Staying active: the key to a strong immune system?
You've probably heard it many times: One of the best ways to stay healthy - especially now that cold and flu season is upon us - is to stay active. This wisdom has been around for a long time, but only recently have researchers been able to back it up with data. Now, scientists studying risk factors related to Covid-19 have found evidence of a link between regular exercise and better immune defense.

New research results: Exercise as protection against Covid-19
When researchers reviewed 16 studies of people who stayed 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. These findings have caused a stir in the sports science community. Experts hope these results could lead to updated physical activity guidelines and health policy that consider exercise as medicine.

Cautious Interpretation: What Experts Say
Experts who study immunology and infectious diseases are more cautious in their interpretation of the results, but they agree that exercise can help protect your health in several ways.

How exercise strengthens the immune system
Exercise could strengthen your immune system on several levels.

Fewer infections through regular exercise
For decades, scientists have observed that people who are fit and active seem to get fewer respiratory infections. And when people who exercise do get sick, they often have milder cases, says David Nieman, professor of health and exercise science at Appalachian State University. "The risk of severe illness and death from colds, flu, pneumonia - all of that is significantly reduced," says Dr. Nieman. "I call it the vaccine-like effect."

The study situation: exercise and Covid-19
A meta-analysis that examined studies between November 2019 and March 2022 found that this effect also applies to Covid-19. Globally, people who exercised regularly had a 36 percent lower risk of being hospitalized and a 43 percent lower risk of dying from Covid compared to those who were inactive. They were also less likely to get Covid in the first place.

How much exercise does your immune system need?
People who followed the guidelines and did at least 150 minutes of moderate or 75 minutes of vigorous activity per week had the best results. But even those who exercised less were better protected against disease than those who did not exercise at all.

Why exercise supports the immune system
Researchers suspect that exercise may help fight off infectious bacteria and viruses by increasing the circulation of immune cells in your blood. In some smaller studies, researchers also found that muscle contraction and movement releases signaling molecules called cytokines that help immune cells fight infection.

Keeping the immune system in top shape
Even though your cytokine and immune cell levels drop two or three hours after exercise, Dr. Nieman said, if you exercise every day, your immune system will be more responsive and able to fight off pathogens more quickly. "Your immune system will be better prepared and in better shape to handle a viral load," he says.

Less inflammation through exercise
In healthy people, physical activity is also associated with lower chronic inflammation. Inflammation can be extremely damaging because it can even cause your own immune cells to turn against your body. This is a known risk factor for Covid-19, Dr. Nieman said. It makes sense, then, that reducing inflammation could improve your chances of fighting off infection.

Exercise and vaccinations: an unbeatable combination?
Research also shows that exercise can enhance the benefits of vaccines. People who exercised right after their Covid-19 vaccination, for example, produced more antibodies. And in studies of older adults vaccinated early in flu season, those who exercised had antibodies that lasted throughout the winter.

Holistic Health Benefits of Exercise
Exercise offers a host of other health benefits that can help reduce the occurrence and severity of illness, says Stuart Ray, MD, an infectious disease expert at Johns Hopkins University School of Medicine. For example, adding a walk, jog, gym session or exercise to your routine has been shown to help reduce obesity, diabetes and heart disease - all risk factors for the flu. Exercise can also help you sleep better, boost your mood and improve your insulin and cardiovascular health, which in turn increase your chances of fighting off the flu and similar illnesses. It's hard to say whether the benefits come from direct changes to the immune system or simply from better overall health, Dr. Ray says.

The Limits of Research: What We Know – and What We Don’t
Research can only help us to a limited extent.

More research needed: The opinion of the experts
Dr. Peter Chin-Hong, an infectious disease specialist at the University of California, San Francisco, agrees that more research is needed before scientists can determine a specific mechanism or causal link. In the meantime, however, it is important not to rely too heavily on it. "You can't say, 'I go to the gym to prevent Covid,'" says Dr. Chin-Hong.

The challenge: to measure movement scientifically
The problem with studying the exact effect of exercise on the immune system is that exercise isn't easily measured on a linear scale, Dr. Ray says. "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 the expectation that exercise is beneficial can have a strong placebo effect. That's why researchers have a hard time saying how much exercise or what type is ideal for immune function. It's also quite possible that people who exercise regularly share other characteristics that help them fight off infection, such as a varied diet or better access to health care, Dr. Ray says.

Too much of a good thing: Can excessive exercise weaken the immune system?
In addition, there's a big debate about whether too much exercise makes you more vulnerable to infection and disease, says Richard Simpson, who studies exercise physiology and immunology at the University of Arizona. Marathon runners often report getting sick after races, Dr. Simpson says, and some researchers think that too-intense exercise could inadvertently overstimulate cytokines and inflammation in the body. Exercise without rest also depletes the body's glycogen stores, which could cause impaired immune function for some people for a few hours or days, depending on their baseline condition. And training in groups or attending intense sports camps could expose athletes to more pathogens. Other experts point out that people who are physically active may simply keep better track of their health.

What you should know: Exercise and protection from disease
Still, early evidence suggests there may be a protective effect against serious disease for the average exerciser. But those who struggle to get enough exercise or are unable to exercise for some reason should not despair, says Dr. Ray. "What helps a person stay healthy is a complex mix of factors."