With the current interest in our general health and immune function, one of the things we’ve seen been asked is if an acute bout of exercise or training session is going to have a negative impact. Are we more likely to become ill or catch an illness after a training session? Here is a quick look into some of the science and research behind this.
These studies are all typically in younger, otherwise healthy individuals. The mechanisms may be different for those in at risk groups or people with an underlying illness. It’s also important to remember the importance of good hygiene and dietary practices.
J-Shaped Curve Of Infection
For anyone that has studied exercise medicine or immunology, this curve will be very familiar. In general, this curve illustrates that people that undertake a moderate amount of exercise, typically have a lower risk of a respiratory infection than people that do not do any exercise. But, the risk increases as exercise amount and intensity increases. What has not been well defined though, is what “moderate” is. One person’s heavy training is another person’s moderate.
The Case For And Against
A recent paper (1) had two groups of experts make the case for and against the notion that exercise affects our susceptibility to infection. The case for pointed to the fact that is well established that different markers of immune function decrease after a heavy bout of exercise. However, it was argued that the occurrence of infectious illnesses appears to be no more prevalent in elite athletes than it does in the general public. One thing that was highlighted was that there does appear to be a group of athletes that seem to be more susceptible to infections as they pick up more illnesses, and that this is reflected in differences in their inflammatory response to exercise. Something that may be due to genetic differences.
Infection or Symptoms?
A classic study that made the case for the argument that intense exercise increases our risk to infection took ultramarathon runners, and examined the number that became ill after the race (2). Compared to a control group of runners that did not run the race, twice as many ultramarathon racers reported being ill after the race. This was long argued to be strong evidence that intense (in this case, very intense) exercise could increase our risk of become ill. However, one of the limitations with studies like this is that the participants are never clinically diagnosed and infections are not confirmed. The studies rely on self reporting of symptoms from participants. It has then been argued that some symptoms such as dry throats or coughs, could come from respiratory inflammation, not infection.
Exercise Or The Accumulation Of Stress?
The ‘for and against’ paper (1) finished with quite an important point - exercise is not the only stress in athletes’ or our own lifes. If there is a time when there maybe an increase in the incidence of infectious illnesses in athletes, it is during major international competitions. This is also a time though when long haul travel, sleep loss, change in diet, psychological stress and other factors come to the fore.
So, should we exercise, and exercise hard, during times of increased risk of infection? In young, otherwise healthy individuals, maintaining exercise habits and training programmes is more than likely beneficial. If there is an exercise duration or intensity that might leave us more susceptible to infectious illness, it is currently not known how severe this is, or if the reductions in immune markers measured in the lab have clinical relevance and make us more susceptible to becoming ill.
1 - Simpson, R. J., Campbell, J. P., Gleeson, M., Krüger, K., Nieman, D. C., Pyne, D. B., ... & Walsh, N. P. (2020). Can exercise affect immune function to increase susceptibility to infection?. Exercise immunology review, 26, 8.
2 - Peters, E. D. (1983). Ultramarathon running and upper respiratory tract infections-an epidemiological survey. South African Medical Journal, 64(16), 582-584.
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