Running is one of the most accessible forms of exercise with a wealth of benefits for both body and mind, says James Witts. But recent research suggests ultra-runners (where you run 50km or more) are potentially at greater risk of colon cancer, raising questions about screening for high-performance athletes.
The study, presented at the 2025 American Society of Clinical Oncology annual meeting in Chicago but yet to be peer reviewed (where a group of scientists assess the validity of a study) and published, found colon polyps in nearly half of the 100 ultra-runners and marathoners who took part in the study.
Nearly 15% had advanced adenomas – pre-cancerous lesions – compared to 1-2% in the general population. The participants were aged between 35 and 50 and had no history of the disease in the family.
“This is the first prospective study we know of to explore whether exercise-induced intestinal stress – specifically bowel ischemia from long-distance running – might increase cancer risk,” said lead author, Dr Timothy Cannon oncologist at Inova Schar Cancer Institute in Virginia, USA.
While the results don’t prove causation, they strongly suggest that extreme endurance exercise could be a meaningful risk factor for colorectal cancer.
How? Potential causes of the polyps could be due to repetitive gut ischemia. During prolonged exercise, blood is diverted from the intestines to the muscle and skin, to aid movement and cooling. This can result in consistent episodes of an ‘ischemia-reperfusion’ injury in the colon, caused by a shortage of bloodflow. Over time, repeated gut injury may lead to mucosal damage, chronic inflammation and possibly increased polyps formation.
Top ultra-runners can train up to 25hrs a week. Amateurs can still train up to 12hrs each week, especially if they’re battling 100km events. That hasn’t deterred record attendance with a 2021 study revealing a 1,676% in participation since the late 70s. Anecdotally, ultra-running has pushed on again since 2021.
Dr Cannon stresses that people shouldn’t stop running. Which is good advice as it improves your cardiovascular health, strengthening your heart and lowering blood pressure; helps manage weight; strengthens bones and joints; plus enhancing mood and reducing stress. Even just five to 10 minutes’ running a day has been linked to longer life expectancy.
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However, he believes the research may help define a new, at-risk population for earlier colorectal cancer screening. “If we can identify these cancers earlier in people who otherwise wouldn’t meet current screening criteria, we can save lives,” he said. “I feel strongly that young runners who have blood in their stool after long runs, a problem that has historically been dismissed as ‘normal’ or inconsequential, should receive screening. The good news is that screening can prevent advanced cancers.”
Ongoing research by the Inova Schar Cancer Institute is examining dietary habits, training routines and other lifestyle factors in the study group to better understand what may be contributing to this trend.
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