In this period of isolation, it is important, for both our physical and psychological health, to stay busy and active, and therefore continue to practice physical activities. It’s even more essential given that exercise is a major stimulant for the production of several neurotransmitters (endorphins, dopamine and serotonin) in the brain which contribute to our well-being. This helps to manage stress in a period like the one we are going through right now. On top of that, regular physical exercise is very beneficial for the body, in addition to improving health, increasing immune surveillance and reducing the risks of illness [10; 14].
In other words, regular physical activity is strongly related to a decrease in the frequency of upper respiratory tract infections (URTI), as well as the severity or duration of the symptomatology [9; 10; 12; 14]. However, if we exceed a certain level of sustained and prolonged intensity (exercise workload), this effect could be offset, even reversed [2; 10; 13; 14; 15]. This would especially be the case for URTI. These infections can be caused by over 200 viruses, primarily the rhinovirus and the coronavirus, the great majority of which are benign. Since COVID-19 is one of them, it is currently of general interest to present part of the information provided by the scientists who have investigated this problem.
Researcher David Nieman, whose expertise is based on these problems, has proposed a model in the form of a “J” (fig. 1) to explain the intensity of exercise and the risks of URTI. The model suggests that regularly practicing moderate physical activity can decrease the risk of URTI below that of a sedentary individual, while an overly intense and prolonged exercise workload could increase the risk [7; 8].
Figure 1 
What explains this curve?
During physical exercise, there is a redistribution of immunoglobulins/leukocytes (white cells) in the blood system. The leukocytes se break away from where they were located to respond to the demand by the muscles and their inflammation during exercise [7; 8; 10; 14]. We also see that moderate regular physical activity is associated with a 20 to 46% decrease in URTI among these people compared to a sedentary individual [5; 9; 10; 14].
However, an overly intense and prolonged effort (exhaustion phase) can cause numerous changes in immunity possibly related to physiological stress and other factors. This immunodepression could be explained by a reduction in the number of leukocyte neutrophils available in the blood and an inhibition of NK (“natural killer”) cell functioning and B- and T- cells, which are antibodies which react to viral attacks and infections. The same applies to the antibodies (immunoglobulins A) in the nasal and mucous membranes and saliva, although a review of recent literature shows that the relationship between exercise and the changes in the concentration of salivary immunoglobulin A remains to be demonstrated .
During this period of immunodepression which can be considered an “open window”, the body can be more vulnerable and more susceptible to being invaded by micro-organisms which get through the first lines of defense more easily . This relationship is demonstrated in most studies conducted on the subject [6; 7; 10; 14] with a few exceptions, which still leads us to be cautious with the results . Especially since the data appear to support the assumption that among elite athletes, presenting a very high fitness level, the curve for the association between the intensity of physical exercise and the risks of URTI tends to flatten, to use a very popular expression these days (fig. 2) [6;9]. Certain researchers continue to doubt these conclusions since there is still a great deal of work to do in order to clearly identify if it is exercise or other confounding variables that would explain these results, and that with the new immunology methods and techniques, future research will provide more answers to our questions [10; 14].
Figure 2 [6;9]
In conclusion, if you adopt a high or extended exercise workload (e.g. 20-30 hours per week; high intensity over more than 90 continuous minutes, etc.), think about modifying the frequency in order to allow your immune system to recover between each training session or simply moderate the activity concerning to continue to do it regularly at a less intense level in order to get the benefits on your immune system and your general health. Several studies also lead to the conclusion that a healthy, balanced diet, taking nutritional supplements (e.g. vitamin C, glutamine and foods rich in carbohydrates such as bananas, etc.), the integration of adapted recovery periods between training periods, as well as a good sleep ethic can counter the effects of immunosuppression associated with prolonged and intense physical exercise [1; 3; 8; 10; 11; 14]. It has also been demonstrated that practicing regular and moderate physical activity could limit the decline of immune functions among seniors .
In summary, it is important to keep in mind that moderately practicing physical exercise is very beneficial for the entire population and that its advantages outweigh its dangers, but you have to be mindful about doing too much. And if you must maintain a very high or prolonged exercise workload in order to keep in shape during this isolation, at least try to respect social distancing!
Patrick Vachon, Ph.D.
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