Can Exercise Make You More Likely To Get Sick?

It has been a long standing belief that intense bouts of exercise and heavy training volumes can suppress your immune system making it easier to get sick. But is this true?

Tell me if you heard this before

“I finished [insert intense form of physical activity] and could not train for the next month because I got so sick.”

I bet this has happened to you.

You are not alone. It appears science is on your side as there are several studies that seem to back this up^{[1],[2],[3],[4]}. Some studies^[3] have even tried to quantify the amount of exercise you need to do that puts you at risk, showing that if you are a runner, running more than 40 miles a week puts you at greater risk for infection. Researchers in a 1994 study^[4] came up with the “J curve” of exercise which seems to show as exercise volume goes up so does the risk of infection.

Fig 1 From Nieman, D. (1994). Exercise, Infection, and Immunity. International Journal of Sports Medicine, 15(S 3), S131–S141. doi:10.1055/s-2007-1021128

This has even been refined into an “S shaped curve”^[5] as it appears that elite athletes can exercise at a high volume but get sick less often.

Malm, C. (2006). Susceptibility to infections in elite athletes: the S-curve. Scandinavian Journal of Medicine and Science in Sports, 16(1), 4–6. doi:10.1111/j.1600-0838.2005.00499.x

In my opinion, this type of refinement of the model, from a “J curve” to an “S curve”, raises a red flag. Are the authors of these studies just trying to make the data fit the hypothesis that exercise can make you more susceptible to getting sick, or is there some other explanation?

Turns out recently there was a review paper^[6] published that sheds new light on some of the data from these studies, and it appears that intense and/or high volume exercise actually DOES NOT correlate to greater chances of getting sick.

The studies^[7],[8] above were conducted on marathoners of which a large percentage self-reported upper respiratory tract infections (URTI) after finishing the race. Since these infections were self-reported and not verified via a clinical test, it calls into question whether the symptoms the race participants were experiencing were URTIs or something else entirely.

It turns out that most of these self-reported URTIs were most likey not actually caused by infections at all, and instead were the result of allergy, asthma, inflammation, or airway trauma. In a 2007 study^[9], researchers not only collected self-reported URTI data but then went the extra step of confirming the infection clinically. Out of the 37 self-reported URTIs only 11 of those actually ended up coming back as infections.

But 11 still had an actual infection, so were those 11 infections caused by the volume and/or exercise intensity?

Most likely not. We have to take into consideration what else these athletes are doing. For one, they are racing in a marathon with hundreds, if not thousands of other people. We know that the likelihood of getting sick goes up when you are surrounded by large crowds. In addition, athletes are notorious for having other confounding factors like much greater stress^[10], nutritional deficiencies^[11], compromised gut health^[12], and cross time zone travel^[13] all of which will suppress immune function.

We could also look at this from another angle, is there anything that we can measure physiologically that might show if excessive exercise volume and/or intensity compromises our immune system? One way to measure the robustness of the immune system is to look at lymphocyte count. Lymphocytes are a type of immune cell, the more you have the better you will be able to fight off infections. If these cells decrease for whatever reason, it makes you more susceptible to getting sick. A 2010 study^[14] looked at lymphocyte counts in healthy male cyclists to see if they could tie exercise to a downregulation in lymphocyte counts and explain why athletes might be more likely to get sick post exercise. This study found that post exercise lymphocytes levels drop below pre-exercise levels and then returned to normal around 24 hours post exercise. This has been referred to as the “open-window hypothesis”, ie a period of time where the athlete is more susceptible to getting sick.

The open window of susceptibility to infection after acute exercise in healthy young male elite athletes M.W. Kakanis1,3, J. Peake2,3, E.W. B

However, recent evidence^[15] suggests that lymphocyte counts in the blood do not drop, but instead some of the lymphocytes are being redeployed post-exercise to different tissues in the body in order to provide an enhanced immune response.

