In fact, the process starts while you're sweating. The moment you start exercising, your body cranks up circulation, at the same time initiating the fight-or-flight response, which produces stress hormones epinephrine and cortisol.
“Both actions activate immunosurveillance, mobilizing immune cells into blood, lymph, and tissues to increase the probability that they come into contact with pathogens,” says Jeffrey Woods, Ph.D., a kinesiology professor at the University of Illinois and a fellow at the American College of Sports Medicine. This search-and-destroy mission to hunt down damaging agents is part of the body’s innate immunity.
Another level of the body’s defense system is adaptive (or memory) immunity. This comprises specialized cells like T cells, which detect body cells that are infected. “Lifelong regular exercise may help maintain healthy numbers of young T cells as we age,” says James Turner, Ph.D., a professor at the University of Bath in England, who studies exercise and immunity.
Your long-term exercise habit may also pump up your innate immunity: In a recent study in Oxidative Medicine and Cellular Longevity, going from the couch to regular cycling workouts for 10 weeks significantly increased the number of two key inflammation-fighting cells. (Related: Functional Strength Workout to Boost Immunity)
The newest research by Turner and his colleague John Campbell, Ph.D., finds that steady cardio and high-intensity workouts are equally good at supporting immunity—so the choice is yours.
“If you look at the evidence, people who exercise more in general get fewer infections and also have lower incidence of cardiovascular disease,” says Campbell. In fact, in the cycling study, exercisers who were split into two groups of differing intensities had similar immunity improvements: The moderate-intensity group pedaled steadily for five workouts a week, and the HIIT group did Spin classes three times a week with speed intervals of 15 to 60 seconds.
The bottom line, Turner says, is that you should aim to meet the exercise guidelines: 150 minutes of moderate exercise or 75 minutes of vigorous intensity weekly. “But accumulating more than these recommendations—for example, doubling or tripling them—would not be harmful to the immune system” as you build up to that, especially if you already do so, he says. (Note: One sports doc argues that, during times where you're at increased risk of infection, you should be careful not to overdo it with HIIT workouts, since they could overstress your system and put you at risk for overtraining. That said, most experts recommend that you save HIIT for a couple of times a week regardless.)
Whether you stick to one intensity or do a mix of the two, Turner stresses maximizing other factors that play into your immunity outside your workouts: adequate sleep, good nutrition, avoiding exposure to contagious people, and maintaining good hygiene.
Shape Magazine, July/August 2020 issue