Many so-called health gurus or experts have perpetuated the notion that strenuous exercise weakens the immune system. A new study has debunked this myth, showing that endurance sports can actually boost the immune system.

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The widespread belief that endurance sports increase the risk of infection can be traced back to a small study carried out in the 1980s at the Los Angeles Marathon. Researchers had asked athletes competing in the marathon whether they had any symptoms of infection in the days and weeks after the race and because many did, the notion that this happens across the board simply stuck.

In a new study, researchers at the University of Bath have reinterpreted the findings of the 1980s study based on the fundamental principles of immunology and exercise physiology. The authors explain that challenging exercise such as running a marathon changes the behavior of immune cells in two distinct ways. Initially, during the physically intensive act, the number of immune cells in the bloodstream can skyrocket up to 10 times their normal amount — this is especially the case for ‘natural killer cells’, which directly tackle infections. After the exercise, however, some immune cells decrease substantially in the bloodstream, sometimes falling below the pre-exercise baseline. This effect can last for several hours.

This fall in immune cells was previously interpreted as the body’s immune-suppression response to strenuous exercise. The British researchers, however, stress that this does not mean that the cells have been ‘lost’ or ‘destroyed’, but rather that they’ve moved to other more vulnerable sites of the body where infections are most common, such as the lungs.

According to Dr. John Campbell from the University of Bath’s Department for Health, there are three pieces of evidence to indicate that the immune cells are not actually destroyed. The first and most obvious reason is that the cells return to normal levels within several hours, which far too quickly for them to be replaced by new cells. Secondly, previous studies showed that it is not only possible but also natural for cells to leave the bloodstream and travel to other sites in the body. Lastly, studies on mice where immune cells were tagged and labeled showed that, following exercise, these tagged cells accumulated in the lungs, as well as other places that are susceptible to infections.

 “It is increasingly clear that changes happening to your immune system after a strenuous bout of exercise do not leave your body immune-suppressed. In fact, evidence now suggests that your immune system is boosted after exercise – for example, we know that exercise can improve your immune response to a flu jab,” Campbell said.

Co-author, Dr James Turner added, “Given the important role exercise has for reducing the risk of cardiovascular disease, cancer and type II diabetes, the findings from our analysis emphasise that people should not be put off exercise for fear that it will dampen their immune system. Clearly, the benefits of exercise, including endurance sports, outweigh any negative effects which people may perceive.”

The authors stress that while a heavy duty workout itself will not increase the likelihood of catching an infection, it’s possible that other factors involved in the act of exercising might. For instance, attending a sports event where large crowds of people gather can expose people to airborne infections. Other factors, like eating an inadequate diet, getting cold and wet, and psychological stress, have all been linked to a greater chance of developing infections.

The findings appeared in the journal Frontiers in Immunology.