Physical Activity and the Immune System: How Are They Related?
The medical community has known for a long time that physical activity is an important part of a healthy lifestyle as it helps to reduce the risk of cardiovascular disease, diabetes, and obesity (Reiner et. al, 2013). Even in 400 BCE, Hippocrates said “Walking is man’s best medicine”. As we live during a global pandemic, you may wonder how exercise/physical activity affects another important system of the human body...the immune system. As our previous post mentioned, there is growing evidence that physical activity protects individuals from severe cases of COVID-19 and the risk of hospitalizations (link to post). The protection that has been seen with COVID-19, is also evident with other attacks to the immune system such as pneumonia and bacterial infections. This post will take a closer a look at how physical activity affects the immune system and to simplify the mechanism behind that relationship; so that you can better communicate and educate those around you.
Interestingly, while researching for this topic I found that in the earlier 2000s and 2010s, the relationship between exercise and one’s immune response was controversial. Studies found that athletes who engaged in high intensity exercise, had higher risks of some infections and weaker immune systems (Campbell & Turner, 2018). However, more recently, researchers are looking to re-evaluate the relationship between physical activity and the immune system by comparing individuals who live a sedentary lifestyle to those who keep physically active. A sedentarylifestyle is where an individualremains inactive for most of the time and who doesnot meet the recommended 150 minutes of physical activity per week that is recommend by the Government of Canada.
Physical Activity and risk of infections
Studies which have looked at the benefits of physical activity have used observational studies to examine their relationship. In an observational study researchers will observe the actions/behaviours/environment of study participants and observe the outcomes of interest. Researchers have observed the difference in risk of infection between those who live a sedentary lifestyle and those who are low-moderately physically active. The incidence and severity of viral infections, like upper respiratory tract infections follow a J-shaped curve, which indicates that they are reduced by the regular moderate physical activity (Pape et. al, 2016). Viral infections would also include COVID-19 which is caused by the SARS-Cov-2 virus. Due to the evidence that physical activity may be able to reduce viral infections, researchers wanted to determine if the same could be said about with bacterial infections. A study conducted in Denmark, used surveys so that participants could self-assess their physical activity and then identified bacterial infections based on antibiotic prescriptions. In total they had over 18,000 participants in their study (Pape et. al, 2016). They found a similar relationship that previous studies found with physical activity and viral infections, where there was a J-shaped relationship where low/moderate levels of physical activity were associated with a lower risk of bacterial infections (Pape et. al, 2016). Limitations to these types of studies is that individuals will self-report these physical activity levels so they may overestimate their physical activity levels. Nevertheless, these results have been seen by other studies, which provides more confidence in their validity.
Physical activity in the elderly
The benefits of physical activity extend to the elderly population as well. In older adults, their immune system becomes weakened, which is commonly known as immunosenescence and can lead to an increased risk of infections and cancer. To show the positive effects of lifelong physical activity, one study compared the immune cell profiles of older adults aged 55-79 who have maintained their physical activity levels and found that they had fewer signs of immunosenescence (Duggal et. al, 2019). One effect of immmunosenescence is decreased efficacy of vaccines in older adults, a number of studies have shown that regular high-level intensity physical activity has shown to improve the immune response to influenza and pneumococcal vaccines (Duggal et. al, 2019).
How does physical activity affect the immune system?
Now how does a physically active lifestyle really improve one’s immune system. Several theories exist to help identify this relationship. It is known that as individuals get older, they lose bone and muscle mass. Research has indicated that skeletal muscle can regulate the immune system through myokines. Myokines are proteins that have a specialized name due to them being secreted from muscle tissue. These myokines possess anti-inflammatory effects which have led researchers to look at the immune benefits of maintaining physical activity (Duggal et. al, 2019; Gleeson et. al, 2011). Skeletal muscle also secretes glutamine, which is a type of amino acid (Rogeri et. al, 2020). This amino acid is an important part of our immune system response as it is used as an energy source by immune cells such as leukocytes and lymphocytes. Studies have provided evidence to show that muscle provides an optimal level of glutamine to help with the immune response (Rogeri et. al, 2020). Furthermore, researchers have used animal models to show a causal relationship between two factors. One study used a rodent model to demonstrate if physical activity can help fight against an infection (Duggal et. al, 2018). They injected the mice with a lethal dose of influenza and saw that moderate exercise reduces the entry of inflammatory cells into their lungs. This is a relatively new theory, and more research will be invested to understand the relationship between these two systems of the human body.
The field of moderate exercise and physical activity among non-athletes is a new field with growing research. Based on the current evidence, physical activity can help maintain the immune system through the maintenance of skeletal muscle and secretion of anti-inflammatory agents. With all of this new information, the main take-away message is to GET UP & GET MOVING. If you are not currently physical active, it is not too late. Take some time for yourself to get moving whether it is a walk, run, bike ride or even dancing in your living room. The less time you spend sitting will be better for you and your immune system!
Feel free to share or comment any questions or thoughts you may have. Stay tuned to further discussion on how physical activity can be a benefit to you!
Campbell, J. P., & Turner, J. E. (2018). Debunking the Myth of Exercise-Induced Immune Suppression: Redefining the Impact of Exercise on Immunological Health Across the Lifespan. Frontiers in immunology, 9, 648. https://doi.org/10.3389/fimmu.2018.00648
Duggal, N. A., Niemiro, G., Harridge, S. D. R., Simpson, R. J., & Lord, J. M. (2019). Can physical activity ameliorate immunosenescence and thereby reduce age-related multi-morbidity? Nature Reviews Immunology, 19(9), 563. https://link.gale.com/apps/doc/A597889119/AONE?u=guel77241&sid=bookmark-AONE&xid=03027d56
Gleeson, M., Bishop, N. C., Stensel, D. J., Lindley, M. R., Mastana, S. S., & Nimmo, M. A. (2011). The anti-inflammatory effects of exercise: mechanisms and implications for the prevention and treatment of disease. Nature Reviews Immunology, 11(9), 607+. https://link.gale.com/apps/doc/A267333263/AONE?u=guel77241&sid=bookmark-AONE&xid=f7406471
Pape, K., Ryttergaard, L., Rotevatn, T. A., Nielsen, B. J., Torp-Pedersen, C., Overgaard, C., & Bøggild, H. (2016). Leisure-Time Physical Activity and the Risk of Suspected Bacterial Infections. Medicine and science in sports and exercise, 48(9), 1737–1744. https://doi.org/10.1249/MSS.0000000000000953
Reiner, M., Niermann, C., Jekauc, D., & Woll, A. (2013). Long-term health benefits of physical activity - a systematic review of longitudinal studies. BMC Public Health, 13(1). https://link.gale.com/apps/doc/A534623774/AONE?u=guel77241&sid=bookmark-AONE&xid=740caad3
Rogeri, P. S., Gasparini, S. O., Martins, G. L., Costa, L., Araujo, C. C., Lugaresi, R., Kopfler, M., & Lancha, A. H., Jr (2020). Crosstalk Between Skeletal Muscle and Immune System: Which Roles Do IL-6 and Glutamine Play?. Frontiers in physiology, 11, 582258. https://doi.org/10.3389/fphys.2020.582258
Moderate physical activity has been shown to reduce the risk of bacterial infections compared to sedentary lifestyle.
Among the elderly population, regular physical activity helps to boost the immune response from influenza and pneumococcal vaccines.
A lifetime of physical activity can protect individual from the effects of an aging immune system.
Moderate physical activity has also been shown to reduce the risk of viral infections compared to sedentary lifestyle.