A gran fondo through the Alps. A trail run at altitude. A brutal CrossFit WOD. While totally different, all of these feats demand Herculean effort. Previous research also suggested they walloped your body’s defenses.
One study published in Frontiers in Physiology concluded CrossFitters who worked out on consecutive days suppressed their immunity, while an older study published in Sports Medicine found marathon training and races heighten stress hormones and can diminish your skin, upper respiratory tract, blood, and muscle health.
When this idea was first incepted in the 1980s, researchers asked runners in the Los Angeles Marathon if they experienced symptoms of infections in the days and weeks post-race. Many did, so, naturally, researchers drew a link between endurance sports increasing infection risk.
Researchers reviewed literature from the last few decades. During endurance sports, immune cells will multiply in the bloodstream as much as 10 times. Post-workout, they’ll decrease—sometimes even lower than before exercise. Previously this was interpreted as immune suppression. But researchers now believe the immune cells are simply moving to other parts of the body, like the lungs, which are at a heightened risk of being infected, since levels are restored after a couple hours.
“It is increasingly clear that changes happening to your immune system after a strenuous bout of exercise do not leave your body immune-suppressed,” lead analysis author John Campbell said in a press release. “In fact, evidence now suggests that your immune system is boosted after exercise—for example, we know that exercise can improve your immune response to a flu jab.”
As for that link between the Los Angeles Marathon and bouts of sickness? You’ve got swarms of people congregating in one area, public transportation (trains, planes, buses, etc.), stress, and, most likely, shoddy sleep. All of these factors can heighten the odds of getting sick.
Case in point: Indiana State Department of Health is recommending people get vaccinated for hepatitis A ahead of the Kentucky Derby because of recent outbreaks.