Marathon Runners Identify Unusual Association with Cancer in Studies
Study Suggests Link Between Extreme Endurance Running and Colorectal Cancer Risk
A recent study led by oncologist Dr. Timothy Cannon at Inova Schar Cancer Institute has raised concerns about the potential link between extreme long-distance running and an increased risk of colorectal cancer. This research challenges the conventional view that high-volume exercise protects against colon cancer.
The study, presented at the 2025 American Society of Clinical Oncology (ASCO) annual meeting, was prompted by cases of advanced colorectal cancer in three young, physically fit endurance athletes under 40, with no known risk factors. Two of these athletes were ultramarathoners, and one had completed many marathons and half-marathons.
The pilot study involved 100 runners aged 35-50 who had completed either two or more ultramarathons (≥50 km) or at least five marathons (26.2 miles). Colonoscopy findings showed that nearly 50% of these athletes had polyps, and 15% had advanced adenomas, which are precancerous lesions with a high potential to develop into cancer. This is significantly higher than the 4.5%-6% expected rate in the general population of similar age, and even higher than 12%, a rate observed in some high-risk groups like Alaska Natives.
The study hypothesizes that extreme endurance exercise could cause repeated colonic ischemia (reduced blood flow) and inflammation, leading to tissue stress and increased likelihood of mutations in colorectal cells, which might drive polyp and cancer development.
While the findings are preliminary and do not yet establish causation, they raise important questions about colorectal cancer screening guidelines for extreme endurance athletes. The researchers note that their work doesn't provide definitive proof that endurance running can cause colorectal cancer, but it does suggest a potential higher risk of advanced adenomas in runners compared to the average risk for people in their 40s.
The volunteers had all run at least two ultramarathons or five regular marathons and had no family history of colorectal cancer or other apparent risk factors. The study was inspired by three young patients with colorectal cancer, all of whom had run ultramarathons.
The researchers went looking for advanced adenomas in the colons of their volunteers and compared the rate of finding these polyps to historical trends. In the study, 15% of runners had advanced adenomas, while nearly half had polyps in general. The study hasn't undergone the formal peer-review process yet.
Regular physical activity, including running, still offers many health benefits, such as a lower risk of at least eight different types of cancer. However, this recent clinical study provides initial evidence that extreme endurance running might be associated with a higher risk of precancerous colorectal lesions, indicating a need for further research and potentially earlier screening in this specific population. Future studies should aim to confirm the findings and untangle the causes and risk factors.
