Exercise is a cornerstone of good health and evidence shows it can even help prevent cancers returning following treatment . But new findings are raising an unexpected question : could very high-volume endurance training carry its own risks?
Author
- Justin Stebbing
Professor of Biomedical Sciences, Anglia Ruskin University
At the 2025 American Society of Clinical Oncology meeting, researchers from Inova Schar Cancer Institute reported that a surprising number of dedicated marathon and ultramarathon runners had precancerous growths in their colons. Among 100 athletes aged 35 to 50, 15% had advanced adenomas - lesions that can develop into bowel cancer - while 41% had at least one adenoma.
The study is small and not yet peer-reviewed, but the signal is strong enough to have captured global attention. Here's what the findings really mean, why experts urge caution in interpreting the results, and what runners should watch for.
At first glance, it seems counterintuitive. Decades of evidence show regular exercise lowers cancer risks , including bowel cancer, and improves outcomes after a cancer diagnosis . This study doesn't overturn that science. Instead, it suggests a narrow group of young, very high-volume endurance athletes might face unique bowel stress that could increase their odds of developing precancerous changes over time.
Young people with colon cancer has been called a new epidemic, and we don't really understand why it's increasing so much.
The Inova study deliberately excluded people with known genetic conditions or bowel disease to focus on runners who otherwise seemed low risk. Yet their screening found more advanced lesions than expected for that age group - a pattern outside experts, commenting in the New York Times, described as worth investigating, not a final answer.
How might heavy endurance training contribute to bowel changes? One theory focuses on temporary blood flow reductions to the gut during prolonged, intense exercise. Distance runners are familiar with runner's colitis - cramping and occasional bleeding after long runs. Repeated cycles of low-oxygen stress, inflammation and tissue repair in the bowel could, theoretically, encourage adenoma development in susceptible people.
The Inova team highlighted this mechanism based on observations and runners' reports of gut symptoms, though the study didn't directly measure blood flow, oxygen or inflammation markers. It also didn't isolate other lifestyle factors that might matter, such as dehydration strategies, anti-inflammatory drug use, specific nutrition practices, or very low body fat levels.
Just as important is what this study doesn't establish. It doesn't prove marathons or ultramarathons cause bowel cancer. It doesn't show most young-onset bowel cancers occur in runners - doctors not involved in the study emphasised that most younger patients with these cancers aren't endurance athletes. And it doesn't address whether more moderate exercise carries similar risks.
The comparison point - the expected rate of advanced adenomas in the late 40s - comes from broader population studies, not from a matched control group. That makes the observed difference notable but still preliminary.
The research is clinically grounded, but its size and design mean it should be seen as a starting point for larger studies rather than a basis for changing general exercise guidance .
Still, there are practical lessons for endurance athletes and doctors. First, persistent blood in stool, changes in bowel habits, unexplained stomach pain, or iron-deficiency anaemia shouldn't be dismissed as "just running".
In a community where gut complaints are common and often normalised, it's easy to miss warning signs. The lead oncologist argued that young runners with bleeding after long runs should be offered screening - a stance grounded in the fact that colonoscopy can remove precancerous lesions and prevent cancers developing. This is more cautious than current guidelines for average-risk adults but aligns with individualised, symptom-driven care.
Second, the study reinforces the difference between exercise as medicine and exercise as extreme sport. For cancer prevention and overall health , the strongest evidence supports regular , moderate-to-vigorous activity , not necessarily repeated ultra-endurance feats.
Careful attention needed - not panic
Recent conferences highlighted data showing structured exercise after bowel cancer treatment improves long-term outcomes, underlining that physical activity remains one of the most powerful, low-cost tools in cancer prevention and care . This runner study doesn't contradict that larger story. It flags a potential exception at the extreme end of training that needs careful attention, not panic.
If future research confirms a link, what might change? Screening recommendations could evolve for a clearly defined group of high-volume endurance athletes, perhaps starting colonoscopy earlier than the current age-45 threshold for average-risk adults.
Athletes and coaches might adapt training, nutrition and recovery to protect gut health - paying attention to hydration, heat stress, gradual progression, and avoiding unnecessary anti-inflammatory medication around long efforts.
Sports medicine and gastroenterology clinics might work together on protocols for evaluating gut bleeding in runners, narrowing the gap between "common" and "concerning" symptoms. But these steps depend on replication in larger, diverse groups and understanding which components of endurance life - intensity, duration, heat, altitude, nutrition - matter most.
For now, a balanced message serves the public best. Endurance running is a profound source of meaning and health for many people, and quitting running isn't the lesson from a single small study.
The key is keeping the proven benefits of exercise in view while being clear-eyed about potential risks at extremes. Listen to your body's signals, especially bleeding. Treat red-flag symptoms as medical, not merely athletic. And discuss personal risk factors and family history with your doctor.
As science probes this signal further, the likely outcome isn't a blanket warning but more nuanced guidance: who might need earlier screening, when to investigate symptoms, and how to train hard with the gut in mind.
The study's real contribution may be cultural as much as clinical: it gives runners and doctors permission to ask a question they've too often waved away, and to catch dangerous lesions before they become cancers.
Justin Stebbing does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.
