Why Marathon Runners Often Fall Ill Post-Race: What Science Reveals About Immune Suppression

In the 1980s, David Nieman, then a graduate student and enthusiastic marathon runner, noticed a troubling trend: runners, including himself, often developed illnesses in the days following a marathon. Motivated by personal experience, Nieman pursued immunology to explore this curious link between endurance sports and post-race sickness.

To investigate further, Nieman and his research team surveyed participants of the 1987 Los Angeles Marathon, asking about their training routines and any subsequent illnesses. Their analysis revealed a clear trend: marathoners were more prone to falling ill in the week after the race compared to those who didn’t participate. The risk was especially pronounced among runners logging over 60 miles per week, suggesting that higher training intensity correlated with increased vulnerability to illness.

Now a leading expert in exercise immunology at Appalachian State University, Nieman has spent decades studying the body’s immune response to physical stress. His work, along with that of other researchers, has shown that while moderate exercise strengthens immune function, intense exertion—like that experienced in marathons and ultramarathons—can temporarily weaken it, especially increasing susceptibility to upper respiratory infections.

In one set of studies, Nieman examined how varying exercise intensities affect immune cells. People who took daily brisk walks for 45 minutes showed heightened immune cell activity and increased presence of natural killer (NK) cells, key players in fighting infections. Regular walkers also tended to have milder respiratory symptoms compared to inactive individuals. These findings aligned with others in the field, which demonstrated that moderate physical activity mobilizes immune cells, allowing them to circulate through the bloodstream and monitor for pathogens.

However, the immune landscape changed dramatically in athletes undertaking prolonged, intense exercise. Nieman’s research showed that running for three continuous hours reduced both the number and effectiveness of NK cells. The exertion also elevated cortisol levels—a stress hormone known to suppress immune responses. Though these changes typically returned to baseline within a day, they created a temporary “open window” during which viruses could take hold and lead to infection.

While some scientists debate whether these post-race symptoms stem from actual infections or are the result of exercise-induced inflammation, there is consensus that intense activity suppresses aspects of immune function. Without adequate recovery, athletes may find themselves more susceptible to common illnesses.

Faced with the surprising downside of an activity he loved, Nieman pivoted his focus toward mitigating these effects. His team discovered that nutrition—especially carbohydrate intake—played a vital role. In studies involving long-distance cyclists, consuming carbohydrates like bananas during exercise reduced inflammation afterward. Carbohydrate consumption also moderated cortisol levels and changes in neutrophil counts, though it had little effect on lymphocytes or NK cells.

Nieman emphasized the importance of balance. “You have to find this sweet spot,” he said, referring to the equilibrium among exercise, rest, and nutrition. Incorporating these insights, he revised his own training strategy and continued participating in endurance races until 2005. Over his athletic career, he completed more than 50 marathons and ultramarathons, achieving a personal best of 2 hours and 37 minutes.

Looking back, Nieman is reflective. Had he known then what his research would later uncover, he might have chosen a different path. “I just think the insult to the immune system is not worth it,” he concluded. “I don't think humans were meant to run marathons or ultramarathons.”

Source:https://www.the-scientist.com/why-do-some-runners-get-sick-after-a-marathon-73060

This is non-financial/medical advice and made using AI so could be wrong.