too much of a nutrient.Scientists from the United States and Canada collaborated on the studies. Additional reports will be issued on DRIs for trace elements and other vitamins; electrolytes and water; protein, carbohydrates, fibre, and fats; and other food components.

Setting new dietary recommendations

The report examines available scientific data on both beneficial and harmful health effects of four nutrients typically found in food: vitamins C and E; selenium; and carotenoids, some of which are sources of vitamin A for human beings. Whether carotenoids, including beta-carotene, function as antioxidants when consumed is still unknown.

Although numerous epidemiological studies have suggested that foods rich in dietary antioxidants have a positive effect on chronic diseases, insufficient evidence exists to conclude that such nutrients, even in very high doses, will reduce the risk of diseases such as cancer; cardiovascular disease; cataracts; age- related macular degeneration, a common form of blindness in elderly people; diabetes mellitus; and neurodegenerative diseases.

In addition to setting daily intakes for vitamins C and E and selenium, the report establishes ``tolerable upper intake levels'' for each of these three nutrients.

The report underscores the finding that upper levels, or ceilings, should not be considered the recommended amount. Instead, they represent the maximum intake of a nutrient that is likely to pose no risk of adverse health effects in almost all individuals in the general population.

Highlights of report

Vitamin C - Recommended intake levels of this nutrient were increased to achieve maximum saturation in the body. Women should consume 75 milligrams per day, and men should consume 90 milligrams daily. Because smokers are more likely to suffer from biological processes that damage cells and deplete vitamin C, they need an additional 35 milligrams per day. Food sources of the nutrient include citrus fruit, potatoes, strawberries, broccoli, and leafy green vegetables.

The report sets the upper level for vitamin C, from both food and supplements, at 2,000 milligrams per day for adults. Intakes above this amount may cause diarrhoea.

Vitamin E - Recommended intake levels also were increased for this nutrient; both women and men should consume 15 milligrams - or 22 International Units (I.U.) - of ``alpha-tocopherol'' from food sources each day. Alpha-tocopherol is only type of vitamin E that human blood can transfer to cells when needed. Food sources include nuts, seeds, liver, and leafy green vegetables.

The upper level, based only on intake from vitamin supplements, is 1,000 milligrams of alpha-tocopherol per day for adults. This amount is equivalent to roughly 1,500 I.U. of ``d-alpha- tocopherol,'' sometimes labelled as ``natural source'' vitamin E, or 1,100 I.U. of ``dl-alpha- tocopherol,'' a synthetic version of vitamin E. People who consume more than this amount place themselves at greater risk of haemorrhagic damage because the nutrient can act as an anticoagulant.

Selenium - The recommended intake level for this nutrient was set at the amount associated with the highest activity of enzymes that guard against oxidants in the body. Women and men should take 55 micrograms per day, the report says.

Food sources include seafood, liver, meat, and grains. The report set the upper intake level for selenium at 400 micrograms per day. The level is based on nutrients from all sources. More than this amount could cause selenosis, a toxic reaction marked by hair loss and nail sloughing.

The report noted that most American and Canadian adults already get sufficient quantities of these three nutrients from their food.

Beta-carotene and other Carotenoids - In laboratory tests, these nutrients have been shown to act as antioxidants, but the results have not been consistently duplicated in humans. In addition, data on the adverse effects of carotenoid overconsumption are contradictory.

For these reasons, the report does not recommend a daily intake level or an upper intake level for consumption of carotenoids. People should use caution before taking them in high doses; the report recommends beta-carotene supplementation only for the prevention and control of vitamin A deficiency.

More research needed

As a general rule, healthy people should not routinely exceed upper intake levels, the report advises. How nutrient consumption above these levels may affect human health should be studied further in well-controlled clinical trials.

Likewise, much more research is needed to investigate the role of dietary antioxidants and carotenoids in the prevention of chronic diseases; to explore the nutrient needs of specific groups of people, including children and the elderly; and to look into how selenium, vitamins C and E, and beta-carotene interact with each other and with other food components.

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