A compound that prevents or retards the oxidation of sensitive molecules found in the body or in foods. Antioxidants occur in many foods naturally as nutrients or non-nutrients, or as synthetic additives. Antioxidants typically block oxidation by preventing damage caused by free radicals, extremely reactive forms of oxygen and other molecules that lack an electron and tear electrons from molecules they meet. In the body, likely targets are DNA, proteins, and lipids (unsaturated fatty acids).

Free radicals form in the body by normal cellular processes. These include phagocytosis (engulfing viruses and bacteria) by immune cells; incomplete reduction of oxygen as mitochondria burn fuels; production of hydrogen peroxide by the breakdown of fatty acids and the generation of nitric oxide, a free radical that functions as a localized vasodilator, a defensive chemical and as neurotransmitter. Free radicals and reactive forms of oxygen occur by chemical modification of pollutants and toxic substances within the liver. Free radical damage may contribute to cancer, cardiovascular disease, and aging; consequently, antioxidants are a current focus of extensive medical research. It is intriguing that certain antioxidants are both anticancer nutrients and antiaging nutrients. Because there is such a large variety of reactive molecules and free radicals, the body requires a wide range of antioxidant defenses. A “pecking order” exists among antioxidants; some are more readily oxidized than others and will be consumed rapidly unless replenished or recycled in the body. Certain antioxidants are “preventive inhibitors,” that is, they block the initiation of free radical attack. Preventive inhibitors include defensive enzymes like catalase and glutathione peroxidase (destroy hydrogen peroxide and lipid peroxides) and superoxide dismutase (destroys superoxide), chelating agents like citric acid that lock up metal ions, proteins that bind metal ions, including albumin, transferrin, and ferritin. Other antioxidants, “chain breakers,” convert free radicals to stable (safe) products, vitamin e and vitamin c are essential chain-breaking antioxidants. It is worth remembering that under certain conditions, an antioxidant may become an oxidant. If the antioxidant becomes a free radical, then it, too, must be disarmed and regenerated.

Antioxidants as Nutrients

Vitamin A, beta-carotene, vitamin C, vitamin E, and selenium are key antioxidant nutrients.

Carotenoids, including beta-carotene, trap free radicals, while vitamin A helps guide normal tissue development. Inadequate carotenoid intake increases the risk of cancers of the lung, bladder, esophagus, stomach, colon, rectum, prostate, and skin. Studies indicate that when used alone, beta-carotene does not prevent cancer or heart disease. Indeed, there are hints that unless beta-carotene is protected by another antioxidant, such as vitamin E, it may actually increase damage. A multitude of studies indicates that the consumption of foods rich in carotenoids protects against cancer, cataracts, and cardiovascular diseases.

Vitamin C destroys water-soluble free radicals and protects against cancer. It is needed for a healthy immune system and it also speeds wound healing. Vitamin C also protects low-density lipoprotein (LDL) cholesterol from oxidation. Evidence for the role of vitamin C in reducing the risk of coronary heart disease is weak. Ongoing clinical trials may help decide whether vitamin C supplementation is beneficial for preventing heart disease. There is some evidence that high dietary vitamin C may lower the risk of several cancers, such as breast cancer and stomach cancer. Evidence does not indicate that high doses of vitamin C decrease cancer risk, however.

Vitamin E acts as a fat-soluble, free radical trap that seems to protect the brain from free radical damage and to partially reverse age-related decline of the immune system in experimental animals. In addition, vitamin E promotes the normal function of smooth muscle cells and reduces platelet adhesion to arterial cells, factors which could reduce the risk of atherosclerosis. Many population studies have found a reduced risk of coronary heart disease with increased intake of vitamin E. However, most clinical studies of vitamin E supplementation for several years found no benefit in reducing heart disease risk.

Selenium works together with vitamin E by helping an enzyme system (glutathione peroxidase) block free radical attack and to disarm reactive lipids. Selenium is also required for a healthy immune system. Selenium deficiency increases the risk of cancer of the esophagus, stomach, and rectum.

