The Myth: For decades, researchers have thought that endurance exercise takes a toll on the immune system. The connection goes back to the 1980s, when they followed runners and found that some got sick in the days and weeks after finishing a race.
A 1983 study showed that one-third of the athletes who ran a 34-mile race in South Africa developed upper respiratory infections afterward. In 1990, scientists found that 13 percent of competitors in the 1987 Los Angeles Marathon developed the same condition post-race.
Around the same time, researchers discovered that marathon runners had lower levels of immune cells in their bloodstreams after completing the 26.2 miles, leading them to blame endurance exercise for suppressing the immune system. The myth has persisted ever since.
The Truth: A new study published in the Frontiers of Immunology questions the idea that endurance sports weaken immunity. When researchers from the University of Bath in Somerset, England, took a closer look at how the body works during strenuous exercise, they found immune cells weren’t dying, as previously thought.
It’s true that there were fewer immune cells in the bloodstream post-workout, but not because they were dying. Instead, it’s probably because they’re moving from the bloodstream to other areas of the body like the lungs, says study author James Turner, Ph.D., a senior lecturer in the university’s Centre for Therapeutic Innovation.
“Since runners do a lot of intense breathing while training or racing, they might inhale infectious particles,” he says. As a result, immune cells may travel to those areas to look for, and stop, potential infections. Separate studies have shown that athletes respond better to vaccines, he says, and strenuous workouts probably have a similar effect.
The reason why marathoners may pick up colds after racing has to do with something more obvious: being in a crowd. “We think getting sick has to do with exposure,” Turner says. Being in large groups of people increases your chance of coming down with something, whether you’re at a race or on a plane or at a wedding. What you’re doing is less important than the environment you’re in. “You’re surrounded by hundreds, maybe thousands, of people who might be coughing and sneezing,” Turner says.
While a 50-mile weekend bike ride or three-hour training run won’t get you sick in and of itself, athletes should still pay attention to their habits. Chronic psychological stress, lack of sleep, dehydration, and extreme heat or cold can increase your risk of infection. Carefully plan your workouts and incorporate enough rest, Turner says.
The Final Word: Whether it’s a HIIT routine or long-lasting steady-state cardio, any bout of movement is healthy. “You should probably be more worried about a lack of personal hygiene and other unhealthy lifestyle habits, not exercise, increasing your risk of infection,” he says.