HOW DOES EXERCISE AFFECT OUR DEFENSES AND IMMUNE FUNCTION?

The immune system is essential to maintaining health, as the COVID-19 epidemic made especially clear. People with weaker immune systems—including older adults, immunocompromised people and those with certain conditions—were the most severely affected by the virus. Immune function is influenced by many factors, including age, nutritional status and disease, but exercise is another major modulator.

Effects of regular exercise on the immune system

Remaining active and exercising regularly provides benefits throughout the body, including the immune system.¹ Substantial evidence shows, for example, that exercise improves the ability of natural killer cells—among the body's main defenders—to attack foreign agents. The cytotoxic activity of these cells is greater in athletes than in sedentary or untrained people.^2,3 Similarly, untrained people who begin an exercise program, such as 8 weeks of aerobic training, improve both the number and cytotoxic activity of their natural killer cells.⁴

Another major component is the adaptive immune system, which received considerable attention during the COVID-19 epidemic. This includes the antibodies of humoral immunity and T cells of cellular immunity that increase to defend the body against the virus. Regular exercise also appears beneficial here: elite athletes show stronger cellular responses, with a greater increase in T cells, and antibody responses after influenza vaccination.⁵  This effect is not limited to athletes. Older adults who exercise regularly—for example, averaging 18.000 steps per day or exercising 3 days per week—also show a better immune response after vaccination.^6,7

Effects of an acute bout of exercise on the immune system

Despite the clear benefits of remaining regularly active, there is debate about whether strenuous exercise, especially when intense or very prolonged, could impair the immune system and increase susceptibility to infection. The hypothesis stems mainly from blood tests after strenuous exercise, which show a temporary decline in some immune cells such as T lymphocytes. This observation gave rise to the post-exercise “open window” theory, describing a period of immunosuppression that would increase infection risk immediately after exercise. More recent evidence suggests that exercise does not truly cause post-exercise immunosuppression. Instead, fewer immune cells are measured in blood because they move to peripheral tissues where they are most needed at that moment, such as the mucous membranes.^8,9

Although some studies have observed a higher prevalence of respiratory infections, such as bronchitis, in athletes than in the general population,¹⁰ which could potentially result from high training loads weakening the immune system, other authors suggest that the higher incidence may instead reflect travel, crowds in stadiums, inadequate nutrition or sleep.⁹

Conclusions

Once again, regular exercise benefits health—in this case by supporting the immune system. Whether strenuous exercise temporarily reduces immune defenses remains controversial, and recent research challenges that hypothesis. Particularly in light of the COVID-19 epidemic, the importance of remaining active for health should not be overlooked.

 AUTHOR

Pedro Valenzuela
Researcher in the Physiology Unit at the University of Alcalá and the Performance Monitoring Unit at the Sports Medicine Center (AEPSAD, Madrid High Performance Center).
Website: www.fissac.com

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References

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