Fighting winter colds may be easier for people who exercise on a daily basis, because regular workouts may boost the immune system and increase resistance to colds and flu.

Excessive exercise, on the other hand, may weaken the immune system and pave the way for upper respiratory tract infections.

"It's a classic case of 'A little is good, a moderate amount is great and too much can be worse than none at all,' " says E. Randy Eichner, professor of medicine at the University of Oklahoma Health Sciences Center in Oklahoma City.

New studies show that people who exercise regularly, in moderation, may get sick less often than those who don't. In a 15-week study, "women who took a brisk, 45-minute walk, five days a week experienced half the days with cold and flu symptoms compared with the non-exercise group," notes David C. Nieman, associate professor of health, leisure and exercise science at Appalachian State University in Boone, N.C.

In the walking group, "natural killer cell activity increased, and these cells play an important role in the body's first-line defense against foreign invaders. Plus, regular walking led to a 20 percent net increase in antibodies that better equip the immune system to rid the body of pathogens." These immune-enhancing effects occurred during the walk and for about one hour afterward, Nieman says. "It's like a housecleaner coming in for a few hours every day, so the house stays clean."

Another reason moderate exercise may boost the immune system is its stress-relieving capacity. Numerous studies indicate that exercise helps reduce stress and enhance mood. Research also shows that people under severe stress have impaired immune system activity. This may be why the physical and psychological stress of excessive exercise puts people at higher risk of colds and flu.

Runners training competitively with high mileage are more likely to contract an upper respiratory tract infection, according to Gregory Heath, an epidemiologist-physiologist at the Centers for Disease Control in Atlanta. "This is particularly true for those who engage in long-distance, competitive races. They seem to be vulnerable the first 12 to 24 hours after a race."

Heath and colleagues at the University of South Carolina School of Public Health studied 564 runners and discovered that both male and female high-mileage runners had a greater likelihood of upper respiratory tract infections.

"People who were running more than 20 miles per week were generally at greater risk," says Heath, whose study will be published early next year in the journal of the American College of Sports Medicine.

"The odds of getting sick after a marathon were six times greater for those who completed the race than for equally experienced runners who did not compete," says Appalachian State's Nieman, who studied 2,300 runners who trained for the 1987 Los Angeles Marathon, measuring those who completed the race with those who didn't participate. In a second study of 10 seasoned marathoners, who ran at their fastest pace for three hours, blood samples showed that the natural killer cell activity of the runners' immune systems was more than 30 percent lower for nearly six hours. "These findings suggest that after an exhausting marathon run, the body's immune system is more vulnerable than normal to bacterial and viral attack," Nieman says.

Both Nieman and Heath advise people who must exercise intensely to take extra precautions, such as spacing vigorous workouts and race events as far apart as possible, following a well-balanced diet, getting adequate sleep and keeping other life stresses to a minimum.

Some researchers theorize that exercise may help the immune system fight more serious ailments. While any conclusions are speculative, notes CDC's Heath, "there may be a link between exercise and immune activity" that could affect diseases ranging from arthritis to cancer.

For now, with winter a sneeze away, experts advise you to keep moving if you want to avoid colds and flu.

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