Working up a sweat at the gym does much more than build muscle and cardiovascular endurance. Here's how exercise and your immune system are connected.

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If it seems like you always have a case of the sniffles, taking an Emergen-C pack to support your immune system shouldn't be the only thing you do. Science shows exercise boosts your immune system and makes your body more resilient long-term.

Here's exactly how it works (spoiler: bodies are freaking cool) and the exact amount of exercise you should do when you're trying to power up your defenses.

How Exercise Boosts Your Immune System

"If you look at the evidence, people who exercise more in general get fewer infections and also have lower incidence of cardiovascular disease," says John Campbell, Ph.D., professor at the University of Bath in England, who studies exercise and immune system function.

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., Campbell's colleague at the University of Bath.

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)

How to Capitalize on the Protective Effects

The newest research by Turner and Campbell finds that both steady cardio and high-intensity workouts are good at supporting immunity — so the choice is yours.

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.

And cardio isn't the only immune system booster; keeping your muscles swole and strong can be beneficial, too. You might think of them as simple generators of brute force, but as your muscles contract, they're also pumping out numerous proteins into the bloodstream, some of which behave as hormones. That infusion can have a cascade effect on your immunity, and the latest science shows it's pretty powerful medicine. (Just add it to the list of other benefits you get from lifting weights.)

A recent study in the journal Aging Cell found that people who bicycled regularly throughout their lives had more plentiful immune cells thanks to their muscle quotient. "They maintained muscle mass, and that active muscle produces hormones that maintain the size of the thymus — the organ that produces T cells — which normally shrinks with age," says one of the lead authors, Janet Lord, Ph.D., an immunology professor at the University of Birmingham in England. Having more new T cells and memory T cells is key, so your body "can detect new infections, like the coronavirus, as well as ones you have encountered before," she says. (Ready to hit the city trails? Grab one of these compact folding bikes.)

Besides stoking the production of immune cells, other proteins secreted by your muscles help protect against cancer. The IL-6 protein, or interleukin-6, "plays a role in directing natural killer cells to the tumor site during exercise," according to research from the University of Copenhagen. The rush of IL-6 that your body gets out of a workout is linked to the amount of muscle mass involved and how long you move. (More here: What Is Inflammaging and What Is It Doing to Your Skin?)

The Optimal Exercise Dose to Support Your Immune System

The bottom line on exercise and immune system function, 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 when 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.)

Strength training two to three days a week should also keep your muscles in germ-fighting form, according to the American College of Sports Medicine. If you're using free weights or weight machines, the ACSM suggests doing sets of eight to 12 reps for your major muscle groups, with loads of at least 60 to 70 percent of the maximum you can lift for one rep of that particular move. (Regular lifters can go with 80 to 95 percent of their max.)

But as the study on bicyclists showed, there are more ways to save your strength. "In a way, resistance training and cardio aren't incredibly different," says exercise scientist Cassandra Forsythe, Ph.D., R.D.N., a physical education professor at Central Connecticut State University. "They both stimulate muscle fibers, the cardiovascular system, and the immune response." She recommends boot camp–style circuit training or even her own routine: yoga.

Whether you stick to a high intensity routine or a low one, strength training or cardio, or a mix of all of the above, 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. In the end, it's always about balance.

Shape Magazine, July/August 2020 issue