Keep in mind, however, that these findings are based on moderate exercise. "We've also shown in the lab that the immune system responds in a positive manner up until about 75 minutes of exertion; after 75 minutes stress hormones rise, and even more so if you continue beyond 90 minutes," Nieman says. Still, you can feel free to dismiss all those naysayers claiming you'll make yourself ill if you run too much—Nieman found in the well-known 1987 L.A. Marathon study of 2,300 runners that people did not experience increased sickness until they went beyond 60 miles a week. So, unless your running partner is Deena Kastor, you can relax and relish the benefits your immune system is reaping.
You are more likely to get sick immediately following a marathon. This isn't exactly breaking news, but what's interesting is that there are strategies marathoners can use to combat illness during the vulnerable period—deemed the open window—which lasts approximately one day after exertion, according to Nieman.
"In the L.A. Marathon study, we found that there was a six-fold increase in the chance of getting sick among runners who finished the marathon compared to runners who registered for the race but didn't run that day for some reason other than illness," Nieman says. It underscored something he had heard repeatedly from fellow marathoners: They felt protected during their normal training, but believed they are more likely to get sick after a race. He explains that this finding led to a series of studies about what happens to the immune system after heavy exertion. "The immune system doesn't function normally because in this 'open window' of immune dysfunction, viruses can multiply at a greater rate than normal," Nieman says.
But take heart, you don't have to steer clear of marathons—or live in a bubble—to stay healthy. Nieman advises marathoners to take charge of all the other things you can control: Get plenty of sleep, reduce mental stress and don't lose weight rapidly during training (defined as more than a pound and a half per week.) Other than taking these precautions, he says your best bet for faring well during the open window is to follow the rules your mother taught you. Wash your hands often; keep your hands away from your nose and mouth; and, of course, eat plenty of fruits and veggies.