© Taiye Salawu
July 6, 2026
Christine Bürg & Marianne Waldenfels
A new US study found an unusually high rate of colon polyps among marathon and ultramarathon runners. What do the findings really mean, and should endurance athletes be concerned? A gastroenterologist explains.
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Running is widely regarded as one of the strongest protective factors against cancer. That makes new findings from the United States all the more surprising: researchers found colon polyps in 41 percent of the marathon and ultramarathon runners they examined, many of them classified as advanced adenomas with a higher risk of progressing to cancer.
The study does not prove that extreme endurance training causes colorectal cancer, but the findings are sparking intense debate among gastroenterologists and sports medicine specialists.
The boom in extreme endurance events shows no signs of slowing down. Athletes such as Arda Saatçi and Rachel Entrekin have become synonymous with extraordinary feats of endurance. Entrekin recently made sporting history by becoming the first woman to win the legendary Cocodona 250 ultramarathon outright, finishing ahead of the entire men's field after more than 400 kilometers, around 12,000 meters of elevation gain, and almost no sleep.
A US study presented at the 2025 Annual Meeting of the American Society of Clinical Oncology (ASCO), however, raises the question of whether extreme endurance exercise may place greater strain on the gut than previously thought. Importantly, the findings have so far only been presented as a conference abstract, have not yet been peer-reviewed, and do not establish a causal relationship.
Running has been synonymous with good health for decades. Regular exercise strengthens the cardiovascular system, improves metabolism, and has been shown to reduce the risk of numerous chronic diseases, including various types of cancer. Endurance sports in particular are often held up as a prime example of a health-promoting lifestyle.
In the course of their work on the US study, researchers found intestinal polyps at a surprisingly high rate in dedicated marathon and ultra-runners, including advanced adenomas, which are considered precursors to colorectal cancer. We asked PD Dr. Holger Seidl, Director of the Department of Gastroenterology at Isarklinikum Munich, for his perspective.
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A 2025 ASCO study found adenomas in 41% of marathon and ultra-endurance runners — far higher than would be expected for this age group. How do you interpret these findings from a specialist's perspective?
Dr. Holger Seidl: It is well known that extreme physical exertion causes the body to maximize blood flow to the muscles at the expense of blood flow to the intestines. In essence, the athlete is triggering a Stone Age reflex: the fight-or-flight response — the urge to flee "from the saber-toothed tiger" — which allows the body to prioritize rapid escape over all other organ functions in the short term.
However, since this extreme demand was designed to last only minutes rather than many hours, the intestine's tolerance for reduced blood flow is eventually exceeded.
The intestinal inflammation known as "runner's colitis" in such situations is well documented; in this study, too, 30% of participants experienced intestinal bleeding due to mucosal damage. It is suspected that this recurring cycle of damage, inflammation, and cell regeneration and scar formation — as with all chronic inflammation — may promote the risk of abnormal cell proliferation.
Conclusion: Yes, the study data are scientifically sound. The validity of the data is supported by the fact that several quality criteria were met: a prospective design, complete colonoscopies in all participants, and the exclusion of known high-risk patients (e.g., those with other chronic inflammatory conditions such as ulcerative colitis, or genetic syndromes).
One limitation is the small sample size of 100 participants. The study also did not examine whether these individuals had other potential risk factors, such as extreme dietary habits, performance-enhancing substances, or sleep deprivation.
A dedicated comparison group was not available, so data from the literature were used for the assessments instead. A key reference here is the Munich Polyp Study, for example, which examined 178,000 screening colonoscopies.
In that study, adenomas were found in 25–40% of participants depending on age and sex. However, the rate of advanced adenomas was "only" 7.4%, compared to approximately 15% in the current runner study.
Should ambitious marathon and ultra-runners adjust their colorectal cancer screening — for example, by starting earlier or having colonoscopies more frequently? From what age would you recommend this?
Dr. Holger Seidl: I believe so, yes. Without a family history, the current screening age for all individuals in Germany is 50 years. The athletes studied were between 35 and 50 years old, with an average age of 42.5 years. I would therefore recommend starting screening between the ages of 40 and 45 at least, as is currently done for patients with other risk factors (e.g., first-degree relatives with polyps or colorectal tumors).
As a preventive screening measure, this would not be covered by statutory health insurance in Germany — it will take some time before the new findings are studied further and potentially incorporated into guidelines. However, a diagnostic colonoscopy for gastrointestinal symptoms is covered by statutory health insurance regardless of age.
