One of the most common questions I get asked as a coach and a full-time emergency physician is, ‘Will training at higher intensity actually contribute to making me sick?’ This question has taken on particular significance in 2020 in the context of the pandemic and the ongoing swath of illness and death that it has around the world. In this article, I will try to clarify how exercise affects the immune system.
Let’s Talk About the Immune System
The immune system is incredibly complex and made up of several different cell types and organs. In addition, numerous hormones modulate the function of the system and can impact susceptibility to infection. At its root, the function of the immune system is to recognize self from non-self. That is to say, the immune system is set up to recognize invaders and repel them using a wide variety of effective, and in many cases violent, measures.
Early research on exercise and immune function showed very clearly that those who exercise regularly tend to have more robust immune systems than those who do not. This can manifest as lower rates of respiratory infections during the cold season, among other types of measures.
High-Intensity & Internal Science
And yet, it has long been recognized that elite athletes have a propensity for developing respiratory illnesses after intense competitions. The reasoning for this was not clearly understood for quite some time, but eventually, researchers recognized that this had something to do with a decrease in the secretion of a non-specific immunoglobulin.
Immunoglobulin A, or IgA, is commonly found in respiratory and gastrointestinal tracts where it binds non-specifically to invading viruses and other infectious organisms. IgA provides only a relatively modest first-line of defense, but when levels of this immunoglobulin fall, the result can be susceptibility to infection.
Other markers of immune function also tend to fall immediately after intense efforts. In each case, recovery to normal levels takes about 24-48 hours. This period of one-to-two days during which immune function is depressed now referred to as the ‘open window’ to infection.
So, Why Do Elite Athletes Get Sick Often?
Putting together these two contrasting pieces of information, we can see that regular, low-to-moderate intensity exercise strengthens the immune system and confers some degree of protection from illness. Additionally, high-intensity efforts lasting more than several hours can cause a transient decrease in immune function and leave athletes susceptible to infection over a brief window of time. At this point, it is important to add that the level of immune function is not the only factor determining whether or not an athlete becomes ill. An individual needs both susceptibility and exposure to develop a respiratory illness.
A Tale of Two Athletes
Consider two athletes. One is training at a low-to-moderate intensity and building a robust immune system from their dedication to their routine. However, their roommate has come down with a nasty cold. And in sharing the same living space, the athlete is exposed repeatedly to viral particles over several days. They eventually catch the virus and become sick.
The second athlete undertakes a specific challenge, say an Everesting cycling attempt. After thirteen grueling hours on the bike, they are spent but content with their accomplishment. For the next two days, this athlete is relatively immunosuppressed. However, this athlete stays at home alone, is unexposed to anyone contagious, and therefore does not become ill.
You can see from this example that the effect on immune function is only one small part of the picture. In order to thoughtfully decide how to train, an athlete needs to consider a more global assessment of risk.
What Your Athletes Can (and Should!) Do To Stay Healthy
So, how can athletes who undertake prolonged high-intensity efforts protect themselves from contracting a contagious disease during the open window of depressed immune function?
Despite what you have heard and what many will continue to say, there are simply no supplements or dietary measures that enhance immune function. No vitamins, minerals or specific foods have ever been shown to effectively prevent infections of any kind (with the exception of clostridium difficile, an entirely different subject). Maintaining a balanced, healthy diet is a always good idea, so doing so after these kinds of efforts is no different, just don’t expect that adding supplements or focusing on blueberries will make any difference.
As we have seen with COVID, and it is true for all contagious illnesses, the only way to effectively prevent infection is to simply to avoid exposure. And when that cannot be done, one should take all the precautions necessary to minimize infection. Social distancing, wearing a mask, meticulous hand-washing and eye protection are all best practices. These will help keep even the most immunosuppressed individuals safer.
The take-home message here is, don’t let the fear of becoming sick get in the way of pursuing big efforts or special challenges—just take precautions to keep yourself healthy afterward. Train hard and train healthy!
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2. Gleeson M. Immune function in sport and exercise. J Appl Physiol. 2007;103(2):693-699. doi:10.1152/japplphysiol.00008.2007
3. Walsh NP, Oliver SJ. Exercise, immune function and respiratory infection: An update on the influence of training and environmental stress. Immunol Cell Biol. 2016;94(2):132-139. doi:10.1038/icb.2015.99
4. Metz JP. Upper respiratory tract infections: who plays, who sits? Curr Sports Med Rep. 2003;2(2):84-90. http://www.ncbi.nlm.nih.gov/pubmed/12831664. Accessed October 15, 2019.