As millions of Americans flock to the gym armed with New Year's resolutions to get in shape, medical experts are offering an additional reason to exercise: Regular workouts may help fight off colds and flu, reduce the risk of certain cancers and chronic diseases and slow the process of aging.

Physical activity has long been known to bestow such benefits as helping to maintain a healthy weight and reduce stress, not to mention tightening those abs. Now, a growing body of research is showing that regular exercise—as simple as a brisk 30 to 45 minute walk five times a week—can boost the body's immune system, increasing the circulation of natural killer cells that fight off viruses and bacteria. And exercise has been shown to improve the body's response to the influenza vaccine, making it more effective at keeping the virus at bay.

"No pill or nutritional supplement has the power of near-daily moderate activity in lowering the number of sick days people take," said David Nieman, director of Appalachian State University's Human Performance Lab in Kannapolis, N.C.

Nieman has conducted several randomized controlled studies showing that people who walked briskly for 45 minutes, five days a week over 12 to 15 weeks had fewer and less severe upper respiratory tract infections, such as colds and flu. These subjects reduced their number of sick days 25 percent to 50 percent compared with sedentary control subjects, he says.

Medical experts say inactivity poses as great a health risk as smoking, contributing to heart disease, diabetes, hypertension, cancer, depression, arthritis and osteoporosis. The Centers for Disease Control and Prevention says 36 percent of U.S. adults didn't engage in any leisure-time physical activity in 2008.

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