Long-Distance Running Linked to Increased Colon Cancer Risk, Study Finds

New Study Links Long-Distance Running to Increased Colon Cancer Risk

A recent study has raised concerns about the potential health risks associated with long-distance running, including marathons and ultramarathons. The findings suggest that extreme endurance athletes may face a higher risk of developing precancerous polyps in the colon. This research, conducted by Dr. Timothy Cannon of the Inova Schar Cancer Institute, was presented at the 2025 American Society of Clinical Oncology (ASCO) Annual Meeting in Chicago.

Dr. Cannon's interest in this topic began after he noticed an unusual number of ultramarathoners under the age of 40 presenting with advanced colon cancer. To investigate further, he initiated a study focusing on runners who had completed at least five marathons or two ultramarathons, while excluding those with a family history of cancer or other known risk factors.

Key Findings from the Study

The study involved 100 participants aged 35 to 50 who had never undergone a colonoscopy but had completed multiple long-distance races. Individuals with hereditary cancer syndromes or inflammatory bowel disease were excluded to ensure the focus remained on the impact of endurance exercise alone.

Each participant underwent a colonoscopy as part of the study, along with a detailed survey about their dietary habits, bowel history, and running patterns. The results were striking: 15% of participants had advanced adenomas, which are precancerous growths in the colon, compared to the typical 1–2% for their age group. Additionally, 41% had at least one adenoma.

The findings were reviewed by a panel of gastroenterologists, pathologists, and oncologists to confirm the presence of advanced adenomas. Dr. Cannon expressed surprise at the high prevalence of these growths among the runners, noting that he expected an increase in adenomas but not at such a significant rate.

Possible Causes and Theories

Experts believe that intense endurance training might cause repeated "gut stress." During prolonged runs, blood is redirected away from the intestines, potentially leading to damage and inflammation. Over time, this could contribute to genetic mutations and the formation of polyps.

However, the study has limitations. It lacks a control group and has not yet been peer-reviewed. Dr. Cannon acknowledged that the participants were primarily from the Washington DC area, which might introduce other common risk factors beyond running. The study also did not account for detailed family histories, which could influence the results.

Recommendations and Next Steps

Despite the concerning findings, experts emphasize that running should not be abandoned entirely. Dr. Cannon stressed the importance of continued physical activity, noting that the risks of inactivity far outweigh those of excessive exercise. Exercise is generally associated with a lower overall cancer risk, even though this study suggests that extreme forms of exercise, like ultramarathons, may carry additional risks.

For those engaging in long-distance running, it is crucial to monitor any symptoms such as bleeding, persistent diarrhea, or abdominal pain. Bleeding after running should not be considered normal and should prompt a visit to a healthcare professional.

Dr. Cannon highlighted that there is no expert consensus suggesting that rectal bleeding or blood in stool is benign. However, some gastroenterologists may dismiss these symptoms as normal in runners, which could lead to delayed diagnosis.

Conclusion

While the study raises important questions about the relationship between long-distance running and colon cancer risk, more research is needed to confirm these findings. For now, maintaining a balanced approach to exercise and being vigilant about health symptoms remains essential for all athletes.