By Knvul Sheikh
You’ve probably heard the advice: One of the best things you can do to keep healthy — especially as cold and flu season creeps up — is stay physically active.
This folk wisdom has been around for ages, but until recently, researchers did not have much data to support the idea. Now, scientists studying risk factors related to COVID-19 have turned up some preliminary evidence about the link between regular exercise and better immune defenses against disease.
When researchers reviewed 16 studies of people who stayed physically active during the pandemic, they found that working out was associated with a lower risk of infection as well as a lower likelihood of severe COVID. The analysis, published last month in The British Journal of Sports Medicine, has generated a lot of enthusiasm among exercise scientists, who say the findings could lead to updated guidelines for physical activity and health care policy that revolves around exercise as medicine.
Experts who study immunology and infectious disease are more cautious in their interpretation of the results. But they agree that exercise can help protect health through several different mechanisms.
Exercise could bolster immunity in a variety of ways.
For decades, scientists have observed that people who are fit and physically active seem to have lower rates of several respiratory tract infections. And when people who work out do get sick, they tend to have less severe disease, said David Nieman, a professor of health and exercise science at Appalachian State University, who was not involved in the recent COVID review.
“The risk of severe outcomes and mortality from the common cold, influenza, pneumonia — they’re all knocked down quite a bit,” Nieman said. “I call it the vaccine-like effect.”
The new meta-analysis, which looked at studies between November 2019 and March 2022, found that this effect extends to COVID. People from across the globe who worked out regularly had a 36% lower risk of hospitalization and a 43% lower risk of death from COVID compared with those who were not active. They also had a lower likelihood of getting COVID at all.
People who followed guidelines recommending at least 150 minutes of moderate activity or 75 minutes of vigorous activity per week seemed to get the most benefit. But even those who exercised less than that were more protected against illness than those who did not work out at all.
Researchers theorize that exercise may help fight off infectious bacteria and viruses by increasing the circulation of immune cells in your blood, for example. In some small studies, researchers have also found that the contraction and movement of muscles releases signaling proteins known as cytokines, which help direct immune cells to find and fight off infection.
Even if your levels of cytokines and immune cells taper off two or three hours after you stop exercising, Nieman said, your immune system becomes more responsive and able to catch pathogens faster over time if you work out every day. “Your immune system is primed, and it is in better fighting shape to cope with a viral load at any given time,” he said.
In healthy humans, physical activity has also been linked to lower chronic inflammation. Widespread inflammation can be extremely damaging, even turning your own immune cells against your body. It is a known risk factor for COVID, Nieman said. Therefore, it makes sense that reducing inflammation could improve your chances of fighting off infection, he said.
Research also shows that exercise may amplify the benefits of some vaccines. People who worked out right after getting their COVID-19 vaccine, for example, seemed to produce more antibodies. And in studies of older adults who were vaccinated early during flu season, those who exercised had antibodies that lasted throughout the winter.
Exercise provides a slew of broader health benefits that may help reduce the incidence and severity of disease, said Dr. Stuart Ray, an infectious diseases specialist at Johns Hopkins University School of Medicine. Building a walk, jog, gym trip or sport of choice into your routine is known to help reduce obesity, diabetes and heart disease, for example, all of which are risk factors for severe influenza and COVID. Working out can help you get more restful sleep, boost your mood and improve your insulin metabolism and cardiovascular health, improving 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 overall better health.
The research can only tell us so much.
Dr. Peter Chin-Hong, an infectious disease specialist at the University of California, San Francisco, agreed that more research was needed before scientists could pinpoint a specific mechanism or causal link. In the meantime, he said, it’s important not to put too much faith in it.
“For now, you can’t say, ‘I’ll go to the gym so that I can prevent getting 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, Ray said. “People exercise in many different ways.”
Study participants typically self-report the amount and intensity of their exercise, which can often be inaccurate. And just expecting exercise to be beneficial can provide a powerful placebo effect. As a result, it can be hard for researchers to tell exactly how much exercise or what type is ideal for immune function. It’s also quite possible that people who work out regularly may share other attributes that help them fight off infections, such as a varied diet or better access to medical care, Ray said.
Beyond that, “there is a huge debate about whether or not too much exercise makes you more susceptible to infection and illness,” said Richard Simpson, who studies exercise physiology and immunology at the University of Arizona.
Marathon runners often report getting sick after races, Simpson said, and some researchers think that too much vigorous exercise could inadvertently overstimulate cytokines and inflammation in the body. Exercising without a break also depletes the body’s glycogen stores, which for some people could lead to impaired immune function for a few hours or a few days, depending on their baseline health, he said. And working out in group settings or attending intense sports training camps could be exposing athletes to more pathogens. Other experts point out that people who are physically active might simply keep closer track of their health.
Still, for the average exerciser, early evidence suggests there may be a protective effect against getting severely ill. But those who have trouble getting enough exercise or can’t exercise at all for some reason shouldn’t despair, Ray said. “What helps one person stay healthy compared to another is a complex mix of factors,” he said.