We have known for some time that moderate exercise is linked to improved immune function. 

Physical activity seems to boost the innate immune system, in part by increasing the production of macrophages. This is particularly true for regular, daily activity, and the results of maintaining a regular amount of physical activity are desirable. For instance, people who walk 40 minutes per day have been shown to have half as many sick days due to upper respiratory infections.

This kind of data is probably a major reason why government officials have continued to allow us to go to parks and take exercise outside, even during the lockdowns. One could argue that being sedentary is actually a risky proposition at a time like this.

But what about strenuous, prolonged exercise? 

You might have heard before that people who exercise very hard are actually more prone to getting sick. As someone who likes doing high-intensity interval training, this notion gave me pause. Was it possible that an ambitious exercise program could actually put someone at greater risk of catching an infectious disease?

The Open-Window hypothesis

The basis for reservations about hard exercise is encompassed by the Open-Window hypothesis, which suggests that the immune system is suppressed for about 3-72 hours after vigorous exercise, which produces a transient increase in the risk of developing an infection. 

So, where did this idea come from?

It has long been thought, based largely on observational evidence, that intense physical activity might result in compromised immunity. Ernst Jokl, a pioneer in the field of sports medicine, wrote in 1977:

Physical fitness established by athletic training does not enhance immunological resistance against infections. At times, it renders athletes more susceptible.

But it wasn’t until relatively recently that studies were performed to determine whether incidence of infectious disease was indeed greater in athletes after bouts of activity.

A persuasive example is an epidemiological study of applicants for the 1987 Los Angeles Marathon. This study cleverly compared individuals who completed the marathon to athletes who withdrew from the race (for reasons other than illness), the latter of whom could serve as a sort of control. They found that 12.9% of those who completed the race reported an infection in the week after the race, compared to 2.2% of those who had withdrawn. In other words, the folks who participated in the race were at nearly six times greater odds of developing an infection. Other similar studies have echoed these findings.

However, some researchers have begun to question how persuasive this evidence really is. 

Are these symptoms actually caused by infectious agents?

One problem with the kinds of studies that I described above is that these are self-reported respiratory infections. In other words, none of them were validated through any kind of objective lab analyses, like PCR or bacterial cultures. 

Fortunately, more recent research has actually performed that kind of testing to confirm the presence of infectious disease, and their findings make the relationship between intense exercise and infectious disease much more murky. In one such study, clinicians collected nasopharyngeal and throat swabs from athletes and sedentary controls who reported upper respiratory symptoms over a 5-month period. Infectious agents were found in just 11 out of 37 episodes of illness. That means only about 30% of people who seemed to have an infection actually had a positive diagnosis for disease of bacterial or viral origin!

You might be wondering what else could be producing these issues, if not a pathogen. Well, the most obvious culprits are asthma and allergies, whose symptoms can definitely resemble a cold. It may also be due to mucosal inflammation or irritation in the airway due to increased ventilation and exposure to cold air. That might sound kind of weird, but if you’ve ever exercised in an indoor training facility in the winter, you’ve probably experienced this phenomenon.

Okay, but before we dismiss this entirely, let’s take another look at those eleven confirmed infections. It’s a small sample, but it sure does look like more of them occurred in the elite athletes (six in that group, versus just two in controls and three in the recreational athletes). 

That would certainly seem to suggest that intense physical activity does result in greater susceptibility to infection, right? That brings me to the next counter-argument.

Is exercise the cause of the observed infections?

There are actually a ton of confounding variables that make it very difficult to say whether intense exercise itself leads to increased risk of infections, even when you do see an increased rate of infections in a group of athletes.

We know, for instance, that stress has a deleterious impact on the immune system, and athletes who experience psychological stress before a bout of exercise are at greater risk of becoming sick. That is a peril for every single one of us, and perhaps all the more so for athletes who spend enormous amounts of time training and are under immense pressure to perform.

There is a whole constellation of different factors that tend to surround training and competition that could make an athlete more likely to get sick – but that aren’t really caused by exercise directly. 

Here’s an obvious one: sporting events that demand mass participation – like marathons – will inevitably result in greater exposure to infectious disease agents simply due to the large crowd of people. This, of course, is something that all of us are painfully aware of right now. 

That alone would increase your risk of falling ill, even if you were just watching the race from the sidelines! To give you an idea of how big of a deal this is, a staggering 39.8% of individuals who attended the Hajj in Saudi Arabia reported that they developed an acute respiratory infection. This is unlikely to be linked to physical activity.

Then consider the impact of travel, particularly for events like the Olympics or other elite competitions. We all know that flying increases exposure to infectious diseases, and air travel is a major predictor of illness symptoms in athletes specifically. This gets even worse in the context of longer flights crossing multiple times zones, which wreaks all kinds of physiological havoc. Athletes who travel to international destinations with a 5 time zone difference from their home country have a 2-3-fold increased risk of illness. There are plenty of reasons for this – circadian misalignment, altered dietary patterns, dehydration, sleep disruption/restriction, and just loads of stress.

Are we even sure that athletes get sick more frequently?

Finally, it’s also not even totally clear that athletes are more likely to get sick than the general population. 

Large cross-sectional surveys of Olympic athletes have found that around 10% of them report symptoms of illness around the time of competition. Yet in representative surveys of the general population, as many as 24% report experiencing a cold over a one month period. Of course, you can find some studies suggesting that competitive athletes get sick more. Overall, epidemiological evidence in this area seems fairly equivocal.

However, there is ample reason to believe that exercise is generally protective against infections. A large prospective cohort study in Sweden found that higher physical activity levels were linked with significantly lower risk of contracting a respiratory infection. 

What about people who train a whole lot, though? 

One population that might merit consideration are ultramarathon runners, who incur a bigger volume of exercise than any other class of athlete I can think of. Definitely more than me, and probably more than you unless you happen to be one of them. 

Sure enough, they miss fewer days of work and school due to illness than most people. One study of 1212 ultramarathon runners found an average number of just 1.5 sick days, which is pretty enviable.

Key Takeaways

To sum up, there is reason to question whether strenuous exercise, at least in a dose that is realistic for most people, is actually likely to lead to increased risk of infection. To the contrary, it may be protective. Regular exercise has been shown in numerous studies to improve immune function and reduce risk of illness. And for the most part, we should probably be more worried about not getting enough activity, rather than overdoing it.

While observational studies have identified a number of factors that are linked to sickness in athletes performing an acute bout of exercise, they probably either don’t apply to you right now (most of us aren’t traveling much nowadays, or even competing), or they are modifiable (like sleep and nutrition). 

There is, however, another reason why you might want to be more careful with your exercise choices these days: avoiding injuries. This is not an ideal time to be forced to go to the hospital for obvious reasons, and it may be harder to find a physical therapist or doctor depending on where you are. So, you might want to reconsider attempting a PR in box jumps or kipping pull-ups for the time being.

Use some common sense, but don’t worry too much about going hard. Keep exercising regularly, make sure you plan time to recover, and if you train outside, maintain social distance. 

Other Immunity Articles

Sleep and Immunity – Things to Know

Does Elderberry Boost Your Immune System?

Immunity Effects of Edible and Medicinal Mushrooms. Podcast with Jeff Chilton