Are You Running Yourself Sick?

How does running affect your health?

By now, you’re experienced enough to know that more doesn’t always equal better (we’re looking at you, third glass of chardonnay). And when it comes to exercising to reduce your risk of illness, the same may be true.

People who exercised at a moderate pace, such as brisk walking, for 30 to 45 minutes per day most days of the week reduced their number of annual sick days by nearly 50 percent, found a 2010 study in the British Medical Journal. However, when you crank up the intensity, the opposite may occur: Several recent studies show that exercising harder and longer may actually have a negative impact on your immune system. In fact, after running a marathon, the risk of coming down with a cold spikes six-fold over the following week, according to a study in the Journal of Athletic Training.

What exactly is going on? Does intense exercise really put your health on the line and your bum on the sidelines? We consulted the experts to find out.

“First and foremost, according to our research, exercise is the single most important positive lifestyle activity you can do to reduce sick days,” says David Nieman, DrPH, director of the Human Performance Laboratory at Appalachian State University. “We’ve shown it’s more important than diet, sleep, and managing stress. It’s better and more powerful than any supplement or any drug that’s been studied.”

How? Physical activity increases the circulation of key immune system cells throughout your body. These cells are part of your body’s innate immune system, which means they’re the first line of defense in recognizing and responding to pathogens, Nieman explains. After exercise, the boost in circulation improves the cells’ ability to detect and destroy viruses and bacteria that could make you sick—an effect that lasts for up to three hours after your sweat session, says Nieman. “It takes going out day after day for the effect to add up to what we call ‘improved surveillance.’” This means your immune system amasses enough defenses against pathogens to decrease your risk of sickness. “In other words, exercise helps your immune system do its job better,” he adds. (And your immune system needs all the help it can get considering these 10 germ hot spots you touch every day.)

Still, there seems to be a threshold at which exercise is no longer helpful and may actually become harmful. For instance, a 2014 study in the Journal of Strength and Conditioning Research found that soccer players who did four 90-minute long intense workouts in a single week had lower levels of an antibody called immunoglobulin A, which helps your body fight off illness, compared to people who participated in lower-intensity activity. In marathon runners, Nieman’s research found that the same protective cells that build up after a brisk walk take a nosedive post-race. As a result, this enables viruses to multiply at a higher rate than normal, putting you at risk for a cold or the flu.

Simply put, too much exercise stresses your body out. The body perceives prolonged activity, such as running a marathon, cycling a century, or playing an intense sport for long periods of time without rest as a stressor, Nieman says. In turn, the body churns out stress hormones such as cortisol and epinephrine and downgrades the immune system since it’s not essential to the fight-or-flight response.

So does this mean you should abandon your long runs? Not necessarily. “Research shows that very intense exercise can suppress the immune system, but most of us aren’t elite athletes doing that kind of physical activity,” says Jessica Chubak, an epidemiologist at Group Health Research Institute who studies the effect of exercise on illness. In fact, a recent study by researchers at the London School of Hygiene and Tropical Medicine found that people who exercised vigorously for at least two and a half hours per week were 10 percent less likely to report flu-like illness.

Still, if you’re training for a race or recently started an intense training program, make sure you’re not overdoing it, especially in the beginning while your body adapts to the new regimen (this 5k training program will help keep you on track). “People who train three to five times per week, with two to three of those workouts being high-intensity sessions, shouldn’t be concerned,” says Hassane Zouhal, PhD, of the University of Rennes in France who authored the soccer-player study. But if you pack in too many high-intensity sessions without adequate rest, your immune system may take a hit, he adds.

Your best bet: Alternate high-intensity and moderate-intensity workouts throughout your week (taking about a day to rest after each high-intensity session in order to allow your body—and your immune system—to recover), and keep your hardcore sessions to 90 minutes or less. “At that point stress hormones go up, the immune system starts to reflect that stress and no longer functions like it should,” says Nieman.