You’ve probably heard the advice: one of the best things you can do to stay healthy — especially during cold and flu season — is to stay physically active.
As popular wisdom this has been circulating for a long time, but until recently researchers did not have much data to support the idea.
Now, scientists studying risk factors related to COVID-19 have found some preliminary tests on the relationship between regular exercise and improved immune defenses against disease.
Exercise as medicine?
When they reviewed 16 studies on people who were physically active during the pandemic, a team of researchers found that exercising was associated with a lower risk of infectionas well as a lower chance of contracting severe COVID.
The analysis, published last month in the journal The British Journal of Sports Medicinehas led to great excitement among exercise scientists, who say these 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 their interpretation of the results. But they do agree that exercise can help protect health through several different mechanisms.
Exercise May Boost Immunity in Several Ways
For decades, scientists have observed that people who are fit and physically active seem to have a lower rate of various respiratory tract infections.
And when exercisers get sick, they tend to have a less serious illnessnotes David Nieman, a professor of health and exercise sciences at Appalachian State University, who was not involved in the recent COVID review.
“The risk of serious consequences and mortality from the common cold, influenza and pneumonia are greatly reducedNieman says. “I call it the vaccine-effect.”
The new meta-analysis, which examined studies conducted between November 2019 and March 2022, found that this effect extends to COVID.
Worldwide, people who exercised regularly had a 36% lower risk of hospitalization and a 43% lower risk of death by COVID, compared to those who were not active. He also experienced a lower chance of contracting COVID overall.
People who followed guidelines that recommend at least 150 minutes of moderate activity or 75 minutes of vigorous activity per week seem to have gotten the biggest benefits. But even those who exercised less were more protected against the disease than those who did nothing.
A better prepared system
The researchers maintain that the theory that exercise can help fight infectious bacteria and viruses is because it increases the circulating immune cells in the blood, for example.
Some small studies have also found that muscle contraction and movement release signaling proteins known as cytokines, which help guide immune cells to find and expel infection.
Even if your levels of cytokines and immune cells decline two to three hours after you stop exercising, Nieman explains, the immune system becomes more responsive and fit to trap pathogens more quickly over time if you exercise every day.
“Your immune system is more prepared and is in better shape to cope with a viral load at any given time,” he adds.
In healthy humans, physical activity has also been linked to less chronic inflammation.
Generalized inflammation can be very damaging and even turn the body’s own immune cells against the body. It is a known risk factor for COVID, Nieman clarifies. So it makes sense that reducing inflammation might improve your chances of fighting infection, he says.
Research also shows that exercise can extend the benefits of some vaccines.
Those who exercised immediately after receiving the COVID-19 vaccine, for example, appeared to produce more antibodies. And according to studies of older adults who got vaccinated early in the flu season, those who exercised had antibodies that lasted all winter.
An infectious disease specialist at Johns Hopkins University School of Medicine, Dr. Stuart Ray argues that exercise provides a number of broader health benefits that can help reduce the incidence and severity of the illness.
Benefits of exercise: fewer risk factors
Incorporating a walk, a jog, a visit to the gym or the sport of your choice into your routine is known to help reduce obesity, diabetes and heart disease, for example, all of which are risk factors in what makes severe flu and COVID.
Practicing exercise can help you have a more restful sleep, boost your mood and improve your insulin metabolism and cardiovascular health, which improves your chances against the flu and COVID.
It’s hard to know, Ray says, whether the benefits come from direct changes in the immune system or simply from a better overall health.
Research can inform us only to a certain extent
Peter Chin-Hong, an infectious disease physician at the University of California, San Francisco, agrees that more research is needed before scientists can pinpoint a specific mechanism or a causal relationship.
In the meantime, says Dr. Chin-Hong, it’s important not to set your expectations too high.
“For now, you can’t say ‘I’m going to the gym to avoid catching COVID,'” he warns.
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, specifies the specialist. “People exercise in many different ways,” adds Dr. Ray.
Study participants often report the amount and intensity of their exercise, which can often be inaccurate.
Furthermore, the mere expectation that exercise will be beneficial can generate a powerful Placebo effect.
That is why it is difficult for researchers to know precisely how much and what type of exercise it is great for immune function.
It’s also quite possible that people who exercise regularly report other factors that help them fight infections, such as a varied diet or better access to medical care, according to what Ray reports.
And the excess?
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 say they get sick after races, Simpson says, and some researchers think exercising too vigorous could inadvertently overstimulate cytokines and inflammation in the body.
Exercise without breaks too depletes glycogen storeswhich in some people could lead to impaired immune function for a few hours to a few days, depending on their preliminary health status, he says.
Also, exercising in a group or going to intense sports training camps could expose athletes to more pathogens. Other experts point out that physically active people may simply be more in control of their health.
However, for the average athlete, early evidence suggests there may be a protective effect against getting seriously ill.
But those who have trouble getting enough exercise or can’t for some reason shouldn’t give up hope, concludes Dr. Stuart Ray. “What helps one person stay healthy compared to another is a complex mix of factors.”
© The New York Times // Translation: Román García Azcárate
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