Does Exercise Actually Help or Hurt Your Immune System?

Your workouts bring a lot of benefits: They relieve stress, improve your heart health, and help you get stronger. But what about exercise and your immune system? Do your workouts help your immune system—or can they actually weaken it?

It’s a question that people are asking more and more as the novel coronavirus spreads. Total cases are continuing to increase, and because it can be transmitted by someone not displaying symptoms, many people are wondering if there’s anything they can do to improve their chances of fighting off the virus, especially if they come into contact with it without even knowing they’ve been exposed.

Hoping for an “immune booster” is understandable, because these are scary times and there are tons of things about the novel coronavirus that we just don’t know yet. But as we reported earlier, there’s no magic pill or supplement that’s going to give your immune system superpowers.

That’s not to say, though, that lifestyle factors like physical activity and exercise don’t play a role in how your immune system works. But it’s just not as simple as “run a mile, fight off a bug.” Here’s what you need to know about exercise and your immune system—especially in the time of the new coronavirus.

How exactly does exercise affect your immune system? Exercise does affect your immune system, but thinking of it as an “immune booster” isn’t exactly correct.

“In response to bouts of exercise, there is an immune response, and that is a normal immune response,” James Turner, Ph.D., an exercise physiology and immunobiology researcher at the University of Bath in the U.K., tells SELF. “It’s probably more accurate to say exercise stimulates or kickstarts some normal immune processes.”

Here’s what’s happening: When you engage in any kind of physical activity that gets your heart rate up for a sustained amount of time—say, a 30-minute brisk walk or jog, a bike ride, or even some tennis volleying—your body senses it as a type of physiological stressor. As a result, it deploys certain types of white blood cells like neutrophils and lymphocytes (particularly T-cells and natural killer cells) from different parts of your body to flood your bloodstream.

“These very specialized, powerful immune cells are like the Army Rangers of the military,” says exercise immunology researcher David Nieman, Dr.Ph., a professor of biology at Appalachian State University and director of the Human Performance Lab at the North Carolina Research Campus. “They come out and circulate during exercise at a higher rate than normal. Any pathogens are more easily detected and destroyed during this process.”

Soon after your workout, these immune cells start to decline in your bloodstream and even go down to below resting levels. Initially, researchers believed this was evidence of immunosuppression, Turner says, but improved lab techniques actually showed that these cells were just being dispatched to other bodily locations where they continue to perform a process called immune surveillance.

“They go off to other tissues in the body, like the lungs or maybe the skin, intestines, or mucosal surfaces, where an infection might be found,” Turner says.

This whole kickstart to the immune system is only temporary—it lasts about three hours, says Nieman—but it occurs after each bout of moderate to vigorous exercise. So if you continue to exercise regularly, you’ll continue to experience those effects after each session.

But do the physiological responses translate to real-world benefits? Research has shown that people who exercise regularly do tend to get sick less frequently. According to a 2010 study of more than 1,000 adults published in the British Journal of Sports Medicine, people who exercised for at least 20 minutes a day, five or more days per week, reported 43% fewer days with upper respiratory tract infection symptoms than those who were sedentary. And when they did get sick, their symptoms tended to be less severe.

That’s not to say, though, that exercise will automatically trigger your immune system to annihilate any germ invader it sees—it’ll just help you improve your odds of fighting it off, Nieman says. It’s not a get-out-of-jail-free card by any means (more on that below).

Is more exercise better—or can too much hurt your immune system? If a moderate amount of exercise can stimulate your immune system, will longer or more vigorous exercise have a greater effect? Or can it actually weaken your immune system?

That’s a question that’s been hotly debated for years, and as with many questions in the field of science, there isn’t exactly a consensus on the answer. According to Nieman, vigorous exercise of a long duration—think 90 minutes or more, like you’re racing a half marathon or a marathon—starts to over-stress your immune system, which can temporarily impair its ability to do its job and leave you more vulnerable to infection during this time. That’s what’s known as the open window hypothesis. (HIIT, on the other hand, isn’t linked to such immune suppression even though it involves super-intense work, probably because of its rest intervals and shorter overall duration, Nieman says.)

Now, evidence does show that some elite athletes get sick with upper respiratory infections (URI) after competition, but Turner and other experts argue that it’s not exactly the exercise that’s to blame: “It is misleading to conclude from existing evidence that exercise is the causative factor of URI among athletes,” Turner’s team wrote in a new debate paper on exercise and immune suppression published in Exercise and Immunology Review this year. After all, Turner tells SELF, even though some immune cell counts are lower after intense exercise, it isn’t because they’ve died off—they’ve just gone off to other tissues in the body to continue their infection patrol.

