The Sick Sense

How does the immune system respond to physical exertion?

The ailment we call a cold is actually an infection of the lining of the nose, sinuses and throat. Caused by a virus, the cold is indeed very common. As the most frequently occurring illness in humans worldwide, colds strike the average American adult two or three times per year and the average American child six to seven times annually. The flu is also a viral infection of the upper respiratory tract, but it makes a person much sicker. In addition to the symptoms typical of a cold (such as sneezing, coughing and congestion), the flu frequently causes a fever, headache and chills. More than 425 million colds and flues occur annually in the United States, according to the Centers for Disease Control and Prevention, resulting in $2.5 billion in lost school days, workdays and medical costs.

More than 200 different viruses cause these upper-respiratory infections, and they are very easily transmitted. For example, if you shake hands with an infected person, then touch your eye, mouth or nose, you could become infected. Whether or not you get sick with a cold after the virus has entered your body depends on many factors, one of which is the strength of your immune system. Certain behaviors and experiences are known to impair immune functioning and increase your risk of infection. These include cigarette smoking, aging, stress, poor nutrition, quick weight loss, fatigue and heavy, prolonged physical exertion. Healthy behaviors that can enhance immune function include eating a nutritious diet, getting adequate sleep, minimizing stress and getting regular, moderate exercise.

Fit people report getting fewer colds than their inactive peers. For example, one survey showed that 61 percent of recreational runners said they'd experienced fewer colds when they'd started to run, compared to only 4 percent who felt they experienced more, according to the American College of Sports Medicine (ACSM). In another survey of 170 experienced runners, 90 percent said they "definitely" or "mostly" agreed with the statement that they "rarely get sick."

Research backs up this perception. Several studies by David C. Nieman of Appalachian State University show that people who walked for 45 minutes 5 days a week for 12 weeks suffered half as many days of cold and flu symptoms as people in a non-exercising control group.

When people exercise, the increased circulation encourages the spread of beneficial immune cells throughout the body, boosting protection against viruses and other pathogens. Exercise can increase the number, activity and delivery of immune cells during physical activity and for about 90 minutes afterward. When people exercise daily, or almost every day, "it's like a house cleaner coming in for a few hours every day," Nieman says. "So the house stays clean."

Exercise may also enhance immunity through its stress-relieving effects. Stress is known to impair immune function, and numerous studies show that exercise helps reduce stress and enhance mood. By helping people balance their emotions and relieve tension, regular exercise may offer another kind of immunological boost.

However, there's a flip side to this relationship between exercise and stress. As noted earlier, excessive exercise--such as running a marathon or training for the Olympics--can itself be a physiological and psychological stress. This may explain why prolonged, heavy exertion appears to suppress the immune system and leave athletes more vulnerable to infection. During the winter and summer Olympic Games, for example, team physicians report that upper-respiratory infections abound.

Nieman suggests thinking about it this way: Moderate exercise, like taking a brisk walk, provides an immunological boost similar to feeding the immune system "police force" some coffee. The molecular "cops" get a caffeine-like "buzz" and start circulating at a greater rate than normal, which increases their chance of running into bad guys. But doing excessive exercise is like giving the police force a keg of beer. The immune system "cops" become confused by the barrage of demands for fuel and repair from the body's exhausted muscles, so they are too befuddled to do their job well.

Exercise Rx for Cold Prevention

The obvious question is this: How much exercise is enough to enhance resistance to colds, but not too much to impair it? Your particular exercise prescription will depend on many factors, such as your health status, previous exercise levels and general fitness. If you've been sedentary or an irregular exerciser, try moving just a little bit more than you currently move. Gradually, but regularly, move just a little bit longer and a littler bit faster. A good goal is the surgeon general's recommendation for health: Perform 30 minutes of moderate activity on most days of the week. Of course, "moderate" is a relative term, since a moderate workout for a competitive athlete may be a killer workout for someone who's been sedentary.