Eating an apple a day could help reduce your risk of getting the flu this year, especially during times of stress. The results of an animal study published by The American Physiological Society found that quercetin, a powerful antioxidant present in apples and other fruits and vegetables, may have protective effects against the influenza virus.

The scientists conducting the study demonstrated that while stressful exercise increased the occurrence of influenza in laboratory mice, quercetin canceled out that negative effect. Vigorous, stressful exercise has been shown to increase the susceptibility to upper respiratory infections not only in mice, but also in humans.

Fortunately, the protective benefits of quercetin appear to apply to people as well as animals. A recent human study showed that individuals taking daily doses of the flu-fighting antioxidant suffered fewer respiratory illnesses following three days of exhaustive exercise compared to those who did not take the plant compound.

You don't have to be a serious athlete to reap the health benefits of quercetin. Like exhausting physical activity, emotional stress can weaken the immune system, increasing susceptibility to infections and illnesses. Experts believe that regular intake of the natural plant compound can help protect stressed-out, sedentary individuals from contracting the flu and other respiratory infections.

In addition to its powerful anti-viral effects, quercetin has important cancer-fighting properties. Researchers at Mayo Clinic reported that it blocks hormone activity in prostate tumor cells, an action that can slow or prevent the spread of the cancer.

Additional studies have shown that the antioxidant compound can inhibit the growth of other types of cancers cells, including those found in tumors of the breast, colon and lung. Quercetin appears to induce apoptosis, a form of cell death that doesn't ordinarily occur in cancer cells.

Researchers at Cornell University in New York found that the naturally occurring antioxidant may protect brain cells from oxidative stress, a tissue-damaging process associated with Alzheimer's disease and other degenerative disorders of the brain. In a laboratory study, the scientists exposed brain cells to hydrogen peroxide, a chemical known to produce the type of oxidative damage that contributes to the development of Alzheimer's disease.

Brain cells treated with quercetin suffered significantly less damage to both cellular proteins and DNA than untreated cells. Although the exact mechanism of action is unknown, many experts speculate that the compound works by blocking the cell-damaging actions of free radicals.

Free radicals are thought to contribute to the development of other degenerative diseases, including cataracts and macular degeneration of the eye. The powerful antioxidant properties of quercetin appear to play an important role in the prevention and treatment of these eye disorders.

Scientists have found that regular quercetin intake can reduce the risk of cardiovascular disease. The compound not only lowers blood pressure and cholesterol levels, it also helps prevent the buildup of plaque in arteries that can lead to heart attack and stroke.

Quercetin is well known for its powerful anti-inflammatory effects, making it useful in the prevention and treatment of arthritis and other chronic inflammatory diseases. Research shows that when patients with rheumatoid arthritis switch to a quercetin-rich diet, they typically experience significant reductions in their symptoms.

For folks with allergies, quercetin may be a viable alternative to some over-the-counter and prescription medications. The antioxidant compound has impressive antihistamine actions, making it useful in the treatment of hay fever, eczema and hives.

Several studies have linked high quercetin intakes with improved lung function. Among people who regularly consume apples, scientists have noted a lower risk of some respiratory disorders, including bronchitis, asthma and emphysema.

Quercetin is naturally found in a variety of plant foods and beverages, including grapes, blueberries, broccoli, tea and red wine. The disease-fighting antioxidant is especially abundant in the outermost layers of red onions and in the skins of apples.

Any type of apple is likely to be a good source of the beneficial plant compound, but red apples tend to have more of the antioxidant than green or yellow varieties. Because apples are generally peeled and cooked when making applesauce or apple juice, these products offer less quercetin than fresh, unpeeled fruit.

Dietary supplements containing the natural plant compound are easy to find in most health food stores, and they're generally safe and well tolerated. But if you'd rather get your daily dose of quercetin the old-fashioned way, enjoying a fresh, ripe apple might be your best bet.

Rallie McAllister is a board-certified family physician, speaker and the author of several books, including "Healthy Lunchbox: The Working Mom's Guide to Keeping You and Your Kids Trim." Her website is www.rallieonhealth.com. To find out more about Rallie McAllister, M.D., and read features by other Creators Syndicate writers and cartoonists, visit the Creators Syndicate Web page at www.creators.com.

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