Exercise immunology: Part 1 | Part 2
Running and COVID-19: Evidence-based training load & immune health
Thomas Solomon, PhD.
This page contains practical, evidence-based guidance on staying active during COVID-19 while prioritising recovery, using exercise immunology to help runners and obstacle race athletes train smart at home.
The content is for endurance athletes who are basically wondering how hard to train during the pandemic—especially if races are cancelled—and how to protect health while maintaining fitness.
Key takeaway: The regular, moderate-to-vigorous activity that matches public health guidelines supports the immune system, and breaking up long sitting bouts (about every 30 minutes) is part of a solid at-home strategy.
Key takeaway: The frequent prolonged hard training (especially sessions longer than ~1 hour, and extreme events around ~3 hours or more) can temporarily disrupt immune markers; stacking that stress without enough sleep, food, and rest can push athletes toward illness, injury, and chronic fatigue.
Key takeaway: If illness hits, stop training—missing a few sessions now can save weeks or months later—and don’t lean on mega-dose supplement fixes.
But, don’t stop there! Scroll down to fully educate yourself on the details, nuances, and nerdy bits.
I hope that everyone is doing well amidst this chaotic pandemic. Since home quarantine laws were imposed, it is an important time to maintain daily physical activity in pursuit of optimising health and maintaining fitness. Today, my goal is to help you learn a little bit about exercise immunology and why it matters now more than ever.
First up…
What is COVID-19?
Viruses are not living organisms; they are pathogens that contain nucleic acids (DNA or RNA) encapsulated in a protein layer. Viruses can infect animals, plants, and microorganisms like bacteria, which all act as hosts to help the virus translate its nucleic acids into proteins, transfer between organisms, and sometimes between species.
COVID-19 is the disease caused by the most-recently discovered coronavirus. Unless you’ve wrapped your head in several thousand layers of toilet roll, you will know that COVID-19 is serious. The common symptoms are fever, tiredness, and a dry cough, which, in some people, leads to severe respiratory problems. Everyone is at risk, but older people and those with underlying diseases (high blood pressure, heart problems or diabetes) are more likely to develop serious illness. While the virus causing COVID-19 is related to that which caused SARS, it is not the same. COVID-19 is more infectious but less deadly; the good news is that about 80% of people are recovering from COVID-19.
To help prevent further spread, the World Health Organisation issued very clear guidelines:
Wash your hands frequently.
Maintain social distancing.
Avoid touching eyes, nose and mouth.
Practice respiratory hygiene.
If you have a fever, cough and difficulty breathing, seek medical care early.
Stay informed and follow advice given by your healthcare provider.
While some countries are being a little slow on the uptake, many governments have imposed quarantine laws to maximise social distancing and help keep folk from being infected with and transferring the virus to others. For you folks who have recently been ramping up your training loads with the goal of “peaking” for spring races, no doubt you are now frustrated and now wondering what the f to do. Before proceeding with your high-volume, high-intensity training in the confines of quarantine law, I encourage you all to learn a little more about the effect of exercise on immunology.
Make physical activity a habit of daily living.
Regular, moderate-to-vigorous-intensity activity is deeply embedded in public health guidelines. Current guidelines recommend adults to accumulate a minimum of 150-minutes per week of moderate-to-vigorous-intensity aerobic activity, strength building activity on at least 2-days per week, and to minimise sedentary behaviour. One of these reasons for reducing inactivity and increasing activity is to support the optimal function of your immune system. Therefore, maintaining daily activity combined with interrupting prolonged (>30-mins) periods of inactivity (sitting or lying down while awake) is an essential part of your COVID-19 home-quarantine strategy. Doing so will not only keep you metabolically- and psychologically-healthy but also immunologically-healthy.
Keep moving. Stay sane.
Exercise causes a healthy acute stress.
Exercise, like any acute stress, such as trauma, surgery, burns, or sepsis, transiently disrupts the body’s homeostasis. I.e. exercise causes a temporary loss of equilibrium in physiological states: heart rate, breathing rate, and blood pressure go up; muscle glycogen levels go down, etc. In the context of immunology, the change in circulating immune markers following a single bout of exercise is dependent on the duration and intensity, such that light bouts tend to induce a “protective” profile while heavy and prolonged bouts tend to cause a temporary “dysfunctional” immune profile.
Since inactivity is a cause of poor metabolic, psychological, and immunological health, think of exercise as the natural medicine against inactivity-induced disease - too little medicine has no effect, while too much medicine is harmful. A loss of homeostasis caused by exercise is a challenge that stimulates the body to learn to become stronger.
Unhealthy chronic stress = Overly frequent and prolonged vigorous exercise combined with inadequate sleep, poor nutrition, and insufficient rest.
