Intense exercise can have harmful effects we didn’t know about

Bad news for all you extreme exercise junkies out there: excessive vigorous exercise can suppress your immune system. At least, that’s what a study of firefighters that analyzed over 4,700 post-exercise fluid molecules suggests.

This can be problematic for workers with consistently physically demanding jobs that require intensive fitness training, such as emergency workers and athletes.

“People who are in good shape may be more prone to a viral respiratory infection immediately after vigorous exercise,” suggests Pacific Northwest National Laboratory (PNNL) biomedical scientist Ernesto Nakayasu. “Less inflammatory activity to fight infection may be one cause.”

Although there is strong evidence to suggest that moderate physical activity among healthy individuals can favor the immune system in the long term, what happens to the immune system immediately after intense exercise is controversial.

There is little reliable evidence to support the claim that intense exercise increases the risk of opportunistic infections, although several previous studies have noted upper respiratory tract infections in athletes, compared with controls, following strenuous activity. It is not known whether these are correlations or causes.

So Nakayasu and colleagues tested the blood plasma, urine, and saliva of 11 firefighters before and after 45 minutes of intense exercise lugging up to 20 kilograms (44 pounds) of gear over hilly terrain.

“We wanted to take a close look at what’s going on in the body and see if we’re able to detect the danger of exhaustion in its earliest stages,” explains PNNL bioanalytical chemist Christine Burnum-Johnson. “We may be able to reduce the risk of strenuous exercise for first responders, athletes and members of the military.”

There’s no doubt that exercise does wonders for our health, from lifting our moods to boosting our immune systems. But, as in previous studies, the new research found possible signs of immune suppression in trained firefighters.

Amid the expected physical changes that help our bodies maintain the increased fluid, energy, and oxygen needed for exercise, there was a decrease in molecules involved in inflammation. This was accompanied by increased opiorphin, a peripheral blood vessel dilator.

What these changes ultimately mean for the short-term function of the immune system is not clear, but researchers have a few ideas.

“[Opiorphin] may increase blood flow to muscles during an exercise regimen to improve oxygen and nutrient delivery,” the team wrote in their paper.

“We hypothesize that the decrease in inflammatory molecules we observed in saliva after exercise may represent an adaptive mechanism to improve gas exchange in response to higher cellular oxygen demand.”

There was also a change in the oral microbiome of the participants. Scientists suspect that this is due to an increase in antimicrobial peptides found in firefighters’ mouths after their intense activity, possibly to compensate for immune suppression, although this conclusion is disputed.

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

However, other scientists argue that some of the observed changes may not be indications of immune suppression, but of “an enhanced state of immune surveillance and immune regulation.”

While the within-subjects comparison reduced the impact of their small sample size, firefighters experience unique exposure to pollutants during fires, which may also alter their immune responses. Furthermore, this study only looked at healthy and active men, the researchers cautioned, so further research in the wider community is needed to confirm their findings.

However, considering previous studies, “there is evidence to support a link between physical demands and a higher incidence of respiratory infections,” conclude Nakayasu and team.

This research was published in Military medical research.