Does Hard Training Increase the Likelihood of Illness?

There are two sides to this debate, which is presented in the paper “Can exercise affect immune function to increase susceptibility to infection?” (Simpson et al., 2020). One side argues that heavy training can impair immunity and increase infection risk. The other side argues that the evidence for exercise-induced immunosuppression is much weaker than many people think.

It remains unclear whether the exercise itself causes those illnesses, or whether other stressors that accompany high-performance sport are responsible. But, we do know from anecdotal evidence that athletes who train extremely hard do experience more illness episodes.

“Yes” Camp

The "Yes" camp believe that prolonged heavy training can compromise immune function and increase illness risk. They point to evidence showing that heavy endurance training can produce reduced salivary IgA, altered NK cell function, altered T-cell function, increased viral reactivation (especially Epstein-Barr virus), and higher rates of upper respiratory illness during heavy training blocks and competitions.

They believe that when training stress becomes excessive, some athletes develop significant immune disturbances which leads to increased illness risk. However, they also note that vulnerability to illness is increased when there is poor sleep, low energy availability, psychological stress, travel, and environmental extremes.

“No” Camp

The “No” camp is interesting because they challenge the “open window” hypothesis, which is a foundational idea in exercise immunology. The “open window” hypothesis states that intense, prolonged exercise creates a temporary period of reduced immune protection after exercise, potentially increasing the risk of infection. However, this camp argues that the observed immune changes reflects immune cells being redistributed into the tissues, thus increased immunosurveillance, rather than true immunosuppression. They believe that less immune cells in the blood does NOT mean weak immunity.

They believe that athletes get sick during competition periods because of air travel, crowds, hotel stays, sleep disruption, psychological stress, nutritional challenges, and increased pathogen exposure.

How does Illness Risk Increase?

High training load + inadequate recovery + additional life stressors accumulate beyond an athlete's ability to adapt

Takeaways

Stop thinking of immunity as simply "suppressed" or "enhanced".

The immune response to training is dynamic and adaptive. Many changes that were historically viewed as negative may actually be beneficial immune surveillance.

Manage total stress, not just training stress.

The major drivers of illness are often: sleep loss, travel, psychological stress, low energy availability, and pathogen exposure. These may be just as important as training load.

The highest-risk athletes are usually the most overloaded athletes.

Pay attention to the following high-workload periods such as training camps, heavy blocks, competition periods, international travel, and exam periods for student-athletes.

The immune system should be viewed like any other physiological system

Coaches manage musculoskeletal stress, neuromuscular fatigue, and endocrine strain. However, coaches should look at the total-stress lens, not just training-load lens. They should also manage immune stress, recovery capacity, and illness risk through sleep, stress, and nutrition management.

References

Simpson, R. J., Campbell, J. P., Gleeson, M., Krüger, K., Nieman, D. C., Pyne, D. B., Turner, J. E., & Walsh, N. P. (2020). Can exercise affect immune function to increase susceptibility to infection? Exercise Immunology Review, 26, 8–22.