At the 2025 American Society of Clinical Oncology (ASCO) conference in Chicago, researchers shared that some ultramarathon runners were being diagnosed with advanced colorectal cancer.
During the research, doctors at the Inova Schar Cancer Institute in Virginia studied the colons of younger people who had run several long-distance races. They discovered these runners had a much higher chance of having risky adenomas than expected for their age, suggesting a link between extreme endurance running and colorectal cancer.
The study was led by Dr. Timothy Cannon, who became interested after treating three young ultramarathon runners with colorectal cancer. All three were fit and much younger than the usual patients, with the oldest only 40.
In their research, the team looked for advanced adenomas, which are larger or unusual polyps that carry a higher risk of becoming cancerous. They compared their findings in athletes to existing medical data and found the rates were unusually high.
At first, this may seem surprising as exercise is usually protective against many diseases, including cancer. However, extreme endurance running may affect the body differently.
The scientists explained that blood flow is directed mainly to muscles and skin during very long races, leaving the gut with less circulation. This lack of blood flow, called bowel ischaemia, can damage the intestinal lining. Over time, repeated injury and inflammation may, create conditions that allow abnormal cells to develop, raising the risk of advanced adenomas.
"Given that many runners describe bleeding after running, and runner's colitis is understood to be related to colonic ischaemia and recurrent inflammation, it made us wonder whether the intense physical stress of endurance training could be contributing to a higher likelihood of mutagenesis causing precancerous polyps," Cannon noted.
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