Sneezing? Snuffling? Wheezing? Aching? Feverish? You've probably fallen victim to one of those annoying ailments, the common cold or flu. There may be no known cure for either, but recent studies indicate that there is a link between exercise and the body's immune system, that army of cells that fights off sickness-causing viruses. Some types of exercise appear to help keep colds away, while others appear to bring them on. For those enjoy music, they even use best sport headphones along their exercising time.
The Marathon Effect
While many exercise buffs believe that regular exercise promotes resistance to colds and flus, there are indications that too much exercise invites them. Many European investigators have reported an increased incidence of infections in highly trained athletes during intensive training. At the Seoul Olympic Games, some of the world's premier athletes including British runner Sebastian Coe-were unable to compete because of illness brought about by overtraining.There has been a flurry of research around the world investigating the connection between exercise and immunity. We looked at the incidence of colds and flus during the two-month period prior to the race (the peak cold and flu season) and the week following the race event.
During the two months before the race, those who trained more than 60 miles a week were twice as likely to get sick as those who trained only 20. And significantly, 13 percent of those who ran the 26.2-mile race became sick the week after, compared to only two percent who trained for the marathon but then decided not to run it.
After all the input was analyzed, we concluded that an athlete's odds for getting sick after running a marathon are six times greater than the odds for an equally experienced athlete who doesn't run the marathon. These research findings suggested to us that the combined physical and psychological stress of running a marathon in some way depresses the immune system.
We next brought 10 seasoned marathoners to our Human Performance Laboratory, and ran them on the treadmill at their fastest pace for three hours while collecting blood samples before, during, and for one day after the run. We found that the stress hormone cortisol which weakens the ability of certain immune cells to function properly increased 50 percent above pre-test levels after the run. The result: Natural killer-cell activity was down more than 30 percent for nearly six hours (natural killer cells play an important role in the body's first-line defense against foreign invaders).
More research must be conducted to confirm our findings, but we propose that after an exhausting marathon run, the immune system is more vulnerable than normal to attack by foreign invaders for at least six hours.
A Walk a Day Keeps the Doctor Away
Our next step was to research whether or not exercise could protect against sickness, as many have claimed. Specifically, we wanted to find out if moderate exercise could strengthen the immune system and reduce the risk of coming down with a cold or flu.We selected 50 women who were not on exercise programs and randomly divided them into exercise and non-exercise groups. For 15 weeks (February through mid-May), the exercise group briskly walked 45 minutes, five days a week, at about 75 percent of their maximum heart rates. Subjects in both groups recorded how they felt using daily logs. This was the first study ever to investigate the relationship between moderate exercise training, immune system function, and upper respiratory tract infections.
We found that the women in the walking group experienced half the days with cold and flu symptoms during the 15-week period as the non-exercise group (5.1 versus 10.8 days). Natural killer-cell activity increased in the walking group from the very beginning of the study, as did the effectiveness of other components of the immune system.
These results suggest that moderate exercise is beneficial in strengthening that system and lessening the onslaught of winter sickness. This, plus our conclusion that strenuous exercise can weaken immunity, is consistent with what other researchers have been finding.
Practical Advice
Although brisk walking was the mode of exercise used in our study, it is likely that moderate jogging, swimming, cycling, and other aerobic exercise are just as beneficial. The important principle is that of moderation. The scientific research in this area has barely begun, but researchers have usually defined moderate exercise as three to five days a week, 20 to 30 minutes a session, at 70 to 85 percent of maximum heart rate. When exercise becomes stressful (high intensity or long and exhausting), cortisol rises strongly in the bloodstream.At what point does moderation turn into stressful exercise? That is a fruitful area for new research. What is stressful to a beginning exerciser, for instance, may be moderate to a well- conditioned one. But in our Los Angeles Marathon study, no matter how well conditioned an athlete was, if he or she trained over 60 miles a week, the risk of getting a cold or the flu increased.
For those athletes who must exercise intensely for competitive reasons, several precautions may help decrease the risk of illness. Sensible practices include spacing vigorous workouts and race events as far apart as possible, eating a well-balanced diet, keeping other life stresses to a minimum, obtaining adequate sleep, and avoiding overtraining (symptoms can include insomnia, irritability, and continual fatigue). By the way, most studies have failed to show that large doses of vitamin C prevent colds and flus.
