by Coach Jeff Sankoff M.D. (a.k.a. the Tri-Doc)

With the completion of the Ironman women’s World Championship in Kona this past weekend more than two thousand weary competitors will begin the process of returning home. As they sit on aircraft filled with other competitors, spectators and vacationers, some will be exposed to pathogens that will make them ill and this will cause many to wonder, ‘Did racing make me sick?’

As a coach whose full-time job is an emergency physician, this is one of the more common questions that I get asked, especially during the fall and winter months when respiratory viruses circulate much more within the population.

This question took on particular significance in the past few years since the emergence of the novel coronavirus Covid-19. Athletes have become much more attuned to anything that might make them more prone to catching that particularly unpleasant pathogen.

So, what then is the answer about how exercise affects our immune system and whether training and racing can make us sick?

How does exercise affect the immune system?

The immune system is very complex and made up of several different cell types and organs. In addition, numerous hormones modulate the function of the system and can impact the susceptibility to infection.

At its root the core function of the immune system is to recognize self from non-self. In other words, the immune system is set up to recognize invaders and repel them using a wide variety of effective and in many cases violent, (albeit on an exceptionally microscopic scale) measures.

A recent review article published in Sport Sciences for Health provided a review of much of the literature on this subject and gave an overview of the current understanding of how exercise and immune function relate to one another.[i]

Early research in this area showed that those who exercise regularly tend to have a lower rate of respiratory infections during cold seasons. Specifically, exercise in this context is described as daily activity of moderate intensity (60-80%of VO2 max) leading to an associated decrease in upper respiratory tract infections of 20-30% over the cold and flu season.i,[ii]

The mechanism by which exercise is believed to improve resistance to infection is by increasing the amount of a specific immunoglobulin in the saliva and respiratory tract.

Immunoglobulin A, or IgA, is commonly found in the respiratory and gastrointestinal tracts where it binds non-specifically to invading viruses and other infectious organisms. IgA provides only a relatively modest first line of defense but when levels of this immunoglobulin are high, there can be heightened levels of protection.

Several studies have shown that both men and women who regularly exercise at moderate intensity have significantly higher levels of salivary IgA than do matched sedentary controls.

However, anecdotally many athletes will relate that they associate races with episodes of illness. As if to confirm this, a study conducted in 1982 on 140 marathon runners in Cape Town, 33% of runners reported URTI symptoms in the 2 weeks after the event, compared with 15% for the same age group who were at home at the same time. Other studies have demonstrated similar findings.

It turns out that in single high intensity, longer duration efforts, immune function rather than being augmented is depressed. In part this has to do with a decrease in IgA levels but there appear to be other mechanisms implicated as well. Specialized white blood cells integral to the host response in warding off infection (neurophils and natural killer cells) are both decreased in number and have stilted function after such events. Recovery to normal levels and function takes about 24-72 hours. This period of one to three days during which immune function is depressed has come to be referred to as the ‘open window’ to infection.

At this point it is important to add that the level of immune function is not the only factor in the equation that determines whether an athlete will become ill. This is because to develop a respiratory illness an individual needs not only to be susceptible but also to be exposed.

Consider two athletes. One is training at low to moderate intensity in their usual fashion and gaining a robust immune system as one of the many benefits from their dedication to their routine. However, their roommate has come down with a nasty cold. And in sharing the same living space, the athlete is exposed repeatedly to viral particles over several days and eventually catches the virus and becomes sick.

The second athlete undertakes a specific challenge, say the Ironman World Championship. After thirteen grueling hours in the lava fields and the Energy Lab they are spent but content at their accomplishment. For the next three days this athlete is relatively immunosuppressed. However, this athlete remains in Kona on holiday, with only her family. She does not immediately fly home and therefor is unexposed to anyone who might be contagious and therefor does not become ill.

You can see from this example that the effect on immune function is only one small part of the picture and to make decisions about how to train or race, an athlete needs to consider a more global assessment of risk.

How then can athletes who undertake prolonged high intensity efforts protect themselves from contracting a contagious disease during the open window of depressed immune function?

Despite what you have heard and what many will continue to say, there are simply no supplements or dietary measures that enhance immune function. No vitamins, minerals or specific foods have ever been shown to effectively prevent infections of any kind (with the exception of clostridium difficile, an entirely different subject). Maintaining a balanced, healthy diet is a good idea under any condition so doing so after these kinds of efforts is no different just don’t expect that adding supplements or focusing on blueberries will make any difference.

As we have seen with Covid, and it is true for all contagious illnesses, the only way to effectively prevent infection is simply to not allow for exposure and when that cannot be done, to then take all the precautions necessary to minimize infection. Social distancing, wearing a mask and meticulous handwashing are all best practices and will keep even the most immunosuppressed individuals safer than not.

The take-home message here is don’t let the fear of becoming sick get in the way of doing big efforts or special challenges just take precautions to keep yourself healthy afterwards.

Train hard, train healthy.

[i] Sabzevari Rad R. The impact of different training intensities on athletes’ immune system function and the management of upper respiratory traction infections: a narrative review. Sport Sci Health. 2023;1:3. doi:10.1007/s11332-023-01110-7

[ii] Simpson RJ, Kunz H, Agha N, Graff R. Exercise and the Regulation of Immune Functions. doi:10.1016/bs.pmbts.2015.08.001