Intense physical activity could promote respiratory infections

Some occupations (e.g., firefighter, soldier) and activities (elite sports) require a very intense level of physical activity, beyond the normal adaptive capacity of the human body, which can lead to exhaustion, decreased performance, and an increase in the susceptibility of workers and athletes to injury or illness. To study the molecular signatures of high-intensity physical activity, researchers carried out proteomic, lipidomic and metabolomic (“multi-omics”) analyses in forest firefighters who agreed to do very intense exercise. The ultimate goal of this work is to increase the safety of firefighters and first responders, by making it possible to better assess the balance between adaptive and adverse responses and to predict early biochemical and physiological changes that lead to exhaustion.

In addition to multi-omics analyses in forest firefighters, the study authors conducted a systematic review of studies on respiratory infections in response to high-intensity physical activity. This review identified 6 articles whose results suggest an increased susceptibility to respiratory infections.

Eleven forest firefighters took part in an intense exercise session that involved moving through mountainous terrain for 45 minutes, carrying their heavy firefighting equipment weighing between 9 and 20 kg. Blood, urine and saliva samples were collected before and immediately after the exercise session. Proteomic, metabolomic and lipidomic analyses were carried out by liquid chromatography coupled with mass spectrometry (LC-MS/MS). These analyses made it possible to identify and quantify 3,835 proteins, 730 lipids, and 182 metabolites in the 3 different types of samples.

Blood plasma analyses reveal a typical signature of tissue damage and repair response, accompanied by an increase in carbohydrate metabolism. The increase in extracellular matrix proteins in blood plasma after intense exercise is the result of cellular breakdown. Proteins involved in the repair of damaged tissues (coagulation system) were found in greater quantities in blood plasma after intense exercise. Finally, a significant change in lipid metabolism, glycolysis and the tricarboxylic acid cycle was observed, probably to meet the body’s increased demand for energy.

Multi-omics analyses in urine showed a strong increase in 6 of the 8 proteins that are part of the renin-angiotensin system and other proteins, supporting increased excretion of catabolites, reabsorption of nutrients, and maintenance of fluid balance.

Saliva analysis showed a decrease in three pro-inflammatory cytokines (IL-36a, IL-18 and IL-1) normally involved in defence against viruses, and an increase in 8 antimicrobial peptides. The authors of the study hypothesize that the reduction of inflammation in the oral cavity and possibly in the respiratory tract could be a temporary adaptation mechanism, to improve respiratory performance during intense physical activity. Thus, a reduction in inflammation would allow people who engage in intense physical activity (firefighters, soldiers, athletes) to breathe more easily and deliver more oxygen to the organs, but it would make the body more susceptible to respiratory infections. Increased expression of antimicrobial peptides and certain other proteins of the immune system (CD14, CD55) would be a compensatory mechanism for improving defence against infection.

There are several methodological limitations to this study, and it does not allow us to precisely explain the underlying molecular mechanisms, but it constitutes an important step towards better understanding the adverse effects of intense activity on the human body.