This Type of Exercise Isn't Good for Your Immune System

What makes you stronger could also leave you wide open to infection!

How Vigorous Exercise May Affect Your Immune System

If you push yourself through intense workouts regularly, especially in a physically demanding profession like firefighting, military service, or athletic competition, you might want to reconsider how much is too much. A recent study involving over 4,700 post-exercise fluid molecules from firefighters suggests that vigorous exercise could temporarily suppress your immune system.

This is particularly important if your job depends on peak physical performance. You are constantly balancing the benefits of fitness with the potential risks of overtraining, especially when it comes to immune resilience.

"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."

Although moderate exercise is well-known to boost your immune function over the long term, the effects immediately following intense workouts remain controversial. Evidence supporting a direct link between extreme exercise and increased vulnerability to infection is limited. Still, some earlier studies have noted higher self-reported rates of upper respiratory infections in athletes after demanding training or competition.

To dig deeper, Nakayasu and his team tested blood plasma, urine, and saliva from 11 firefighters before and after they performed 45 minutes of strenuous exercise, hauling up to 20 kilograms of gear across hilly terrain.

"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."

While exercise clearly improves your health, the study revealed possible markers of immune suppression in the firefighters after exertion. The expected physiological changes that allow your body to maintain fluid balance, energy, and oxygen supply during exercise were also accompanied by a decrease in molecules that typically promote inflammation. At the same time, levels of opiorphin, a molecule that dilates peripheral blood vessels, went up.

"[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."

There were also changes in the participants’ oral microbiome. After their intense workout, the firefighters showed increased levels of antimicrobial peptides in their mouths. The researchers believe this might be a compensatory response to immune suppression, although the evidence remains inconclusive.

"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."

Still, not all experts agree on what these changes indicate. Some argue that these responses could reflect a "heightened state of immune surveillance and immune regulation" rather than outright suppression.

The researchers acknowledge the limitations of their work. Although their within-subject comparison helps offset the small sample size, firefighters may experience unique environmental exposures that affect their immune response. Also, the study only examined healthy, active men, meaning the results cannot yet be generalized to a broader population.

Even so, when combined with previous research, the findings do point to a link between intense physical demands and an increased incidence of respiratory infections.

"There is evidence supporting a relationship between physical demands and a higher incidence of respiratory infections," Nakayasu and team conclude.

Your call to action for vigorous exercise and immune system

If your lifestyle includes frequent intense training, it is crucial to allow proper recovery and monitor your health closely. Make sure you are getting enough rest, fueling your body with adequate nutrition, and varying your workout intensity to avoid chronic immune stress. Recognize the signs of overtraining and give your body time to adapt and strengthen. Staying fit is essential, but staying healthy should always be the priority.

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