Exercise and Immune Function
By Eric Carter, PhD.
With contributions by Ryan Kerrigan BSc., Assaf Yogev MSc., & Jem Arnold MPt.
In the last several weeks, many athletes have found themselves with extra time to train, or using exercise as a stress management tool. An abundance of anecdotal information indicates that intense exercise may suppress immune system function and increase risk of viral and bacterial infection. However, this link does not appear to be borne out in the most recent scientific literature.
The connection between exercise and immune function is often described as a J-curve (Fig 1; ). Compared to a sedentary lifestyle, regular cardiovascular exercise results in a nearly 50% reduction in upper-respiratory tract infections (URTI). Regular moderate exercise is related to a host of additional positive health outcomes as well. It’s clear that physical activity is an important component in overall health.
With no exercise, and moderate exercise represented on the left side of the J-curve, the question becomes, at what point does an acute bout of high intensity and duration exercise become detrimental, increasing the risk of infection? To answer this, we’ll look at some more of the existing research.
Mass Participation Events
There is convincing evidence that mass participation in events (i.e. ultras, marathons, ironman, etc.) results in increased rates of URTIs. There’s methodological flaws in some of these studies but generally, they provide evidence for increased risk of infection at mass participation events. This applies to non-sporting events as well; when religious pilgrims gather at large festivals, URTI rates rise as high as found at exercise events .
One commonly cited study examined 141 runners who participated in a 56km ultra-marathon compared to 124 control participants . In the two-weeks following completion of the event, the runners reported two-times greater incidence of illness (33% vs. 15%). Another followed 1,828 athletes participating in the Los Angeles Marathon and compared them to 134 control participants . In the week following the competition, runners reported a six-fold increase in self-reported infectious episodes (13% vs. 2%)
At first glance, it seems these studies indicate a relationship between intense exercise and increased risk of infection. It’s important to remember however that these studies contain a confounding variable – participation in the race itself. The connection between mass participation events and an increased infection risk is logical. We can easily imagine all the touchpoints for transmission that occur at a big race: package pickup, start corral, M&M jars at aid stations, and so on. There are additional confounding variables of travel, sleep disruption, and stress that have been shown to impact immune function . The problem with using these studies as evidence against intense exercise however is that they do not separate the effects of the mass participation event and the individual bout of intense exercise. These studies merely tell us that risk for infection is increased due to participation in the event. To answer our question, more specific research is needed to separate event participation from acute exercise.
Acute Intense Exercise and Immune Markers
There’s also significant (and high quality) evidence that very high intensity bouts of exercise result in redistribution of immune markers in the body (salivary immunoglobin-A, lymphocyte functional capacity, NK cells, etc.; [6,7]). I won’t describe individual studies here but due to the nature of measuring immune markers, these studies take place in a controlled laboratory setting, isolated from many of the possible transmission points of a race or event. Athletes of various levels of training are asked to perform a fixed work task of various intensities and duration. The majority of studies show a transient change in immune markers immediately post-exercise. This is what was originally termed the “open window effect” and the basis for the recommendation that exercise compromises immune function .
More recently however, researches have called into question the immune significance of this redistribution. Is this a reduced availability, indicating immune suppression? Or as proposed by Campbell et al. , is this increased immunosurveillance, indicating a boost in immune function?
Photo by Mark Smiley.
Link to Infection Risk
To reach a conclusion, it’s necessary to connect the high intensity bout of exercise and resulting redistribution of immune markers described above with a change in risk of infection but independent of participation in a large event (Fig 2). Unfortunately, none of the existing literature has examined this in a rigorous manner, so it is our conclusion that evidence does not support the theory that high intensity bouts of exercise alone increase risk of viral or bacterial infection in otherwise healthy and well-trained athletes. This is confirmed by the several recent reviews of exercise and immune function [5,9,10] as well as a recent Spotlight report on athlete health during COVID-19 published in The Lancet .
Overtraining and Immune Function
But what about very high intensity/duration? This is the very top right of our J curve in Figure 1. Chronic overtraining has been linked to changes in immune function and is associate with several illnesses. Training should always be periodized and structured to avoid overtraining – an inherent decrement in performance. A single very-intense bout of exercise, even a hard marathon or ultra, for a sufficiently trained athlete will not lead to overtraining.
Even in normal times, athletes should not complete intense exercise while exhibiting symptoms of infection. If for no other reason, that it will not be possible to achieve the desired training (and more importantly recovery) goals of the session while ill.
For healthy athletes in the age of COVID-19, we recommended that you:
- Maintain physical distancing. Avoid mass participation events, mass transit, and touch-points where viruses may be transmitted.
- Wash your hands, especially after visiting high-traffic areas.
- Continue to train at levels that you are accustomed to. Ensure proper periodization and structure to avoid overtraining.
- Fuel well. Carbohydrate availability has significant impact on immune function . If you are worried about risk of infection, it’s not the time to be trying running fasted or eliminating carbohydrates from your training diet.
- Emphasize recovery. Prioritize sleep duration and quality, use short walks, and ensure a comfortable working environment. Your training load may need to adapt to account for changes in non-training stress.
- Maintain consistency. Working from home can offer greater control over your daily training routine. Consistency in training is another sure way to minimize training stress balance, without having to worry about making it to the office on time.
Sources: Nieman DC. Exercise, Infection, and Immunity. Int J Sports Med. 1994;15(S 3):S131–41.  Choudhry AJ, Mudaimegh Al KS, Turkistani AM, Hamdan Al NA. Hajj-associated acute respiratory infection among hajjis from Riyadh. EMHJ. 2006;12(3/4).  Peters EM, Bateman ED. Ultramarathon running and upper respiratory tract infections-an epidemiological survey. SA Medical Journal. 1963;64(1):582–4.  Nieman DC, Johanssen LM, Lee JW, Arabatzis K. Infectious episodes in runners before and after the Los Angeles Marathon. J Sports Med Phys Fitness. 1990;30(3):316–28.  Campbell JP, Turner JE. Debunking the Myth of Exercise-Induced Immune Suppression: Redefining the Impact of Exercise on Immunological Health Across the Lifespan. Front Immunol. 2018;9:541.  Peake JM, Neubauer O, Walsh NP, Simpson RJ. Recovery of the immune system after exercise. J Appl Physiol. 2017;122(5):1077–87.  Nieman DC. Immune response to heavy exertion. JAP. 1997;82(5):1385–94.  Nieman DC, Pedersen BK. Exercise and immune function. Recent developments. Sports Med. 1999;27(2):73–80.  Nieman DC, Wentz LM. The compelling link between physical activity and the body’s defense system. Journal of Sport and Health Science. 2019;8(3):201–17.  Simpson RJ, Campbell JP, Gleeson M, Krüger K, Nieman DC, Pyne DB, et al. Can exercise affect immune function to increase susceptibility to infection? Exercise Immunology Review. 2020;26:8–22.  Hull JH, Loosemore M, Schwellnus M. Respiratory health in athletes: facing the COVID-19 challenge. Lancet Respir Med. 2020;0(0).
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