It’s long been commonly believed that endurance exercise — such as running a marathon — suppresses the immune system, making you more susceptible to catching an infection, like a cold or the flu.
I remember being repeatedly warned of it back in my marathon-training days.
Well, it seems that such warnings are misguided. A new study — one that reviewed more than three decades of research on the topic — reports that instead of depleting the immune system, endurance exercise appears to give it a boost.
“Regular physical activity and frequent exercise are beneficial, or at the very least, are not detrimental to immunological health,” write the study’s authors. Indeed, as they point out in their paper, the evidence strongly suggests that regular and frequent physical activity may limit or delay immunological aging.
The authors conclude “that leading an active lifestyle is likely to be beneficial, rather than detrimental, to immune function, which may have implications for health and disease in older age.”
How the myth began
For the study, James Turner and John Campbell of the University of Bath’s Department of Health examined research published over the past 30-plus years on immunity and endurance exercise. It was three decades ago — during the 1980s — when studies began reporting that marathon and ultramarathon runners were at a higher-than-expected risk of developing symptoms of an infection in the days or weeks after the event. As a result, it became widely assumed that endurance sports made infections more possible by suppressing the immune system.
But, as Turner and Campbell point out, those studies did not confirm the infections in a lab. Recent research has found that most symptoms reported by runners after a marathon are not caused by infections, but by other factors, such as allergies.
“These earlier studies put the cause of the increased risk of ‘infection’ down to a suppressed immune system,” write Turner and Campbell in an article on their research for The Conversation. “Indeed, exercise does have a profound effect on immunity, but we now know that these changes have been misinterpreted.”
Scientists have identified two major ways that exercise changes immune cells, as Turner and Campbell explain:
Initially, during exercise, the number of immune cells in the bloodstream increases dramatically. Some cells, like natural killer cells, which deal with infections, increase tenfold. Then, when exercise finishes, some immune cells in the bloodstream decrease in number substantially, sometimes falling to lower levels than before exercise started for several hours.
Many scientists interpreted this fall in immune cells after exercise to be immune suppression. But we now know that fewer immune cells in the bloodstream for several hours after exercise does not mean that these cells have been lost or destroyed. Instead, they move to sites in the body that are likely to become infected.
One example is the lungs, because faster and deeper breathing during exercise increases the chance of inhaling something infectious. So a low number of immune cells in the bloodstream in the hours after exercise is not immune suppression. Instead, immune cells, primed by exercise, are looking for infections in other parts of the body.
Another reason scientists thought that immunity was compromised after exercise was because some studies reported lower levels of antibacterial and antiviral proteins in saliva after marathons. These proteins, such as immunoglobulin-A (IgA), are the first line of defence against bacteria and viruses entering the body through the mouth and nose. However, many studies at the time did not consider technical problems with the measurement of IgA. For example, IgA levels change in saliva whether you have exercised or not, due to psychological stress, diet, oral health and even a “dry mouth”. But most studies did not consider these factors.
Precautions still needed
Although running a marathon or engaging in some other strenuous exercise is not likely to increase your chances of “catching” a cold or other infection, other factors related to the activity might.
“First, attending any event where there is a large gathering of people increases you chance of infection,” Turner and Campbell point out. “Second, public transport, particularly airline travel over long distances, where sleep is disrupted, may also increase your infection risk. Other factors, like eating an inadequate diet, getting cold and wet, and psychological stress, have all been linked to a greater chance of developing infections.”
So, if you’re planning to run a marathon or compete in another type of strenuous sporting activity, you should take some precautions — just as you would if you’re participating in any large public event.
“It is important to maintain good hygiene,” advise Turner and Campbell. “Wash your hands or use antibacterial and antiviral hand gel. Avoid touching your mouth, eyes and nose. Don’t share water bottles, and minimize contact with people who may have an infection.”
What you should not do is avoid physical activity.
“Given the important role exercise has for reducing the risk of cardiovascular disease, cancer and type II diabetes, the findings from our analysis emphasise that people should not be put off exercise for fear that it will dampen their immune system,” says Turner in a released statement. “Clearly, the benefits of exercise, including endurance sports, outweigh any negative effects which people may perceive.”