Long-distance running is widely celebrated for its cardiovascular, metabolic, and mental health benefits. Marathon runners, in particular, are often viewed as models of optimal physical fitness and healthy living. However, emerging research presented at the American Society of Clinical Oncology (ASCO) 2025 Annual Meeting suggests that intensive, long-distance running may be associated with an increased risk of advanced adenomas (AAs) of the colon, a known precursor to colorectal cancer.

These findings challenge long-held assumptions that high levels of endurance exercise are universally protective against cancer and underscore the need for more nuanced screening and prevention strategies in this unique population.

Understanding Advanced Adenomas and Colorectal Cancer Risk

Advanced adenomas are colorectal polyps characterized by features such as larger size, villous histology, or high-grade dysplasia. They are clinically significant because they represent a critical intermediate step in the progression from benign lesions to invasive colorectal cancer. Early detection and removal of advanced adenomas through screening colonoscopy significantly reduce colorectal cancer incidence and mortality.

While moderate physical activity has consistently been linked to a reduced risk of colorectal cancer, the effects of extreme or prolonged endurance exercise on colon health have remained largely unexplored until recently.

Key Findings from the ASCO 2025 Study

The prospective study presented at ASCO 2025 examined a cohort of intensive long-distance runners, including marathon and ultramarathon participants, who underwent colonoscopic evaluation. Investigators observed a higher-than-expected prevalence of advanced adenomas compared with general population benchmarks.

According to Timothy L. Cannon, MD, an oncologist at the Inova Schar Cancer Institute in Fairfax, Virginia, and lead investigator of the study, the findings raise important questions about how extreme endurance training may influence colorectal pathology.

Although the study was relatively small and not designed to establish causation, its prospective design and focused population provide valuable early insights into a previously understudied risk group.

Potential Biological Mechanisms

Several hypotheses have been proposed to explain why intensive long-distance running could increase the risk of advanced adenomas:

• Chronic gastrointestinal ischemia: Prolonged endurance exercise can divert blood flow away from the gastrointestinal tract, potentially leading to repeated episodes of mucosal injury and impaired healing.

• Inflammation and oxidative stress: High-intensity, long-duration exercise may induce systemic inflammation and oxidative stress, which could promote abnormal cellular changes in the colonic epithelium.

• Alterations in gut microbiota: Endurance training has been shown to alter the composition of the gut microbiome, which may influence colorectal carcinogenesis.

• Nutritional factors: Marathon runners may adopt specialized diets, including high carbohydrate intake or frequent use of supplements, which could interact with individual genetic or metabolic risk factors.

Importantly, these mechanisms remain speculative, and further pathologic and epidemiologic research is needed to clarify causation.

Implications for Screening and Prevention

Given the study’s findings, the authors suggest that refined colorectal cancer screening strategies may be warranted for individuals engaged in intensive long-distance running. This does not imply that marathon runners should stop exercising, but rather that one-size-fits-all screening recommendations may not adequately address their unique risk profile.

Potential considerations include:

• Earlier initiation of colonoscopy screening

• Shorter screening intervals for high-mileage runners

• Greater attention to gastrointestinal symptoms often dismissed as exercise-related

Dr. Cannon and colleagues emphasize that clinicians should maintain a high index of suspicion when evaluating endurance athletes who report persistent gastrointestinal complaints.

Balancing Benefits and Risks of Endurance Exercise

It is important to place these findings in context. Long-distance running remains associated with numerous health benefits, including reduced risk of cardiovascular disease, type 2 diabetes, obesity, and overall mortality. The potential increase in advanced adenoma risk does not negate these advantages but highlights the complexity of human physiology under extreme physical stress.

The study also reinforces the concept that more exercise is not always better, and that the health effects of physical activity likely follow a dose-dependent relationship that varies across organ systems.

Future Research Directions

The authors call for larger, multicenter studies to validate these findings and explore:

• Dose-response relationships between running intensity, duration, and colon pathology

• Genetic and lifestyle modifiers of risk

• Long-term colorectal cancer outcomes in endurance athletes

Such research could ultimately inform evidence-based, personalized screening guidelines for athletes engaged in high-intensity endurance sports.

Conclusion

The ASCO 2025 findings suggest that intensive long-distance running may be associated with an increased risk of advanced adenomas of the colon, highlighting a previously unrecognized health consideration for marathon runners. While further research is needed to confirm these results and elucidate underlying mechanisms, the study underscores the importance of tailored screening approaches and ongoing vigilance in populations traditionally viewed as low risk.

As our understanding of exercise biology evolves, so too must our strategies for maximizing health benefits while minimizing unintended risks.