Exercise and Bowel Health

Exercise plays a vital role in maintaining health, and there’s evidence suggesting it can help prevent cancer recurrence after treatment. However, new findings are raising an unexpected concern: could engaging in very high-volume endurance training have its own risks?

During the 2025 American Society of Clinical Oncology meeting, researchers from Inova Schar Cancer Institute shared surprising results indicating that numerous dedicated marathon and ultramarathon runners had precancerous growths in their colons.

Among a group of 100 athletes aged 35 to 50, 15% were found to have advanced adenomas—which can lead to bowel cancer—while 41% had at least one adenoma.

This study is small and still awaiting peer review, but the findings have enough weight to have captured international attention. What does this really mean? Experts urge caution in interpreting results and point out what runners should keep an eye on.

At first, the results appear counterintuitive. Extensive research shows regular physical activity lowers cancer risks, including bowel cancer, and enhances outcomes after a cancer diagnosis.

This study doesn’t negate existing science, rather it indicates that a small, specific group of young, high-volume endurance athletes may experience unique bowel stress that could raise their risk for precancerous changes over time.

Young individuals with colon cancer have been termed a new epidemic, and there remains uncertainty regarding the reasons behind its increase.

The Inova study specifically excluded participants with known genetic predispositions or pre-existing bowel diseases, aiming to focus on runners who were otherwise considered low risk. Despite this, they detected more advanced lesions than anticipated in that age group—a trend that experts have commented on in the New York Times, suggesting that it warrants further investigation, rather than serving as a definitive conclusion.

So, how might the rigors of endurance training contribute to bowel changes? One proposed explanation involves temporary reductions in blood flow to the gut during prolonged, intense workouts.

Runner’s colitis, with symptoms like cramping and occasional bleeding following long runs, is a familiar experience for many distance runners. Repetitive cycles of low oxygen, inflammation, and tissue repair in the bowel could logically lead to adenoma development in susceptible individuals.

The Inova team highlighted this idea based on anecdotal evidence and runners’ experiences with gut issues, though the study did not directly assess blood flow, oxygen levels, or inflammation markers. It also did not consider other lifestyle factors such as hydration strategies, anti-inflammatory medication use, specific dietary practices, or very low body fat rates.

Importantly, the study does not establish a causal relationship. It does not demonstrate that marathons or ultramarathons are responsible for bowel cancer. Moreover, it clarifies that most younger patients diagnosed with these cancers aren’t endurance athletes, as medics not involved in the study noted. It doesn’t address whether moderate exercise might carry similar risks either.

The expected prevalence of advanced adenomas in the late 40s is derived from widespread population studies rather than a matched control group, making the observed differences relevant, yet still preliminary.

This research is clinically relevant, but due to its size and design, it should be considered a foundation for larger studies, rather than a basis for altering general exercise recommendations.

Nonetheless, there are actionable insights for endurance athletes and healthcare professionals. First, persistent blood in stool, abrupt changes in bowel habits, unexplained abdominal pain, or iron-deficiency anemia should not be dismissed as mere “running issues.”

In a community where gastrointestinal complaints are frequent and often normalized, it can be easy to overlook warning signs. The lead oncologist suggested that young runners experiencing bleeding after long runs should receive screening—a view grounded in the understanding that colonoscopy can remove precancerous lesions and stave off cancer development.

This recommendation is more cautious than current guidelines for adults at average risk, but aligns with a personalized, symptom-driven approach to care.

Secondly, this study underscores the distinction between exercising as a form of treatment and pushing the limits of extreme sports. For cancer prevention and overall well-being, there is a strong suggestion for regular, moderate-to-vigorous activity, rather than repeated ultra-endurance challenges.

Proceed with Caution

Recent symposiums have highlighted data indicating that structured exercise post-bowel cancer treatment enhances long-term outcomes, emphasizing that physical activity is one of the most effective, cost-efficient tools in cancer prevention and management.

This study pertaining to runners does not contradict the broader narrative; rather, it points out a potential anomaly at the extreme end of exercise that calls for careful attention instead of alarm.

If further research validates this connection, what changes might occur? Screening criteria could adapt for a clearly delineated group of high-volume endurance athletes, possibly initiating colonoscopy at earlier ages than the current threshold of 45 for average-risk adults.

Athletes and coaches might modify training, nutrition, and recovery practices to safeguard gut health—focusing on hydration, managing heat stress, ensuring gradual progression, and avoiding unnecessary anti-inflammatory medications during long efforts.

Sports medicine and gastroenterology clinics could collaborate to form a protocol for addressing gastrointestinal bleeding in runners, helping distinguish between ordinary and concerning symptoms.

However, pursuing these changes will depend on replication of findings in larger, diverse populations and discerning which elements of endurance training—like intensity, duration, heat, altitude, or nutrition—are most impactful.

For the time being, a balanced perspective is essential. Endurance running brings significant meaning and health benefits to numerous individuals, and a single small study shouldn’t deter anyone from participating.

The priority should be on recognizing the well-established benefits of physical activity while also being realistic about potential risks at extremes. Listen to your body, especially regarding unusual bleeding. Treat concerning symptoms seriously, not merely as athletic fatigue. And make sure to discuss personal risks and family history with your healthcare provider.

As science delves deeper into these findings, the likely outcome won’t be an overarching warning but rather more tailored guidance; for instance, identifying who may need earlier screenings, when to investigate symptoms, and how to pursue training while considering gut health.

Ultimately, the study’s significant contribution may be more cultural than clinical: it provides runners and healthcare professionals with the permission to pose questions that have often been brushed aside, allowing for the early detection of dangerous lesions before they progress to cancer.