Antioxidants as Nonnutrients in Food

In addition to vitamins, trace minerals, fiber, and carotenoids, vegetables and fruits provide many other ingredients important for long-term health. Vegetables and fruits contain orange-red and yellow pigments called carotenoids. They include carotenes such as beta-carotene and lycopene (from tomatoes) and xanthophylls, oxygen-containing derivatives such as zeazanthin and lutein. Xanthophylls occur at high levels in dark green leafy vegetables. Though relatively few carotenoids serve as sources of vitamin A, they help protect the body as versatile antioxidants, and they enhance the immune system, complementing the actions of beta-carotene. Fruits, vegetables, seasoning, spices, and herbs (tea) possess a wide range of complex-molecules called polyphenols (flavonoids) and phenolic acids that are complex ring structures. Flavonoids include isoflavones (soybean), flavones (such as quercetin from tea, berries, fruits) and flavonones (such as naringenin and hesperidin from citrus), flavanonols (such as catechins, condensed and hydrolyzable tannins), anthocyanins (purple, red, and blue pigments of fruits and berries), coumarins (from citrus), ellagic acid (from grapes), and others. In general, flavonoids possess multiple properties; thus they can quench free radicals, inhibit inflammation, strengthen capillary walls, and reduce oxidative damage to serum cholesterol. The optimal intake of flavonoids and carotenoids is not known and the long-term effects of supplementation with large amounts of phytochemicals has not been studied. It should be pointed out that beta-carotene, vitamin C, and even vitamin E under the appropriate conditions can become oxidants (prooxidants). Certain flavonoids also exhibit prooxidant properties. Chelated (complexed) iron in the presence of vitamin C can generate free radicals spontaneously in the test tube and this could be a potential problem in the body with iron overload diseases. Finally, certain flavonoids can specifically block the thyroid hormone-generating enzyme in thyroid cells. As with many dietary constituents, a little may be beneficial, while a lot could be harmful.

Foods rich in vitamin A and beta-carotene and related carotenoids include orange-colored vegetables like carrots and squash plus dark green leafy vegetables like chard, kale, and spinach. Fresh fruit, frozen juice concentrate, and vegetables like green pepper and broccoli supply vitamin C. Vegetable oil, wheat germ, and nuts provide vitamin E. Selenium occurs in whole grains, seafood, cabbage, onions, and garlic. Fruits and vegetables also provide flavonoids.

Antioxidants Made by the Body

Glutathione is a sulfur-containing antioxidant present in very large amounts in the cytoplasm. Besides helping to keep proteins reduced, it assists amino acid transport, helps regulate the internal oxidation state of the cell, maintains vitamin E in a reduced state, and detoxifies potentially harmful substances.

Coenzyme Q assists mitochondria to burn fat and carbohydrate for energy and it functions as a lipid soluble membrane antioxidant together with vitamin E, which it protects. Coenzyme Q production declines with age and the heart may become deficient in this nutrient.

Uric acid is found in the blood. It is a nitrogen-containing waste product from the breakdown of DNA and RNA.

Citric acid, succinic acid, and other complex organic acids generated by metabolism can bind iron and copper, preventing them from catalyzing of free radical-generating reactions.

Melatonin, a hormone produced by the pineal gland, possesses strong antioxidant properties.

Bilirubin, a breakdown product of hemoglobin, acts as an antioxidant in blood.

Antioxidants as Food Additives

Antioxidants are extensively utilized to prevent or retard deterioration that produces off-flavors or color changes in foods, making them less appetizing or less nutritious. Oxidation can also be promoted by enzymes in foods when exposed to air. This explains why apples, bananas, pears, peaches, and potatoes darken after being sliced. The food industry often employs synthetic antioxidants, particularly butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), ethylene diaminetetracetic acid (EDTA), and propyl gallate, as well as vitamin C, as preservatives to extend the shelf life of processed foods by preventing free radical damage.

Spontaneous oxidation of fats and oils in the presence of oxygen, sunlight, and metal ions causes rancidity unless blocked by antioxidants. (BHA) and BHT are used to prevent rancidity in fats and oils, particularly in baked goods like crackers and cookies. Their safety has been questioned. EDTA is a common additive in salad dressings, margarine, mayonnaise, sandwich spreads, pureed fruits, and vegetables, as well as cured shellfish, beer, and soft drinks. EDTA is judged to be a safe food additive. Propyl gallate retards spoilage of fats and oils and is often used with BHA and BHT to maximize their antioxidant effects. Several studies with experimental animals suggest that propyl gallate may cause tumors. A close relative of vitamin C, erythroboric acid, is a common antioxidant used in the preservation of processed meats such as bologna, frankfurters, and bacon, sulfites are used as antioxidants to prevent discoloration of fruit and vegetables. Spices and herbs, including thyme, rosemary, and sage, are sometimes used as food additives to retard spoilage. (See also atherosclerosis.)

Fairfield, K. M., and R. H. Fletcher. “Vitamins for Chronic Disease Prevention in Adults.” Journal of the American Medical Association 23, no. 287 (June 19, 2002): 3,116-3,126.

Singh, Ram B. et al. “Effect of Antioxidant Rich Foods on Plasma Ascorbic Acid, Cardiac Enzyme and Lipid Peroxide Levels in Patients Hospitalized with Acute Myocardial Infarction,” Journal of the American Dietetic Association 95, no. 7 (July 1995): 775-780.

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