Antioxidants are substances that convert free radicals to harmless waste products that can be eliminated before they cause damage. The major antioxidant vitamins include beta-carotene (forms vitamin A), vitamin C, and vitamin E. The mineral, selenium, is also a major antioxidant. There are many other antioxidants not listed here.
What are free radicals? Free radicals are oxygen and nitrogen by-products resulting from the body’s normal operation and from environmental factors such as cigarette smoke, burns, and ultraviolet light. Free radicals can react so easily with other molecules that they frequently interfere with normal metabolism and can cause cell injury. This is called oxidative stress. Oxidative stress can lead to illness including cancer, cardiovascular disease, cataracts, and arthritis.
A report was released April 2000 by the Institute of Medicine on Dietary Reference Intakes for Vitamin C, Vitamin E, Selenium, and Carotenoids (Norman I. Krinsky, PhD., Chair, Panel on Dietary Antioxidants and Related Compounds). Dr. Krinsky stated "the question of whether dietary antioxidants can prevent chronic disease still remains unanswerable." Scientific studies have shown that populations eating a diet rich in foods containing antioxidants have a lower incidence of certain chronic diseases. But it is not known whether the antioxidants or other factors are the reason for the lower incidence. On the other hand, it is known that taking large quantities of these nutrients can be dangerous. The following are the revised recommendations for adult daily consumption by The Panel.
Vitamin C

• Recommend Daily Allowance:
• Women: 75 milligrams
• Men: 90 milligrams
• Smokers: add 35 milligrams
• Food Sources: broccoli, citrus fruits, leafy green vegetables, peppers, potatoes, strawberries
• Maximum Daily Intake: 2000 milligrams
• Effects of Excess Intake: diarrhea in some – excess likely excreted in urine unused

Vitamin E (alpha-tocopherol)

• Recommended Daily Allowance: Adults: 15 milligrams (22 International Units natural-source vitamin E or 33 I.U.s of the synthetic form)
• Food Sources: nuts, seeds, leafy green vegetables, some vegetable oils
• Maximum Daily Intake:  1000 milligrams (1500 IU d-alpha tocopherol or 1100 IU dl-alpha tocopherol)
• Effects of Excess Intake:  increases risk of bleeding especially when taken with prescription blood thinners, aspirin, gingko baloba, and other products that affect blood clotting

Selenium

• Recommended Daily Allowance:  Adults: 55 micrograms
• Food Sources: seafood, meat, and grains
• Maximum Daily Intake: 400 micrograms
• Effects of Excess Intake: hair loss and nail damage

Beta-carotene (and other carotenoids) – The antioxidant activity in humans is uncertain. Carotenoids are precursors of vitamin A. The data on the effects of excess intake of carotenoids is contradictory. Carotenoids and vitamin A will be addressed in a future report by The Panel. Excess intake of vitamin A can cause birth defects.
More detail on antioxidants and other nutritional issues can be found at The National Academies of Science at www.iom.edu.The current recommendation is that most nutrients be consumed through a healthy balanced diet. Excess consumption of supplements may not only be ineffective but also harmful.