Can exercise boost your immunity? Yes, but overexercise could reduce it

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 lore has been around for a long time, but until recently, researchers didn’t have a lot of data to back up the idea. Now, scientists studying risk factors related to COVID-19 have found preliminary evidence of a link between regular exercise and better immune defenses against disease.

When researchers reviewed 16 studies of people who were physically active during the pandemic, they found that exercising was associated with a lower risk of infection, as well as a lower chance of severe Covid. The analysis, published in the British Journal of Sports Medicinehas generated much excitement among exercise scientists, who say the findings could lead to updated guidelines for physical activity and health care policy revolving around exercise as medicine.

Experts who study immunology and infectious diseases are more cautious in interpreting the results. But they do agree that exercise can help protect health through several different mechanisms.

Exercise May Boost Immunity

For decades, scientists have observed that people who are fit and physically active appear to have lower rates of various respiratory tract infections. And when people who exercise do get sick, they tend to have less severe illness, said David Nieman, a professor of exercise and health sciences at Appalachian State University, who was not involved in the recent Covid review. “The risk of serious outcomes and mortality from the common cold, influenza, pneumonia, they’re all reduced quite a bit,” Professor Nieman said. “I call it the vaccine effect.”

The new meta-analysis, which looked at studies between November 2019 and March 2022, found that this effect extends to Covid. 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 people who were not active. They also had a lower chance of contracting Covid.

People who followed guidelines that recommended 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 were more protected against disease than those who did nothing.

Researchers theorize that exercise may help fight infectious bacteria and viruses by increasing the circulation of immune cells in the blood, for example. In a few small studies, researchers also found that muscle contraction and movement release signaling proteins known as cytokines, which help immune cells find and fight infection.

Even if your levels of cytokines and immune cells decline two to three hours after you stop exercising, Professor Nieman said, your immune system becomes more receptive and able to catch pathogens faster over time if you exercise every day. “His immune system is primed and in better shape to deal with a viral load at any 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, Professor Nieman said. So it makes sense that reducing inflammation might improve the chances of fighting infection, he said.

Research also shows that exercise can amplify the benefits of some vaccines. People who exercised right after receiving the Covid-19 vaccine, for example, seemed to produce more antibodies. And in studies of older adults who got 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 Dr. Stuart Ray, an infectious disease specialist at Johns Hopkins University School of Medicine. Including a walk, jog, trip to the gym, or the sport of your choice in your routine is known to help reduce obesity, diabetes, and heart disease, for example, all of which are risk factors for severe influenza and the covid. Exercising can help you get more restful sleep, improve your mood, and improve your insulin metabolism and cardiovascular health, improving your chances against the flu and covid. It’s hard to know, Dr. Ray said, whether the benefits come from direct changes in the immune system or simply better overall health.

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 is needed before scientists can pinpoint a specific mechanism or causal link. In the meantime, he told himself, it’s important not to rely on it too much. “Right now, you can’t say, ‘I’m going to the gym so I can avoid getting Covid,’” Dr. Chin-Hong said. The problem with studying the precise effect of physical activity on immunity is that exercise isn’t something that scientists can easily measure on a linear scale, Dr. Ray said. “People exercise in many different ways.”

Study participants often 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 difficult for researchers to say exactly how much exercise or what type is ideal for immune function. It’s also quite possible that people who exercise regularly share other attributes that help them fight infection, such as a varied diet or better access to health care, Dr. Ray said.

Beyond that, “there’s a lot of debate about whether too much exercise makes you more susceptible to infection and disease,” 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 believe that too much vigorous exercise could inadvertently overstimulate cytokines and inflammation in the body. Exercising without rest also depletes the body’s glycogen stores, which for some people could lead to impaired immune function for a few hours or days, depending on their initial health, she said. And exercising in groups or attending intense sports training camps could expose athletes to more pathogens. Other experts point out that people who are physically active may simply want to monitor their health more closely.

Still, for the average athlete, preliminary evidence suggests there may be a protective effect against serious illness. But those who have trouble getting enough exercise or who can’t do anything for some reason shouldn’t despair, Dr. Ray said. “What helps one person stay healthy compared to another is a complex combination of factors,” he said. – This article originally appeared on the New York Times