“Here, these immune cells are thought to identify and eradicate other cells infected with pathogens, or those that have become damaged or malignant, termed the acute stress/exercise immune-enhancement hypothesis”^[16]

As someone who exercises daily and likes to push their body to new levels, should you be concerned about the effects of exercise on your immune function? It appears there is plenty of evidence that exercise alone will not put you at any greater risk of getting sick. However, if you eat poorly, don’t get enough sleep, have terrible gut health, disrupt your circadian rhythm, and put yourself in large crowds and continue to exercise at a high volume or with great intensity, there is a high likelihood that you could fall ill. So before you go and destroy yourself on a 2k row on the Concept 2, be sure you have the “big rocks” nailed down first. Check in with yourself, have you been sleeping good? Diet on point? How has your 💩been? What about your HRV, how does that look? If anything seems off, scrap the 2k row and go for a walk instead. You can thank me later when you are able to crush that 2k row later down the line instead of being laid up in bed for weeks on end fighting off an illness 😉

1. “Infectious episodes in runners before and after the Los … – NCBI.” https://www.ncbi.nlm.nih.gov/pubmed/2266764. Accessed 1 Apr. 2020. ↑
2. “Ultramarathon running and upper respiratory tract … – NCBI.” https://www.ncbi.nlm.nih.gov/pubmed/6623247. Accessed 1 Apr. 2020. ↑
3. “Exercise-Induced Immunodepression in Endurance Athletes ….” https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3475230/. Accessed 1 Apr. 2020. ↑
4. “Exercise, infection, and immunity. – NCBI.” https://www.ncbi.nlm.nih.gov/pubmed/7883395. Accessed 1 Apr. 2020. ↑
5. “Susceptibility to infections in elite athletes: the S-curve. – NCBI.” https://www.ncbi.nlm.nih.gov/pubmed/16430674. Accessed 1 Apr. 2020. ↑
6. “Debunking the Myth of Exercise-Induced Immune Suppression.” 16 Apr. 2018, https://pdfs.semanticscholar.org/5b29/070589a66a780d8e4e097fe19cda6313fb4f.pdf. Accessed 1 Apr. 2020. ↑
7. “Infectious episodes in runners before and after the Los … – NCBI.” https://www.ncbi.nlm.nih.gov/pubmed/2266764. Accessed 1 Apr. 2020. ↑
8. “Ultramarathon running and upper respiratory tract … – NCBI.” https://www.ncbi.nlm.nih.gov/pubmed/6623247. Accessed 1 Apr. 2020. ↑
9. “Incidence, etiology, and symptomatology of upper … – NCBI.” https://www.ncbi.nlm.nih.gov/pubmed/17414793. Accessed 1 Apr. 2020. ↑
10. “Anxiety and perceived psychological stress play … – UC Digitalis.” https://digitalis.uc.pt/pt-pt/artigo/anxiety_and_perceived_psychological_stress_play_important_role_immune_response_after_exercise. Accessed 1 Apr. 2020. ↑
11. “Consensus Statement Immunonutrition and Exercise. | LJMU ….” http://researchonline.ljmu.ac.uk/id/eprint/5771/. Accessed 1 Apr. 2020. ↑
12. “Endurance exercise and gut microbiota – ScienceDirect.com.” https://www.sciencedirect.com/science/article/pii/S2095254616300163. Accessed 1 Apr. 2020. ↑
13. “Elite athletes travelling to international destinations >5 time ….” https://bjsm.bmj.com/content/46/11/816. Accessed 1 Apr. 2020. ↑
14. “The open window of susceptibility to infection after acute ….” http://eir-isei.de/2010/eir-2010-119-article.pdf. Accessed 1 Apr. 2020. ↑
15. “Effects of stress on immune function: the good … – NCBI – NIH.” https://www.ncbi.nlm.nih.gov/pubmed/24798553. Accessed 1 Apr. 2020. ↑
16. “Debunking the Myth of Exercise-Induced Immune Suppression.” 16 Apr. 2018, https://pdfs.semanticscholar.org/5b29/070589a66a780d8e4e097fe19cda6313fb4f.pdf. Accessed 1 Apr. 2020. ↑