Many athletes dismiss gastrointestinal symptoms — such as blood in the stool, diarrhea, and cramps — as a normal consequence of training. What warning signs should athletes watch for that warrant immediate medical evaluation?
Dr. Holger Seidl: That ties in directly with question 2. Polyps cause no symptoms in their early stages. Advanced polyps on the border of colorectal cancer, or early colorectal cancer itself, can present with blood in the stool. A moderate athlete would be alarmed by this, whereas an extreme athlete might dismiss it as a training effect. We should therefore get the word out: at the latest when intestinal bleeding occurs, it should prompt a workup with colonoscopy.
The study examined 100 marathon and ultra-runners between the ages of 35 and 50. Participants were required to have an extensive running history, including at least five completed marathons or two ultramarathons of over 50 kilometers. Individuals with known hereditary risk factors for colorectal cancer or chronic inflammatory bowel disease were excluded.
The full analysis reveals what findings the researchers recorded among the marathon and ultra-marathon runners they examined:
41% of participants had intestinal polyps or adenomas.
15% had advanced adenomas with elevated malignant potential.
30% reported rectal bleeding after running.
Also worth noting: participants with advanced adenomas reported rectal bleeding significantly more often than those without such findings (53% versus 22%). The results suggest that blood in the stool following intense physical exertion should not be too quickly dismissed as a harmless training effect.
Short answer: No. The study merely shows a statistical association between intensive endurance training and intestinal polyps — not proof that running marathons or ultramarathons causes colorectal cancer. The study has so far only been presented as a conference abstract, has not yet been peer-reviewed, and had no dedicated control group. Larger studies are now needed to determine whether the results can be confirmed.
The study does not offer definitive explanations. The most plausible mechanism, as described by Dr. Holger Seidl, is reduced intestinal blood flow under extreme exertion, which can lead to mucosal damage and the well-known "runner's colitis."
The study authors also address dietary influences: some research links highly processed foods and certain additives to an increased risk of colorectal cancer, and low-fiber diets could further compromise gut health — a factor that may be relevant for endurance athletes who rely heavily on energy gels and sports nutrition products.
These findings come against the backdrop of a global trend: colorectal cancer is increasingly affecting younger adults. In the United States in particular, incidence rates among those under 50 have risen sharply since the 1990s, and similar trends are now being observed across Europe as well.
Despite the ongoing debate, experts are emphatic: regular exercise remains one of the most important protective factors against cancer and other chronic diseases. The new study does not undermine decades of evidence on the health benefits of physical activity. Rather, it suggests that extreme forms of endurance training may carry their own health risks.
• Preliminary study – presented only as an ASCO Meeting Abstract 2025, not yet peer-reviewed
• No causality – there is no proof that running causes colorectal cancer
• No control group – comparison only with general population data
• Small sample size – only 100 participants; larger studies are needed
• Relevant only for extreme athletes – does not apply to recreational joggers or amateur marathon runners
Recreational runners have no reason to panic. The changes observed primarily affect individuals who have maintained a very high training volume over many years. That said, the study also shows that even highly active individuals are not automatically protected from illness.
Blood in the stool, persistent diarrhea, constipation, abdominal cramps, unexplained weight loss, and changes in bowel habits are all warning signs that warrant medical evaluation — regardless of how well those affected feel. Highly athletic individuals in particular tend to dismiss such symptoms as a normal consequence of training, even though they may point to polyps or early colorectal tumors.
According to PD Dr. Seidl, people with many years of very intensive endurance training should consider a preventive colonoscopy as early as age 40 to 45 — well before Germany's standard threshold of 50. In cases of intestinal bleeding, the rule applies regardless of age: seek medical evaluation immediately, as a diagnostic colonoscopy for symptoms is always covered by statutory health insurance.
The new study does not prove that running marathons causes colorectal cancer. However, given the preliminary nature of the data, the small sample size, and the lack of a control group, it does suggest that extreme endurance exercise may have more complex effects on the body than previously assumed.
At the same time, the study serves as a reminder that health cannot be taken for granted, even in highly active individuals. Physical activity remains a cornerstone of disease prevention — but even dedicated athletes benefit from awareness, early detection, and quality medical care.
The extreme demands of races like the Cocodona 250 — over 400 kilometers and around 12,000 meters of elevation gain — illustrate just how far some athletes are willing to push their limits. It is precisely in this territory, as the preliminary data suggest, that serious health risks may also begin to emerge.
