Excessive intense exercise might suppress the immune system, a potential concern for those with physically demanding jobs such as emergency responders and athletes. A 2023 study involving over 4,700 post-exercise fluid molecules from firefighters supports this notion.
Ernesto Nakayasu, a biomedical scientist at Pacific Northwest National Laboratory (PNNL), explains, “People who are very fit might be more prone to viral respiratory infection immediately after vigorous exercise. Having less inflammatory activity to fight off an infection could be one cause.”
Moderate physical activity is known to benefit the immune system over time. However, the immediate impact of vigorous exercise on the immune system is debated. Although some prior studies have reported upper respiratory tract infections in athletes after strenuous activity, there is little concrete evidence linking intense exercise directly to an increased risk of opportunistic infections.
Nakayasu and colleagues conducted an experiment on 11 firefighters, analyzing their blood plasma, urine, and saliva before and after a 45-minute intense exercise session, which involved carrying up to 20 kilograms (44 pounds) of gear over hilly terrain.
Kristin Burnum-Johnson, a bioanalytical chemist at PNNL, said, “We wanted to take an in-depth look at what’s happening in the body and see if we’re able to detect danger from exhaustion in its earliest stages. Perhaps we can reduce the risk of strenuous exercise for first responders, athletes, and members of the military.”
Exercise undoubtedly benefits health, from mood enhancement to immune system strengthening. However, similar to previous studies, this new research found signs of possible immune suppression in the firefighters post-exercise.
Amid the expected physical adaptations to meet the demands of increased fluids, energy, and oxygen during exercise, a decrease in inflammation-related molecules was observed, alongside an increase in opiorphin, which dilates peripheral blood vessels.
The implications of these changes for short-term immune function remain unclear, though the researchers offer hypotheses. “[Opiorphin] may increase blood flow to muscles during the exercise regimen to improve the delivery of oxygen and nutrients,” the researchers write. They suggest the decrease in inflammatory molecules after exercise could be an adaptive response to improve gas exchange due to higher cellular oxygen demand.
The participants’ oral microbiome also changed, possibly due to an increase in antimicrobial peptides in their mouths after intense activity. This increase may compensate for immune suppression, though this is debated. “However, this increase in antimicrobial peptides had no effect on inhibiting E. coli growth,” Nakayasu and his team note, indicating a limited protective capacity of these peptides against infections in the oral cavity.
Other scientists argue that some observed changes might indicate a “heightened state of immune surveillance and immune regulation” rather than immune suppression.
While the study’s within-subject comparison minimized the small sample size impact, firefighters’ unique exposure to pollutants during fires might also alter their immune responses.
Moreover, the study only involved healthy, active men, prompting researchers to call for broader research to confirm their findings. Despite these limitations, Nakayasu and his team conclude, “There is evidence supporting a relationship between physical demands and a higher incidence of respiratory infections.”
