Since exercise has so many benefits, it’s tempting to assume that it will strengthen immunity, and that by becoming physically fit you’ll make your immune system more robust. On the other hand, you may have heard that heavy-duty exercise can increase susceptibility to colds and other infections. Over the past 20 years, hundreds of studies have tried to clarify the potentially positive—and negative—effects of exercise on the immune system.
Most research has confirmed that moderate exercise— usually aerobic activity, such as running or cycling, but possibly strength training and tai chi as well—tends to improve various aspects of immunity and has anti-inflammatory effects. This could be especially beneficial in older people, who tend to experience a decline in immunity. In contrast, prolonged and/or very intense exercise (such as training for a marathon) has generally been found to depress immunity and elevate inflammation, at least for several hours.
The effect of exercise on immunity is not easy to study, however, because there are so many variables. Age, fitness level, overall health, genetics, type and duration of workouts and environmental factors can all play a role. Moreover, the immune system is very complex, and there isn’t a single test or marker that indicates how exercise is affecting your overall immune status. Researchers have to settle for the measurement of just one or a few components—usually a type of immune cell, protein or inflammatory compound—right after exercise or hours later. Interpreting the results is complicated by the fact that most of the detected changes (good or bad) are short-lasting.
Exercise and immunity: real-world effects
Many athletes report that they get more colds after intensive training and competition. High-intensity or prolonged exercise does step up the output of cortisol and adrenaline, two stress hormones that can depress various elements of the immune system. It can also boost production of free radicals and pro-inflammatory compounds. But while some studies show that people who run more than, say, 20 or 40 miles a week, tend to get more colds than those who run less, other studies have not found this.
There’s also some debate about how much moderate exercise actually boosts real-world immunity. Even if it improves aspects of immune function, no one knows whether this has any clinical significance. For instance, several recent studies have found that people who exercise before (or sometimes after) getting the flu shot tend to produce more antibodies, a sign of a more robust immune response. That’s promising, and it can’t hurt to exercise before or after getting vaccinated, but it doesn’t prove that people who do this are less likely to get the flu.
One thing is clear: Physical fitness in general is strongly associated with a reduced risk of various infections. For instance, a large study from Appalachian State University in North Carolina in 2010 observed that people who exercised aerobically at least five days a week were about half as likely to get colds or other upper respiratory tract infections as those who rarely exercised, and their colds tended to be milder. The researchers controlled for age, weight, and several other factors, but this may still be the result of the “healthy user effect,” which is a pitfall of such observational studies. That is, the type of person who exercises a lot tends to be healthy (and have a healthy immune system) to begin with, so the exercise may be a result of good health, not its cause. Or it may be a “virtuous circle.”
Bottom line: Regardless of its short- and long-term effects on the immune system, exercise is one of the best things you can do for your overall health. “There is no drug in current or prospective use that holds as much promise for sustained health as a lifetime program of physical exercise.” That was said 30 years ago by Walter Bortz, M.D., a leading researcher on aging and exercise at Stanford University—and it’s just as true today.