By Gerald F. Combs, Jr.

Not many people think of oxygen as being toxic, but it's true. Of course, we need oxygen to stay alive--that's why we breathe. And we need more of it when we are doing physical work or strenuous exercise. That's why we breathe harder and faster--to get more oxygen to enable our cells to produce more energy. But it turns out that some of the oxygen that we inhale is--through our own metabolism--turned into highly reactive forms that can interfere with essential functions within cells.

Research has shown that these reactive forms of oxygen are also necessary for certain essential functions. We use reactive oxygen to kill bacteria and to signal functional changes within blood vessels. But when produced in excess, reactive oxygen can also damage proteins, fats and DNA inside of cells. In this way, adverse effects of oxygen have been linked with cardiovascular disease and cancer, and also with aging, arthritis, cataracts, diabetes and impaired immune function. Because these reactions involve oxygen, they are called "oxidations".

On the other hand, there are also ways to protect cells against such oxidative damage by blocking these oxidation reactions. Factors that do this are called "anti-oxidants". Because blocking oxidative damage may lead to reduced risks of chronic disease, there is enormous interest in the health roles of antioxidants.

The most powerful antioxidants in cells are vitamins E and C, but other plant components also function as antioxidants. Because we obtain these antioxidants from foods, protection from oxidative damage depends upon what we eat.

The most important dietary sources of vitamin E are vegetable oils. Wheat germ oil is among the richest sources as are foods such as margarine and baked products made from these oils. To a lesser extent, nuts and cereal grains can be sources of vitamin E.

Fruits and vegetables are generally the best sources of vitamin C--some berries have particularly high levels, and citrus fruits and juices are important sources. Meats in general contain little vitamin C, but organ meats such as liver and kidney can be good sources of this antioxidant.

Plant foods also contain other types of antioxidants. These include pigments in brightly colored fruits and vegetables such as citrus, yellow squash, sweet potato and tomato; compounds that repel insect feeders in broccoli, Brussels sprouts, and mustards; and other compounds in onions, apples, chocolate, grapefruit and tea.

This large and varied group of food antioxidants has one property in common: they all block or interrupt the damaging oxidation reactions in our cells, preventing adverse effects of unchecked production of reactive forms of oxygen. These antioxidant effects are thought to explain the observation that, in general, a diet rich in fruits and vegetables is associated with a lower overall mortality rate, and lower death rates to cardiovascular disease and some forms of cancer.

For this reason, contemporary dietary guidelines emphasize the importance of fruits and vegetables--key sources of antioxidants--in healthy diets. You can learn more about these rich sources of antioxidants, and get help in creating such a balanced diet containing them by going to MyPyramid.gov, sponsored by the U.S. Department of Agriculture. At this website, you can enter information about yourself to get a personalized guide in pyramid form, showing the amounts of various food types to include in your diet.