Why Intense Exercise May Lead to a Cold

Can too much of a healthy habit become bad?

Lots of evidence shows that regular exercise wards off respiratory infections like colds, flu, and COVID-19. However, very vigorous exercise may lead to these infections by triggering immune changes that increase risk, according to a new study.

The findings come as we enter another possible tripledemic this winter, with an increase in COVID, flu, and respiratory syncytial virus (RSV). Public health officials are on alert for a potentially severe flu season, following high flu activity this year in Australia (which can help predict how bad the US flu season will be).

Studies show that the risk for acute respiratory infections is lower in people who exercise regularly. Physically active people are also less likely to suffer severe outcomes from COVID.

But while inactivity has emerged as a potential risk factor for respiratory infections, scientists have long proposed that too much activity, particularly of a prolonged and highly intense nature, may also increase susceptibility.

"The theory suggests that a short-term suppression of the immune system following intense exercise leads to an increase in susceptibility to infection, especially upper respiratory illness," said Choukri Ben Mamoun, PhD, professor of medicine (infectious diseases) and microbial pathogenesis at the Yale Institute for Global Health, New Haven, Connecticut. Researchers have documented a greater incidence of upper respiratory illness "among both highly trained and healthy untrained individuals following increased activity during competition or heaving training blocks."

That's important if you treat athletes or patients with physically demanding jobs that push them to their physical limits, such as firefighters, police officers, or military personnel.

The new study was small but sheds light on a possible mechanism. Researchers tested blood, saliva, and urine samples from 11 firefighters before and 10 minutes after intense exercise designed to mimic wildfire fighting. The firefighters hiked over hilly terrain for 45 minutes in humid weather wearing up to 44 pounds of wildland gear.

After the workout, subjects had fewer proinflammatory cytokines and ceramides, and more antimicrobial peptides, changes that indicate a greater susceptibility to infection, researchers said. A systematic review adds weight to their findings, revealing a handful of studies in marathon runners, firefighters, soldiers, and soccer players that found an increase in respiratory symptoms after strenuous workouts.

"The relationship between exercise and the immune system is complex and varies from person to person," said Mamoun, who was not part of the study. "Physicians can use this study's findings to provide individualized exercise recommendations."

An Adaptive Mechanism Gone Awry

During intense exercise, the body may reduce airway inflammation to help you breathe, say the authors. The boost in antimicrobial peptides found in the saliva samples could be the body's way of compensating for the diminished immune function.

Antimicrobial peptides are part of the immune response but they're "usually not very effective for viral infections," said lead author Ernesto Nakayasu, PhD, senior research scientist at the Pacific Northwest National Laboratory, a US Department of Energy lab in Richland, Washington. "That's why we think it may make you more exposed to respiratory infections."

The drop in proinflammatory molecules had an inverse relationship with opiorphin, a peripheral tissue vasodilator thought to increase blood flow and improve oxygen delivery to the muscles during exercise. This may be an adaptive mechanism to improve gas exchange in response to greater oxygen demand.

But as with many adaptive mechanisms, this one may have an unintended consequence. Fewer proinflammatory molecules on patrol may leave you more vulnerable to infection. Plus, during intense exercise, people tend to breathe through their mouths, bypassing the nasal barriers and allowing more microbes — including viruses — to penetrate and deposit in the distal airways of the lungs.

Advice for Patients

More research is needed to know exactly how long and how strenuously one needs to exercise to trigger these immune changes, Nakayasu said.

As shown by their lactate accumulation (an indicator of anaerobic metabolism), the firefighters in the study outpaced the average person's aerobic respiratory capacity, meaning the average person doing moderate exercise likely wouldn't trigger these changes.

"Regular moderate exercise is generally associated with better health outcomes [and] improved immune function," said Mamoun. For those who exercise to the extreme, proper rest and recovery are "essential for maintaining a robust immune system," Mamoun said.

And of course, you can encourage patients to get vaccinated. Young, healthy patients may assume they don't need COVID-19 or flu shots, as indicated by a recent survey that found one third of Americans feel they don't need these vaccinations if they're not high risk.

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