In the 1980s, David Nieman, a young health science graduate researcher at Loma Linda University and an avid marathon runner, observed an intriguing pattern. “I had noticed, both in myself and in other marathon runners, that they tended to get sick during the week or two after running a marathon,” said Nieman. Fascinated by the observation, he decided to plunge into the field of immunology and dig into the underlying factors.
Nieman and his team mailed questionnaires about training patterns and sickness episodes to people who had signed up to participate in the 1987 Los Angeles marathon. Their results revealed a link between exercise load and illness: Runners were more likely to fall sick the week after the race than non-participants.1 The risk was higher among athletes who trained more than 60 miles a week, indicating that greater exercise intensity raised the chance of illness.
Nieman, now an exercise immunologist at Appalachian State University, explained that these findings led to broader investigations in the lab. He and others in the field have since shown that while people draw a multitude of benefits from moderate exercise, heavy exertion during endurance sports, such as marathons and ultra marathons, triggers transient immune dysfunction and increased risk of upper respiratory illness.2-4
Nieman and his colleagues began by investigating how different training intensities reshaped the immune cell landscape. They observed that people who took daily 45-minute brisk walks had increased circulation of some immune cells and enhanced activity of the body’s natural killer (NK) cells.5,6 Those who regularly walked also experienced less severe symptoms of respiratory infections compared to sedentary people. Other researchers have consistently reported similar observations: Bouts of moderate exercise send immune cells out of the tissues they reside in and into the bloodstream, where they patrol to spot and strike any invading pathogens.7,8
In contrast, when Nieman and his team monitored athletes who ran for three hours, mimicking heavy exertion, they observed reduced numbers and activity of NK cells.9 This high-intensity exercise also increased the levels of the stress hormone cortisol, which can weaken the immune response.10 The levels returned to baseline in about a day. “But it's still enough of an interruption in normal immunity such that the omnipresent viruses then can multiply, gain a foothold, and then increase infection rates,” explained Nieman.
However, some researchers have questioned whether this “open window” is sufficient to cause infections, debating whether athletes suffer from illness symptoms due to infections or simply as a result of exercise-associated inflammation.11 Nevertheless, researchers largely agree that heavy exercise temporarily suppresses some immune functions, which may compromise athletes’ resistance to minor illnesses if they do not rest and recover sufficiently between exercise sessions.12
For Nieman, this was a bitter pill to swallow. “I thought [marathon running] was supposed to be healthy…I thought it was going to do so much good for me,” he recalled. “And then my own research is showing that the runners were getting sick.”
So, he focused on mitigating these effects of a highly intense workout. He and his team found that carbohydrates are a key nutrient in this process; in studies with long-distance cyclists, the researchers observed that consuming carbohydrates, such as bananas, during the activity reduced post-exercise inflammation.13 Loading up on carbohydrates also led to attenuated exercise-induced changes in the levels of plasma cortisol and some immune cells like neutrophils, but did not affect lymphocyte or NK cell counts.14 “You have to find this sweet spot where you're doing your activity and you have sleep and nutrition, and all of this is in a balance that promotes health,” remarked Nieman.
With emerging research, Nieman incorporated nutrition and altered his training plan. He continued running marathons and ultramarathons until 2005. By then, he racked up more than 50 races under his belt and achieved a personal best time of two hours and 37 minutes. He now remains active by walking, building trails, or splitting firewood.
Would he have run marathons since the 1980s if he knew then what he knows now? Nieman does not think so. “I just think the insult to the immune system is not worth it,” he admitted. “I don't think humans were meant to run marathons or ultramarathons.”
- Nieman DC, et al. Infectious episodes in runners before and after the Los Angeles Marathon. J Sports Med Phys Fitness. 1990;30(3):316-328.
- Walzik D, et al. Molecular insights of exercise therapy in disease prevention and treatment. Signal Transduct Target Ther. 2024;9(1):138.
- Yu Y, et al. Single-cell sequencing of immune cells after marathon and symptom-limited cardiopulmonary exercise. iScience. 2023;26(4):106532.
- Gleeson M, et al. Influence of training load on upper respiratory tract infection incidence and antigen-stimulated cytokine production. Scand J Med Sci Sports. 2013;23(4):451-457.
- Nehlsen-Cannarella SL, et al. The effects of moderate exercise training on immune response. Med Sci Sports Exerc. 1991;23(1):64-70.
- Nieman DC, et al. The effects of moderate exercise training on natural killer cells and acute upper respiratory tract infections. Int J Sports Med. 1990;11(6):467-473.
- Campbell JP, et al. Acute exercise mobilises CD8+ T lymphocytes exhibiting an effector-memory phenotype. Brain Behav Immun. 2009;23(6):767-775.
- Bigley AB, et al. Acute exercise preferentially redeploys NK-cells with a highly-differentiated phenotype and augments cytotoxicity against lymphoma and multiple myeloma target cells. Brain Behav Immun. 2014;39:160-171.
- Berk LS, et al. The effect of long endurance running on natural killer cells in marathoners. Med Sci Sports Exerc. 1990;22(2):207-212.
- Alotiby A. Immunology of stress: A review article. J Clin Med. 2024;13(21):6394.
- Campbell JP, Turner JE. Debunking the myth of exercise-induced immune suppression: Redefining the impact of exercise on immunological health across the lifespan. Front Immunol. 2018;9:648.
- Walsh NP, et al. Position statement part one: Immune function and exercise. Exerc Immunol Rev. 2011;17:6-63.
- Nieman DC, et al. Metabolic recovery from heavy exertion following banana compared to sugar beverage or water only ingestion: A randomized, crossover trial. PLoS One. 2018;13(3):e0194843.
- Nieman DC, et al. Influence of carbohydrate on immune function following 2 h cycling. Res Sports Med. 2006;14(3):225-237.
