Vigorous Exercise May Temporarily Weaken the Immune System

Bad news for fans of extreme workouts: excessive vigorous exercise may briefly reduce the efficiency of the immune system.

Vigorous exercise and immune function

That, at least, is what the available evidence indicates. A 2023 analysis of more than 4,700 fluid molecules collected after exercise from firefighters suggests that people whose work or training demands intense fitness - including emergency responders and athletes - could be more vulnerable to 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 video below for an overview of the study's findings:

Although robust evidence indicates that moderate physical activity can support the immune system over the long term in healthy people, the immediate effects of vigorous exercise on immunity remain disputed.

There is limited reliable evidence for the idea that intense exercise raises the likelihood of opportunistic infections. However, several earlier studies have recorded self-reported upper respiratory tract infections among athletes after strenuous activity, compared with control groups. It remains unclear whether this reflects correlation or causation.

Firefighters tested after intense training

To investigate, Nakayasu and his colleagues examined blood plasma, urine and saliva from 11 firefighters before and after 45 minutes of demanding exercise, during which they carried up to 20 kilograms 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."

Exercise unquestionably brings major health benefits, from improving mood to reinforcing immune systems. Yet, as earlier research has found, this study also identified potential signs of immune suppression in the firefighters after exertion.

Alongside the anticipated physiological shifts that help the body accommodate exercise's greater requirements for fluids, energy and oxygen, the researchers found lower levels of molecules linked to inflammation. Levels of opiorphin, which dilates peripheral blood vessels, rose at the same time.

Changes in inflammation and the oral microbiome

Exactly what these changes mean for short-term immune-system performance is uncertain, though the researchers offered several possible explanations.

"[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 microbiomes also changed. The scientists believe this may have resulted from increased antimicrobial peptides in the firefighters' mouths after the intense session, potentially offsetting immune suppression, although that interpretation is contested.

"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."

Other researchers, however, contend that certain observed changes could indicate not immune suppression, but a "heightened state of immune surveillance and immune regulation".

Comparing each participant with themselves helped lessen the effect of the study's small sample size. Nevertheless, firefighters face particular exposure to pollutants at fires, which could also alter their immune responses.

The researchers also warned that the study included only healthy, active men. Further research involving a wider population will therefore be needed to verify the findings.

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

The research was published in Military Medical Research.

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