Regular moderate-to-vigorous physical activity in line with public health guidelines enhances immune surveillance by increasing the exchange of immune cells between the circulation and peripheral tissues. This process improves health by protecting against the risk of infection and subsequent illness. For in-depth reviews of the immunological mechanisms, please see the position statement in Exercise Immunology Reviews, here and here, or read the textbook title, Exercise Immunology, edited by Dr Mike Gleeson.
Following a S.H.I.T. (short, high-intensity training) session and following prolonged endurance exercise longer than ~1-hour, stress hormones like cortisol and several markers of immune function are altered for several hours and even several days, in the case of extreme duration exercise of ~3-hours or more. Accordingly, some studies show that the risk of upper respiratory tract infection is elevated in high-performance athletes when compared to the general population. Several lines of evidence also report an association between high training load and increased incidence of illness, and elevated nasal passage colony counts of upper respiratory tract infection-causing pathogens have been reported following prolonged, very vigorous exercise. Such findings have led to a popular view that training loads greatly exceeding recommended activity guidelines can suppress immunity and increase infection risk.
However, scientists have found great difficulty in separating high training load from the many other factors that are not only present in high-performance athletes but are also independent causes of immune suppression and increased infection susceptibility. These include fasting prior to exercise, low muscle glycogen, dehydration, anxiety, low mood, poor sleep, low energy availability, and environmental extremes. Therefore, it is uncertain whether large amounts of exercise are a direct cause of immunosuppression. For an excellent read, I refer you to an academic debate published earlier this year in Exercise immunology review.
Whether exercise is a direct cause of immunosuppression or not, prolonged and/or high-intensity exercise does cause a transient suppression of immunological markers, which, if not allowed to recover, will lead to immunosuppression. Furthermore, exposure to airborne pathogens is increased during exercise due to a deeper and higher rate of breathing. In my experience, the single most abused part of people’s training is their recovery. I am always amazed at how common it is for people to try to squeeze more workouts into a week by sleeping less, rushing meal times, and omitting rest from their days. A constantly high training load combined with poor sleep, poor nutrition, inadequate rest, and other factors causing psychological stress, like family life or work, is a fast-track route to loss of fitness, injury, illness, and chronic fatigue.
Planned periods of “functional” overreaching with unusually high training loads for about seven days once or twice a year are a useful way to induce a peak in fitness at the opportune time. But, overreaching only succeeds when the increased training load is coupled with more sleep and nutritious food, and then followed by a prolonged period of tapering and rest. Inappropriate or nonfunctional overreaching i.e. under-recovery is a cause of overtraining syndrome, which you might have already guessed is characterised by severely compromised immune function and increased infection risk. Overtraining syndrome takes weeks or months to overcome, and some athletes never recover. In this era of COVID-19, don’t run down that trail!
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Should you train when you’re sick?
Stuck at home and not wanting to become a COVID-19 statistic?
Wondering if you should train?
Public health physical activity guidelines keep people healthy. To win an Olympic Gold medal, an athlete’s exercise dose must exceed recommended levels. If you are a performance athlete, you must train hard to win but you must also train smart. When your exercise stimuli are too large and too frequent and combined with inadequate sleep and nutrition and insufficient time to recover and adapt, your body will become incapable of keeping up with the stress and your health and fitness will deteriorate. Given that all races are off and given that a high training load may prevent your immune system developing an adequate defence against pathogens such as the new coronavirus, now is the prime time to carefully monitor your training load and prioritise recovery.
While we all endure this viral extravaganza from the safety of our homes, here are some useful exercise guidelines from the world of exercise immunology:
Interrupt any prolonged sitting every 30-minutes by standing up and walking around, ideally up and down a flight of stairs.
Keep active by accumulating at least 30-minutes of daily movement aimed at maintaining cardiovascular fitness and strength. To help maintain activity during quarantine, please use this free, daily-updating, home work-out tool. Share this tool with anyone in need of a little nudge to help them use their home as a workout facility. There is no equipment needed, and it is primarily aimed at those who are new to exercise and wanting to maintain/improve their fitness. If you’re an athlete, use this free resource, which contains home gym and hotel room exercises for runners and endurance athletes who want to strength train and keep their running fitness when trapped at home (or when travelling, on holiday/vacation, or trapped in a hotel room).
The best thing you can do to support optimal immune function is to stay active, sleep lots, and eat well. “Boosting your immune system” does not mean mega-dosing with echinacea and vitamin C, or any other bloody nonsense the social media “dark lords” are throwing around. Yes, vitamin C is an “essential” vitamin that must be consumed in our diet, but this can easily be achieved by eating tasty vitamin C-containing foods like citrus fruits, blackcurrants, and bell peppers. Systematic reviews from the Cochrane group show that Echinacea is useless and that Vitamin C supplementation does not prevent or treat the common cold virus or pneumonia, nor was it useful against SARS.
If you are a performance athlete, also consider the following:
Maintain your training as best you can but keep your exercise frequency, intensity, and time under control. Reduce the duration and frequency of time spent in the pain cave. In this age of popularised S.H.I.T. (short, high-intensity training), ease off the gas and aim to maintain rather than gain your fitness.
