Are you immune? How your workouts can prevent—or promote—infection

AFTER COMPLETING HER FIRST HALF-MARATHON, Johanna Decal was feeling great. She'd run about 20 miles a week, in addition to cross training on the elliptical machine and lifting weights twice a week, and it all came together beautifully. After the 13.1-mile race, held near Decal's hometown on Long Island, NY, Decal was feeling proud, fit and healthy.

Buoyed by her success in the half-marathon, Decal, 24, then set her sights on a full marathon. She doubled her training mileage to prepare for the 26.2-mile race and started doing more intense workouts. At the same time, she was juggling night classes with her demanding full-time job at a local medical center and getting no more than five or six hours of sleep a night.

Suddenly, Decal wasn't feeling so terrific. "When I was training for the half-marathon, I didn't get sick once," she says. "Since I started training for the full, I've been sick three times already." Congestion, coughs and sore throats have hampered her progress. Although she still intends to complete her marathon, she may be reaching for a box of tissues not long after she reaches the finish line.

EXERCISE AND IMMUNITY

Decal, who runs with a group of men and women as part of the Leukemia and Lymphoma Society's Team In Training fundraising program, has noticed a pattern. "It seems like a lot of us also got sick more this season than when we were training for the half," she says.

The experience of Decal and her training partners illuminates a basic truth about exercise and its relationship to immunity: Work out regularly and moderately and you'll fire up your immune system, reducing your risk of catching a cold. Exercise really hard and increase your training volume quickly, and you may end up flat on your back with an upper respiratory infection.

The evidence is strong. For instance, a study conducted by researchers at the University of South Carolina and the University of Massachusetts concluded that regular participation in physical activity reduces the risk of infection by nearly 25 percent. In another trial, David Nieman, PhD, and his colleagues from Appalachian State University in Boone, NC, found that women who walked briskly for 35 to 45 minutes five days a week experienced about half the days with cold symptoms as those who were inactive.

"Studies show that regular exercisers have anywhere from one-quarter to a half fewer sick days than sedentary individuals," Nieman says. "I think that is remarkable. There is simply no medication that comes close to that."

At the same time, research shows that long, intense workouts may have the opposite effect on the immune system. In another study, Nieman monitored 100 competitors in the Western States 100, a punishing 100-mile running race in the Sierra Nevada mountains that took participants about 27 hours to complete. Blood and saliva samples taken before, during and after the event indicated the racers' immune systems had been radically suppressed. In the two weeks following the race, a full 25 percent of the runners reported upper-respiratory tract infections. The runners who had the lowest levels of a certain throat-protecting antibody were the most likely to get sick.

STAYING HEALTHY

Why does moderate exercise seem to be so effective in preventing colds while long, intense exercise seems to actually promote infection? The answer lies in the lymphocytes, the body's disease-fighting cells. These are small white blood cells produced in the lymph system, and they tend to congregate in certain parts of the body, such as the liver and spleen. However, aerobic activities and large-muscle-group weight training send these cells charging into action to fight the evildoer pathogens that invade your body in hopes of infecting you.

This increase in the circulation of disease-fighting cells happens almost as soon as you start working out. But just as quickly, when you stop, the cells tend to go back to their original hangouts, thereby reducing the immunity effect. "Near daily activity is what it takes," Nieman says. "Every time you get out and get active, you're revving up your immunity. But don't rest on your laurels."

Exercising moderately for 30 to 45 minutes seems to be just right in terms of recruiting the maximum number of lymphocytes. However, if you go significantly longer and harder, you might be heading into trouble. "Right around 90 minutes, things go awry," Nieman says. "Stress hormones begin pouring out in large numbers."

Although the mechanism is not perfectly understood, a drop in blood-sugar levels during long, hard workouts seems to trigger the brain to order the adrenal glands to emit stress hormones. If your training program calls for intense workouts lasting more than 90 minutes, is there anything you can do to lower your risk of getting sick? Yes: Consume carbohydrate while you work out. In his research, Nieman has found that exercisers who drink a liter of Gatorade or some other sugared replacement drink every 60 minutes are able to keep their blood sugar up, thereby keeping their immune systems humming.