Exercise, Immunity, and COVID-19
This post is a response to noone in particular (well…maybe), but a response to a variety of tweets and (mis)information that I’ve seen delivered to athletes during this unique epidemic time.
The the impetus for this response was actually one particular tweet I saw a few days ago (see below).
Now…there is nothing inherently WRONG with this tweet, but it made me a bit skeptical. For one, the word “irresponsible” sat with me wrong. It seemed a bit harsh.
But the main reason this tweet resonated with me was the fact that I didn’t really know whether there was any truth to it or not. Like most physiologists and athletes, I had often heard myself that vigorous exercise compromises your immune system.
In effect, the consensus is that doing an extremely hard race or workout leaves you “immunocompromised” — putting you at a higher risk for an infection or illness in the hours and days after stressful physical activity.
We aren’t talking about exercise “training” here. Exercise training is known to boost immune system function (so long as you don’t overtrain) and people who are “fitter” have better immune systems.
Rather, we are talking about how a single bout of exercise influences immune system function in the subsequent ~24 hours.
I guess it’s an idea worth considering for athletes and anyone looking to keep up an exercise routine (or start one) during the COVID-19 epidemic. Sure, exercise is good, but could doing “too much” leave you more at risk for Corona (or any) virus and potentially place your health, and that of others, in peril?
Should high-level athletes tone down their training regimen to reduce their infection risk?
I did some digging, and discovered an interesting review article published recently (2018) on the very topic. Even better was the fact that this review set out to DEBUNK the “myth” that exercise compromises the immune system.
I decided to give this review article a read, and the rest of this post is a summary of the evidence found within.
The title of this review, published in Frontiers in Immunology, is quite convicting: “Debunking the Myth of Exercise-Induced Immune Suppression: Redefining the Impact of Exercise on Immunological Health Across the Lifespan.”
The review is split into three parts, two of which this article will focus on. The first part (A) dives into the origins of the idea that exercise is “immune suppressing” — where did we get this concept from anyway? How did this idea proliferate? All that is covered here.
Part B provides an overview of some convincing experimental studies that look at how exercise actually influences certain cells in the immune system, whether or not these cells are suppressed, and perhaps if exercise can actually BOOST our response to an infection.
Part C deals with how regular exercise (exercise training) improves immune health and function, particularly in aging. Tl;dr — exercise helps.
Let’s start with part A: “Is it Time to Close the Shutters on the Open Window Hypothesis?”
What is the “open window hypothesis”?
This is the idea (ahem, “myth”) that a single strenuous and prolonged bout of (aerobic) exercise is detrimental to immune health.
The “open window” refers to the hours and days after exercise where immune system function is suppressed; representing a time when you might be more susceptible to opportunistic infections of bacterial or viral origin.
This hypothesis formed because of three (mistaken) principles that have been long-accepted in the research community based on observational and some experimental evidence. There is the belief that:
- Infection risk is increased after a bout of prolonged and vigorous exercise
- Strenuous exercise reduces levels of something called immunoglobulin A (IgA) in the saliva. IgA is one of our primary antibodies that mounts a fight against foreign invaders (bacteria and viruses)
- Reduced immune cells in the blood following exercise represent a period of immune system suppression
But does the research ACTUALLY support these principles?
Idea #1 originated from studies looking at whether there was a greater incidence of infections in athletes during the weeks after they participated in a strenuous running event (i.e. marathon or ultramarathon). Some of these studies found that a higher percentage of athletes experienced an upper-respiratory tract infection (URTI) in the weeks after an ultramarathon vs. a “control” population who didn’t race.
Even more, the faster runners had a higher incidence of URTI symptoms.
Other studies found that marathon runners, compared to a group of non-runners, had a higher prevalence of infections in the weeks after the race.
These studies have flaws. For one, the infections in the runners were never “validated” by any sort of medical or laboratory examination. Self-reported symptoms are hardly the “gold standard” and really limit the robustness of these findings.
The authors of this review also point out that several external factors may be responsible for more infections independent of exercise. For instance; anxiety, psychological stress, nutritional deficiencies, travel, poor sleep, and being around thousands of people on race day could also up your risk of infection (social distancing, people!!!)
Ergo, these studies are hardly definitive evidence that extreme endurance activity actually increases one’s susceptibility to an infection.
Conclusions:
“…it is unlikely that vigorous and prolonged exercise heighten the risk of infections and should not be considered a deterrent to those seeking to become more physically active.”
Next, the discussion turns to how levels of immune cells are influenced by acute exercise.
As mentioned before, immunoglobulin A (IgA) is an immune cell that inhibits inhibiting invading pathogens that find their way into our body.
Studies show that following exercise, IgA levels in the the saliva are reduced vs. pre-exercise levels; though not all studies report a reduction. This might suggest that reduced IgA represents a temporarily impaired immune system.
However, while not getting into specifics, the authors note that there are many limitations with simply measuring salivary IgA and making a broad conclusion about overall systemic immune function. It’s probably not a comprehensive picture of immunity, and therefore any conclusions made on how exercise influences overall immune function per se, are limited by this measurement.
Another type of immune cells are called lymphocytes. DURING exercise, lymphocytes in the blood are increased. AFTER exercise, lymphocyte levels fall. This indicates an acute immune system stimulation during exercise, and a fall back to baseline/below baseline (i.e. suppression) after exercise ends.
This reduction in lymphocytes represents the “open window” of increased infection risk; occurring about 1–2 hours after exercise ends, with immune cell numbers returning to normal after around 24 hours.
