2023 firefighter study suggests vigorous exercise may briefly weaken the immune system

Bad news for anyone who lives for all-out training: doing too much vigorous exercise may, for a short period, leave your immune system working less efficiently.

The evidence points that way, at least. A 2023 analysis drawing on more than 4,700 post-exercise fluid molecules from firefighters indicates that people whose jobs or lifestyles demand hard fitness work - including emergency workers and athletes - could be putting themselves at greater risk of certain illnesses.

"People who are very fit might be more prone to viral respiratory infection immediately after vigorous exercise," said Pacific Northwest National Laboratory (PNNL) biomedical scientist Ernesto Nakayasu. "Having less inflammatory activity to fight off an infection could be one cause."

Watch the following video for a summary of the study's findings:

While there is strong evidence that, over time, moderate physical activity in healthy people can benefit the immune system, what happens to immunity immediately after vigorous exercise remains debated.

There is also little robust evidence to back the idea that intense exercise increases the risk of opportunistic infections. Still, a small number of earlier studies have reported self-declared upper respiratory tract infections in athletes - compared with control groups - after strenuous bouts of activity. Whether those links reflect correlation or causation is not clear.

What the 2023 firefighter analysis examined

To probe the question more closely, Nakayasu and colleagues analysed blood plasma, urine and saliva from 11 firefighters collected before and after a 45-minute session of intense exercise. The workout involved hauling as much as 20 kilograms (44 pounds) of equipment across hilly ground.

"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," explained PNNL bioanalytical chemist Kristin Burnum-Johnson. "Perhaps we can reduce the risk of strenuous exercise for first responders, athletes, and members of the military."

What shifted in the body after vigorous exercise

Exercise is unquestionably good for health - improving mood and, in many cases, strengthening immune defences - but, consistent with earlier work, this research picked up potential signals of immune suppression in the firefighters after exertion.

Alongside expected physical adjustments that help the body cope with exercise-driven demands for fluids, energy and oxygen, the researchers observed a drop in molecules tied to inflammation. At the same time, opiorphin levels rose; opiorphin is known to dilate peripheral blood vessels.

Exactly what these short-lived changes mean for immune performance over the immediate post-exercise period is uncertain, although the team offered several possible interpretations.

"[Opiorphin] may increase blood flow to muscles during the exercise regimen to improve the delivery of oxygen and nutrients," the team writes in their paper.

"We postulate that the decrease in inflammatory molecules we observed in the saliva after exercise might represent an adaptive mechanism to improve gas exchange in response to higher cellular oxygen demand."

The participants’ oral microbiome also shifted. The scientists suspect this may have been driven by the rise in antimicrobial peptides detected in the firefighters’ mouths after the intense activity, potentially acting to offset immune suppression - though that inference is disputed.

"However, this increase in antimicrobial peptides had no effect on inhibiting E. coli growth," Nakayasu and colleagues elaborate, "suggesting a limited capacity of antimicrobial peptides within the oral cavity to protect against host infections."

Caveats and an alternative interpretation

Some researchers contend that the observed changes may not point to immune suppression at all, but instead reflect a "heightened state of immune surveillance and immune regulation".

Although comparing participants with themselves helped reduce some of the problems created by a small sample, firefighters also face unusual exposures to pollutants during fires, which could influence immune responses.

In addition, the authors noted that the study included only healthy, active men, meaning further research in a broader population will be needed to verify the results.

Even so, when considered alongside earlier studies, "there is evidence supporting a relationship between physical demands and a higher incidence of respiratory infections," Nakayasu and team conclude.

This research was published in Military Medical Research.

An earlier version of this article was published in November 2023.