The more likely reason for sickness after intense competition like marathons—whether for elite athletes or recreational exercisers—Turner believes, is not just about the exercise. It likely also has to do with the environment, he says. Think of marathon corrals at the starting line at big-city races: They’re shoulder-to-shoulder people, and the course itself doesn’t really clear up much after, either.

“You are exposed to thousands of people there,” Turner says—so those who are sick with viral or bacterial infections can subsequently transmit them to you. That can be directly through nasal droplets that enter your nose or mouth, like through another person’s cough or sneeze, or by touching a surface that a person who is sick has touched and then touching your own mouth, nose, or eyes.

Those mass-participation events can obviously be a nightmare for virus spread—which is why races like the Boston Marathon have postponed their competition among coronavirus fears (even before the state’s stay-at-home advisory was issued)—but they’re not the only factor likely involved, either. Other issues that could cause immune suppression could have been at play leading up to the big event too.

“Everything is multifactorial with the body and with the immune system,” says Nieman. “The physiological effects of heavy exertion on the immune system is one factor, but then if they travel and get on planes, then they don’t sleep well, that introduces other stressors to the immune system. And then if they are undergoing psychological stress, that’s another one.”

What does this mean for exercise and immune function as coronavirus spreads? Poor sleep and psychological stress? For many of us, those are unhappily all too familiar during this time of coronavirus fears, as many of us are sheltering in place and stressing about our jobs, our family, our safety, and the incongruence of our new normal.

That means that even without taking our exercise habits into account, our immune systems may already not be running as smoothly as they should be. According to a 2016 study of more than 22,000 adults published in JAMA Internal Medicine, people who reported having trouble sleeping were 29% more likely to develop a cold than those who had no issues snoozing—and short sleepers were also more likely to become ill than those who logged between seven and eight hours a night. As for mental stress, a meta-analysis of 27 studies published in the journal Psychosomatic Medicine concluded that psychological stress can make you more susceptible to developing an URI.

So really pushing yourself with your workouts now—even if you’re not exercising at half marathon or marathon intensities—could be a little riskier than usual, considering the other factors that may be going on in your life. Consider it a constellation of stressors that might combine to make you a little more vulnerable.

“Now is not the time to be overtraining,” says Nieman. “I’m sure you’ve heard about people running marathons in their backyards, that sort of thing. I would not recommend that at all right now.”

What he does recommend is continuing to make movement a regular part of your day. The U.S. Department of Health and Human Services recommends at least 150 minutes of moderate intensity or 75 minutes of vigorous physical activity each week, preferably spread throughout the week.

“That’s the sweet spot for activity, and the immune system responds really well to it,” Nieman says.

Your exercise can be an outdoor run or bike ride (if you can maintain social distance to exercise safely outside) or an at-home workout. If you don’t feel mentally up to your regular routine, a brisk walk outside (with that social distancing) is a perfectly fine way to get that in. (But don’t sweat it either if you can’t hit that amount regularly right now, whether it’s due to additional responsibilities that are taking precedence, a lack of space, or other factors—these are trying times when we’re all doing the best we can with circumstance.)

While regular exercise, proper sleep, and working to reduce stress are certainly helpful for proper immune system function, they shouldn’t be the main prevention factors you focus on when trying to reduce your chances of developing COVID-19, the illness caused by the novel coronavirus, he says. Especially since it’s a new pathogen, reducing exposure to it is paramount to staying safe.

That means staying away from as many people as possible, adhering to stay-at-home orders and social distancing recommendations, and washing your hands regularly, says Turner—these prevention strategies should be your main focus.

Consider your workouts a bolstering factor, something that can supplement those tried-and-true prevention strategies and give your immune system a solid foundation to help it do its job.

“People should aim to exercise regularly, and that will, over a long period, benefit immune function overall,” Turner says.

The guideline to exercise regularly holds true for most days—except the ones when you’re sick, even if you don’t know for sure you have COVID-19. You definitely should not try to “sweat it out” if you don’t feel well, says Nieman. Exercising with systemic, fever-causing illness—say, the flu or COVID-19—is never a good idea.

“You get a huge inflammatory response to [COVID-19], and exercise can make you feel much worse,” Turner says.

In that case, lots of rest (and hydration!) is going to be the best strategy, Nieman says. (Here’s how to care for COVID-19 if you or a loved one has it.) While it may be hard for some people used to exercising regularly to hit pause on their routine, it’s the smartest way to deal with sickness—especially if it’s caused by a pathogen with as many question marks as the new coronavirus.

“Now is the time to think about your health, not your fitness,” says Nieman. “We just need to wait this out and slowly get back to normal, and just be smart about it and not impair or hurt our health in the process.”

Find more fitness articles at SELF.