But, this acute “reduction” in lymphocyte number after exercise might not be a true “reduction” after all.
That’s right. In fact, the authors claim that immune system surveillance is actually enhanced after exercise. Rather than a decrease in cell numbers, exercise actually redistributes immune cells to other areas in the body.
This is termed the “acute exercise immune-enhancement hypothesis.”
Why would the body “redistribute” immune cells after exercise? One hypothesis is that immune cells are deployed to areas where there has been some sort of damage. In particular, the gut, lung, and bone marrow seem to experience a rise in lymphocytes after exercise.
Based on this evidence…one must be cautious in making the grand claims that acute intense exercise depresses immune cell function. In fact, it might do just the opposite.
Conclusions:
“…strong evidence implies that a reduction in the frequency and function of lymphocytes and other immune cells in peripheral blood during the hours following vigorous and prolonged exercise does not reflect immune suppression. Instead, the observed lymphopenia represents a heightened state of immune surveillance and immune regulation driven by a preferential mobilization of cells to peripheral tissues.”
Thus far, we’ve presented some interesting evidence that, rather than depress immune function, exercise could perhaps enhance it. What we KNOW to be true is that physically active and cardiovascularly fit people have better immune system function vs. sedentary people.
But to really make the conclusions about what happens to immune function after exercise, we need experimental studies. The next section in the paper covers this area — with a nice review of some studies that look at how a single bout of exercise influences the body’s response to an infection.
First, let’s talk about the methods behind this. How do you “infect” the body in an experimental study?
This is traditionally done using a vaccine — something like a typhoid vaccine — that gets injected into research participants in order to study the immune response. As we know, a vaccine is really just a low dose of a viral toxin, so this technique simulates the body’s exposure to a virus.
If you have participants complete a bout of exercise, then inject them with a vaccine (or vice-versa) and study the response over the next hours to weeks, you get an idea of how exercise may have influenced the immune response.
Below I summarize three interesting studies on the topic.
Contrary to popular thinking, all of these show that a single exercise bout ENHANCES (or otherwise has no depressive effect) on the immune system response to vaccination in healthy and older adults.
No effect, in this case, can be seen as exercise “doing no harm” on the immune system. I.e. it’s SAFE.
In older adults (55–75 years old), 40 minutes of aerobic exercise had no effect on vaccine efficacy (left).
But two other investigations REALLY studied the effects of strenuous exercise on the immune response to a vaccination. In one, marathon runners were injected with a vaccine immediately after completing the race. Immune system responses were ENHANCED in these runners vs. a control group. While there were only 4 marathoners in this study…it’s still provocative data.
Even in triathletes who had just completed a triathlon and received a vaccination, immune response was no different than that of a non-exercise control group. Immunity wasn’t enhanced…but it wasn’t depressed either.
What’s interesting about these two investigations is that they really study exercise at the extremes! Their results fly in the face of the belief that strenuous prolonged exercise results in temporary immune system suppression.
Of course, these are only two studies…and there are definitely conflicting data out there. It is important to remember that evidence exists for both sides of this hypothesis, and I (and the authors of the review) are presenting only one of them.
Conclusions:
“…there is growing evidence from a powerful array of studies in humans and rodents, indicating that exercise enhances, or at least does not suppress immune responses to in vivo challenge in younger and older individuals. These observations — which contradict those predicated by the “open window” hypothesis — support the contention that an acute bout of exercise has no detrimental immune consequences for health.”
A lot was covered here, so let’s go over some of the concluding remarks and provide a bulleted summary of what the evidence in this review can “conclude”.
- Being physically active and exercising regularly reduces the incidence of both communicable and non-communicable disease in younger and older adults. (I.e. exercise keeps you healthy!)
- There is very limited evidence to support the previous claims that exercise impairs the function of the immune system
- Exercise does NOT heighten the risk of infections
- Exercise may actually ENHANCE immune system responses to bacterial and viral challenges
What does all of this mean in the context of COVID-19? Perhaps nothing…
I’m NOT saying you should go out and exercise to boost immunity against this virus…or any virus for that matter.
I’m also NOT saying you should immediately discard the advice of some to “take it easy” during this time.
What I AM saying is to question advice, especially during this odd period in modern humanity.
What I want to do with this article is provide another side of the coin. There are a lot of inflammatory, hyperbolic, and perhaps unjustified statements going around on Twitter lately (and elsewhere).
I found that telling athletes that training hard is “irresponsible” to be…well…irresponsible. Sure, we are all trying to stay safe and healthy during this time, but fear-inducing statements about doing something you really enjoy are unnecessary, perhaps even stress-inducing.
Maybe a temporary reduction in training intensity could be beneficial or protective. I’m not sure. However, given the data, it seems that if the one outlet you have during this time is a long, hard run or an intense training session…then you should go for it.
Along with staying healthy, we are all trying to stay sane.
References
**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;9:648.
Peters EM, Bateman ED. Ultramarathon running and upper respiratory tract infections. An epidemiological survey. S Afr Med J. 1983;64(15):582–4.
Pedersen BK, Toft AD. Effects of exercise on lymphocytes and cytokines. Br J Sports Med. 2000;34(4):246–51.
Ranadive SM, Cook M, Kappus RM, et al. Effect of acute aerobic exercise on vaccine efficacy in older adults. Med Sci Sports Exerc. 2014;46(3):455–61.
Bruunsgaard H, Hartkopp A, Mohr T, et al. In vivo cell-mediated immunity and vaccination response following prolonged, intense exercise. Med Sci Sports Exerc. 1997;29(9):1176–81.
