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It’s hard to survive a trip through the endurance exercise corner of the Internet without stumbling onto cautionary tales of immune system suppression following a particularly hard or long workout. The actual studies behind those cautionary tales are a bit more nuanced and  leave all of us open to the influence of the anecdotal experiences of headline-driven media and click-driven bloggers. Fortunately, you are reading neither the former nor the latter as a quick perusal of this blog’s analytics shows that I am clearly not a click…driven…blogger…

…Moving on…

Several studies have looked at specific immune system responses to shorter and longer term training. In longer term training (>90 minutes), studies found immune functions like natural killer cell activity and salivary immunoglobulin A (and others) suppressed for several hours. The studies I have read do not conclusively link suppression in any of those systems to a heightened susceptibility to infection, but they hypothesize that end result.

Epidemiological studies have looked at data from marathon runners in and around an event to try to draw conclusions about immune function in the days and weeks following a race, but in my opinion, that pursuit seems a bit misguided. At a major marathon, we arrive with several thousand other people from around the world, all of us carrying with us potential sources of infection, possibly new to others around us. We then crowd shoulder to shoulder in stalls before traipsing through a city, taking food and drink from random strangers every few miles. In other words, there are so many more possible avenues of exposure to infection for a major marathon runner than the average person that comparisons between infection rates seems misguided, at best.

In an animal study, specifically mice, scientists demonstrated that the incidence of infection between sedentary mice and “marathon” mice was similar. Mice undergoing moderate exercise, however, showed substantial immune benefits. Similarly, mice already infected with the flu and undergoing moderate exercise had better outcomes than sedentary or marathon mice. Interestingly, in these mice studies, the sedentary mice often had better results than the marathon mice.

Looking at all of the studies, though, how well were the various subjects controlled for differences in underlying health conditions, nutrition, vitamin and mineral deficiencies, etc. As an endurance athlete, I’ve had a few nutritional issues come up over the years, and they often occur in and around training for or competing in an endurance event. Did any respiratory or other infection I encountered at the time derive from suppressed immune function surrounding a particularly hard or long bout of training, or is it possible a nutritional deficiency or maybe even sleep deprivation was the underlying culprit? 

So how can we apply what appears to be conflicting information to our behavior during covid-19? 

It’s clear from all of these studies that exercise is definitely a net positive for immune function. My own personal experience reinforces that view. Aside from some of the nutritional issues I’ve encountered along the way, my body’s ability to resist infection and recover from infection has drastically improved with more frequent, longer, and more intense exercise. That said, I always give my body time to recover from longer and more intense exercise. You won’t see me out the day after my long run, and if I’m still rundown the second day after, I’ll rearrange my week accordingly.

The one area where all of these studies seem to agree is that moderate exercise is beneficial, both in preventing infection following exposure and in recovering from illness. 

The question you have to ask yourself is what moderate means for your body. As a runner, moderate for me means runs of 10K or less at a fairly comfortable pace (8 min/mile). I might push hard for a mile on a 5K, but most of the time, I’m running at a conversational pace. 

Moderate for you might be walking a 5K or it might be 25K of mountain trails. In the end, it’s a judgement call. If you find yourself rundown at the end of a workout, maybe it’s time to be a bit germaphobic and enhance your social distancing.

References and Further Reading

Moderate exercise protects mice from death due to influenza virus
Thomas Lowder, David A.Padgett, Jeffrey A.Woods
Immune response to heavy exertion
David  C. Neiman
Incidence, etiology, and symptomatology of upper respiratory illness in elite athletes.
Luke Spence, Wendy J. Brown, David B. Pyne, Michael D. Nissen, Theo P. Sloots, Joseph G. McCormack, A. Simon Locke, Peter A. Fricker
Debunking the Myths of Exercise-Induced Immune Suppression
Campbell J.P., Turner J.E.
There is limited existing evidence to support the common assumption that strenuous exercise bouts impair immune competencyCampbell J.P., Turner J.E.