Similar to a fire that requires oxygen in order to burn, every cell in your body needs a steady supply of oxygen to convert digested food into energy.

But burning oxygen comes with a price; it also releases free radicals, unstable molecules that damage healthy cells as they careen through the body. Free radicals contain at least one unpaired or negatively charged electron, which makes them highly reactive. As soon as they are produced, free radicals start searching for positively charged molecules with which they can react, or oxidize.

Although all healthy cells require small amounts of free radicals, excessive bombardment by these molecules damages cellular DNA and other genetic material. However, human cells (which are exposed to dozens of free radical assaults a day) have protective enzymes that repair 99 percent of oxidative damage. But oxygen metabolism is not the only source of oxidative damage; it can come from X-rays, the sun’s ultraviolet rays, radon, tobacco smoke, automobile exhaust, and other environmental pollutants. Over time, the cumulative effect can cause irreversible cellular changes, or mutations, that can result in cancer and other diseases.

The body’s immune system seeks out and destroys these mutated cells, in much the same way as it eliminates invading bacteria and other foreign organism; this mechanism lessens with age, however. The body then becomes more vulnerable to free-radical damage, and the incident of degenerative disorders increases. Consequences range from harmless pigmented skin patches, or “liver spots,” to more serious disorders, such as cataracts, cancer, and a host of degenerative diseases.

Antioxidants are positively charged molecules that combine with the negatively charged free-radicals, making them harmless. The major antioxidants are vitamin C and E; beta-carotene, which the body converts to vitamin A; and selenium. Bioflavonoid, substances found in citrus fruits, grapes, and other fresh fruits and vegetables, have antioxidant properties, as do some phytochemicals, protective substances in many plants.

Numerous studies show a reduced incident of cancer and heart attacks in people who eat plenty of fresh fruits, vegetables, and whole-grain products—the best source of antioxidants. Recent research indicates that antioxidants may prevent heart disease by interfering with the oxidation of LDLs (low-density lipoproteins), the harmful cholesterol. Unoxidzed LDLs are relatively benign, but after oxidation they promote development of artery clogging plague. The antioxidants in vitamin E interact with cholesterol and other lipids, reducing LDL oxidation. Oxidation also facilitates the uptake of LDLs into artery walls; this can be blocked by beta carotene.

Less is known about how antioxidants hinder cancer; researchers theorize that prevention of DNA damage is a factor. Recent studies indicate that vitamin C may protect against skin cancer and melanoma.

Antioxidant Supplements

Doctors discourage taking high-dose antioxidant supplements. When taken in large amounts, some nutrients that are normally antioxidants may have the opposite effect and actually increase oxidation. For example, when taken by someone who has large iron reserves, high doses of vitamin C become pro-oxidant. Similarly, recent studies failed to find any benefit from high-dose beta-carotene supplements.

Supplements exceeding the RDAs should be taken only under medical supervision. High doses of vitamin E, for example, can interfere with blood clotting and increase the risk of a bleeding emergency. Even so, supplements may be prescribed for some heart patients, because it is impossible to get protective amounts (200mg to 400mg a day) from diet alone.

Protective Plant Chemicals

Because plants are also susceptible to cancer and viruses, they have developed their own protective substances, called phyrtochemicals. Mounting research shows that many phytochemicals also protect human against cancer and other diseases.

Phyto-Chemical Functions Sources
Bioflavonoids Antioxidant; inhibits cancer promoting hormones Most fresh fruits and vegetable
Curcumin Protects against tobacco-induced carcinogens Turmeric, cumin
Isoflanoes Inhibit estrogen uptake; destroy cancer enzymes Beans, peanuts and other legumes, peas
Lignans Inhibit estrogen and block prostaglandin Fatty fish, flaxseeds, walnuts
Omega-3 fatty acids Inhibit estrogen; reduce inflammation Canola oil, flaxseed, walnut, fatty fish
Protease Destroys enzyme inhibitors that promote cancer spread Soybeans
Indoles Inhibit estrogen, which stimulates some cancers; induce protective   enzymes Broccoli, cabbage, cauliflower, mustard greens
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