Exercise and immunity: How much is just right?

With the pandemic confining millions of us to our homes and bringing not just our health but also our very mortality into the spotlight, exercise is something that we have to be more deliberate about than ever before.

Most everyone would accept that exercise has wide-ranging health benefits. World Health Organization (WHO) guidelines recommend 150 min/week of moderate aerobic exercise, or 75 min/week of vigorous exercise, along with two or more muscle-strengthening activities per week.

These guidelines, however, are primarily geared towards maintaining healthy body mass, bone density and reducing the risk of chronic conditions like diabetes, high-blood pressure, depression and heart disease. But what about infectious diseases, and particularly respiratory tract infections like Covid-19?

It is widely accepted among immunologists that moderate exercise improves immune function in comparison with sedentary lifestyles. This is supported at the level of biochemistry and population studies.

As noted in Exercise Immunology Review, “Moderate aerobic exercise results in a transient increase in both innate […] and specific […] cells of the immune system. The effector cells of the innate immune system are monocytes, macrophages, neutrophils, and a subset of lymphocytes called natural killer (NK) cells. These cells represent the first line of defence against infections.”

Reference: https://www.ncbi.nlm.nih.gov/pubmed/21446352

In the general population, a study published in the journal Preventive Medicine in 2019 found that increased risk of infectious disease was associated with physical inactivity, as measured against the aforementioned WHO guidelines.

Reference: https://www.sciencedirect.com/science/article/pii/S0091743519300775?via%3Dihub

Similar results were found even in a study carried out during winter and autumn in Wisconsin. Participants who did aerobic exercise five or more times per week experienced 43% fewer days of respiratory tract infection than inactive participants, and the severity of symptoms was also reduced.

Reference: https://bjsm.bmj.com/content/45/12/987

The twist in the story arises with prolonged strenuous exercise. According to Professor Mike Gleeson, “Acute bouts of prolonged strenuous exercise can cause a temporary depression of various aspects of immune function and this is most pronounced when the exercise is continuous, prolonged, strenuous and performed without food intake.”

Reference: https://www.ncbi.nlm.nih.gov/pubmed/17053416

Returning again to Exercise Immunology Review, the suppression of immune function is related to both the duration and intensity of exercise, particularly where muscle damage is provoked, relative to the fitness of the individual.

Reference: https://www.ncbi.nlm.nih.gov/pubmed/21446352

Our study of the scientific literature suggests that regular moderate exercise is undoubtedly beneficial in resisting infectious disease. But while more intense exercise levels typical of professional athletes are important for improving overall fitness, our review of the scientific literature suggests such activities can provoke a temporary period of immune suppression. That’s something for susceptible people to keep in mind during these particularly high-stakes times.