You’ve probably heard the advice: one of the best things you can do to stay healthy — especially as cold and flu season approaches — is to stay physically active.
This folk wisdom is very old, but until recently researchers didn’t have much data to back up the idea. Now, scientists studying risk factors related to Covid-19 have found some preliminary evidence about the link between regular exercise and better immune defenses against disease.
When researchers reviewed 16 studies of people who remained physically active during the pandemic, they found that exercise was associated with a lower risk of infection, as well as a lower likelihood of severe Covid-19.
The analysis, published last month in the British Journal of Sports Medicine, has generated great enthusiasm among exercise scientists that the findings could lead to updated guidelines for physical activity and health policy revolving around exercise as a medicine.
Specialists who study immunology and infectious diseases are more cautious when interpreting the results. But they agree that exercise can help protect health through a number of different mechanisms.
Exercise can boost immunity in a number of ways
For decades, scientists have observed that fit and physically active people appear to have lower rates of various respiratory tract infections. And when people who exercise get sick tend to have less serious problems, said David Nieman, a professor of health and exercise science at Appalachian State University who was not involved in the recent Covid-19 review.
“The risk of serious outcomes and mortality from the common cold, flu, pneumonia is greatly reduced in everyone,” Nieman said. “I call it the vaccine-like effect.”
The new meta-analysis, which analyzed studies between November 2019 and March 2022, found that this effect extends to Covid-19. People around the world who exercised regularly had a 36% lower risk of hospitalization and a 43% lower risk of death from Covid compared to those who were not active. They were also less likely to contract the disease.
People who followed guidelines that recommend at least 150 minutes of moderate activity or 75 minutes of vigorous activity a week seemed to see the most benefit. But even those who exercised less were better protected from disease than those who didn’t.
Researchers theorize that exercise may help fight off infectious bacteria and viruses by increasing the circulation of immune cells in the blood, for example.
In some smaller studies, researchers have also found that muscle contraction and movement release signaling proteins known as cytokines, which help direct immune cells to find and fight infection.
Even if your levels of cytokines and immune cells decline two or three hours after you stop exercising, Nieman said, your immune system becomes more responsive and able to detect pathogens more quickly over time if you exercise every day.
“Your immune system is primed and in better shape to fight a viral load at any given time,” he said.
In healthy people, physical activity has also been linked to less chronic inflammation. Generalized inflammation can be extremely harmful, even turning your own immune cells against your body. It’s a known risk factor for Covid-19, Nieman said. So it makes sense that reducing inflammation could improve your chances of fighting infection, he said.
Research also shows that exercise can amplify the benefits of some vaccines. People who exercised right after getting the Covid vaccine, for example, seem to have produced more antibodies. In studies of seniors who were vaccinated early in the flu season, those who exercised had antibodies that lasted all winter.
Exercise provides a number of broader health benefits that can help reduce the incidence and severity of disease, said Stuart Ray, an infectious disease specialist at the Johns Hopkins University School of Medicine.
Including a walk, run, workout at the gym or sport of your choice in your routine helps to reduce obesity, diabetes and heart disease, for example, all risk factors for serious flu and Covid-19.
Exercising outdoors can help you sleep better, improve your mood, your insulin metabolism and your cardiovascular health, increasing your chances against the flu and Covid. It’s hard to know, Ray said, whether the benefits come from direct changes to the immune system or just better overall health.
Peter Chin-Hong, an infectious disease specialist at the University of California at San Francisco, agreed that more research is needed so scientists can identify a specific mechanism or cause-and-effect relationship. In the meantime, he said, it’s important not to believe this idea too much.
“For now, you can’t say, ‘I’m going to the gym to avoid Covid,'” Chin-Hong said. The problem with studying the precise effect of physical activity on immunity is that exercise is not something that scientists can easily measure on a linear scale, ay said. “People exercise in many different ways.”
Study participants often report the amount and intensity of exercise performed, which can often be inaccurate. And just hoping that exercise will be beneficial can produce a powerful placebo effect.
As a result, it can be difficult for researchers to say exactly how much exercise or what type is optimal for immune function. It’s also very possible that people who exercise regularly have other attributes in common that help them fight off infections, such as a varied diet or better access to medical care, Ray said.
In addition, “there is a lot of discussion about whether too much exercise makes a person more susceptible to infections and disease,” said Richard Simpson, who studies exercise physiology and immunology at the University of Arizona.
Marathoners often report getting sick after running, Simpson said, and some researchers think that too much vigorous exercise can inadvertently overstimulate cytokines and inflammation in the body.
Exercising without breaks also depletes the body’s glycogen stores, which for some people can impair immune function for a few hours or days, depending on their basic health, he said.
And working out in groups or participating in intense sports training camps can expose athletes to more pathogens. Other experts point out that people who are physically active can simply monitor their health more closely.
Still, for the average practitioner, early evidence suggests there may be a protective effect against serious illness.
But those who have trouble getting enough exercise or can’t exercise for some reason shouldn’t despair, Ray said. “What helps one person stay healthy compared to another is a complex combination of factors.”
Translated by Luiz Roberto M. Gonçalves
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