Prioritise enhancing your sleep, nutrition, and restful habits. Use the time you now have to embed these powerful tools as habits of daily living. Then, when you resume real training, you will have added another feather to your bow.
Do not plan any overreaching blocks over the next few months until we know how this story plays out. It would be better to perform poorly in your autumn races as opposed to watching them from the afterlife.
Exercising with low carbohydrate availability (i.e. exercising before breakfast or exercising with low muscle glycogen due to a low-carb diet) enlarges the inflammatory response to an exercise bout and transiently lowers immune function. Carbohydrate ingestion before, during, or after exercise attenuates the rise in cortisol and leads to fewer perturbations in blood immune cell counts. So, avoiding fasted exercise and aiming to train all sessions with a high carbohydrate availability during this quarantine period will potentially help you reduce the exercise-induced transient decrease in immune function. This doesn’t mean gorging on sugar, but simply maintaining a healthy level of carbohydrate in your diet and avoiding fasted exercise.
If you get sick, stop training. A few missed sessions now, will be weeks, months, or even a whole athletic career gained later.
NOTE: For an in-depth review of these practices, please read the two-part position statement from the guns in the field of exercise immunology, part 1 and part 2.
I hope to have succeeded in helping you learn about how sedentary behaviours and your training load may affect your immune system. Increased amounts of both will increase your risk of infection. So, be sensible and do the right thing. What we know so far about COVID-19 is that it is literally life or death for some people. Like your mother always taught you: don’t play with fire, especially when it has its own DNA and the infamous words of Samuel L. Jackson’s wallet tattooed on its outer protein shell.
Until next time, stay nerdy and keep empowering yourself to be the best athlete you can be by training smart.
Important To stay on top of the latest COVID-19 info and prevention strategies, visit the World Health Organisation (WHO) and consult your local healthcare resources for local guidelines. If you are confused about the role of social distancing and herd immunity, the Figure below summarises that when people continue mingling, the virus freely infects people. Those people either die or acquire immunity and survive. Social distancing helps reduce the number of infected people, which reduces the burden on health care, reduces the number of occupied ICU beds, and reduces the number of deaths. Social distancing also allows time for a vaccine to be developed. If a vaccine can be developed, it can be used to give immunity to those who have not acquired immunity from having been exposed to the virus. Herd immunity occurs when a large percentage of the population has become immune to the virus, either through previous infection or via vaccination, resulting in far fewer cases, fewer deaths, more open ICU beds, and relief for health care to deal with other urgencies. If you are still confused, just stay at home and watch World War Z. 
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Who is Thomas Solomon?
My knowledge has been honed following 20+ years of running, cycling, hiking, cross-country skiing, lifting, and climbing, 15+ years of academic research at world-leading universities and hospitals, and 10+ years advising and coaching in athletic performance and lifestyle change.
I have a BSc in Biochemistry, a PhD in Exercise Science, and over 90 peer-reviewed publications in medical journals.
I'm also an ACSM-certified Exercise Physiologist (ACSM-EP), an ACSM-certified Personal Trainer (ACSM-CPT), a VDOT-certified Distance Running Coach, and a UKVRN Registered Nutritionist (RNutr).
Since 2002, I’ve conducted biomedical research in exercise and nutrition and have taught and led university courses in exercise physiology, nutrition, biochemistry, and molecular medicine.
And, with my personal experience of competing on the track (800m to 10,000m), the road (5 k to marathon), on the trails, and in the mountains, by foot, bicycle, cross-country ski, and during obstacle course races (OCR), I deeply understand what it's like to train and compete — I've been there, done it, and gotten sweat, mud, and tears on my t-shirt.
Disclaimer: I occasionally mention brands and products, but it is important to know that I don't sell recovery products, supplements, or ad space, and I'm not affiliated with / sponsored by / an ambassador for / receiving advertisement royalties from any brands. I have conducted biomedical research for which I’ve received research money from publicly-funded national research councils and medical charities, and also from private companies, including Novo Nordisk Foundation, AstraZeneca, Amylin, the A.P. Møller Foundation, and the Augustinus Foundation. I’ve also consulted for Boost Treadmills and Gu Energy on R&D grant applications, and I provide research and scientific writing services for Examine.com. Some of my articles contain links to information provided by Examine.com, but I do not receive any royalties or bonuses from those links. Importantly, none of the companies described above have had any control over the research design, data analysis, or publication outcomes of my work. I research and write my content using state-of-the-art, consensus, peer-reviewed, and published scientific evidence, combined with my empirical evidence observed in practice and feedback from athletes. My advice is, and always will be, based on my own views and opinions shaped by the scientific evidence available. The information I provide is not medical advice. Before making any changes to your habits of daily living based on any information I provide, always ensure it is safe for you to do so and consult your doctor if you are unsure.
