Are Ultramarathon Runners at Increased Risk of Bowel Cancer?

Recent research has brought to light new insights into the relationship between extreme endurance training and bowel health. While regular exercise is widely recognized for its protective effects against various cancers, including bowel cancer, a small study suggests that very high-volume endurance athletes might face specific risks.

A study presented at the 2025 American Society of Clinical Oncology meeting investigated the prevalence of precancerous bowel lesions among dedicated marathon and ultramarathon runners. The findings indicated that within a sample of 100 athletes aged 35 to 50, 15% had advanced adenomas—lesions known to potentially develop into bowel cancer—and 41% had at least one adenoma. Although the study was small and not yet peer-reviewed, these results have garnered significant attention, prompting a reevaluation of assumptions regarding exercise and cancer risk.

The paradoxical nature of these findings lies in the established understanding that regular physical activity reduces cancer risk and improves treatment outcomes. The new data, however, suggest that at a certain threshold of intense endurance exercise, there could be unique effects on bowel tissue. The proposed mechanism involves temporary reductions in blood flow to the gut during prolonged, intense exercise, which may result in inflammation and tissue stress, potentially encouraging the development of precancerous changes.

It's important to note that the research doesn't establish causality—meaning, it doesn't prove that ultramarathon running directly causes bowel cancer. Most young individuals with bowel cancer are not endurance athletes, and the study specifically excluded those with known genetic conditions or bowel diseases. The observed increase in lesions may be related to metabolic or vascular stresses unique to extreme endurance sports.

Further limitations include the study’s small size and lack of a control group, making the results preliminary. Experts emphasize that the overall science still strongly supports exercise as a key element of cancer prevention and health promotion. The findings highlight the importance of monitoring symptoms such as blood in stool, bowel habit changes, or unexplained stomach pain. Clinicians advise that persistent concerns should lead to appropriate screening, including colonoscopy, which can effectively identify and remove precancerous lesions.

Practical recommendations for athletes include paying close attention to hydration, avoiding unnecessary anti-inflammatory medication, and gradually increasing training intensity. The goal is to balance the immense benefits of physical activity with caution at the extremes of endurance efforts. This research advocates for a nuanced approach—recognizing the health benefits of exercise while remaining vigilant about potential risks in high-intensity endurance sports.

As further studies are conducted, guidelines for screening and training adaptations might evolve, especially for those engaging in ultra-endurance activities. For now, maintaining regular, moderate exercise remains a cornerstone of health, and any concerning symptoms should be discussed with a healthcare provider to ensure early detection and prevention.