A new prospective study from the Inova Schar Cancer Institute is prompting both curiosity and caution—suggesting that very high-volume endurance running might be linked to an increased risk of precancerous colon lesions. Dr. Timothy Cannon, co-director of the Gastrointestinal Cancer Program at Inova, noticed an unusual trend: ultramarathon runners under age 40 were presenting with advanced-stage colorectal cancer.
Troubled by this pattern, he initiated a clinical study to investigate if prolonged endurance running could be a factor. “These were otherwise healthy athletes with no known genetic predisposition or inflammatory conditions,” Dr. Cannon said. “Given that many runners describe bleeding after running … the intense physical stress of endurance training could be contributing to a higher likelihood of mutagenesis causing precancerous polyps,” he said.
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Between October 2022 and December 2024, the study recruited 100 runners aged 35–50—individuals free of hereditary cancer syndromes or inflammatory bowel disease—who had completed at least five marathons or two ultramarathons. Each underwent a screening colonoscopy, with findings carefully evaluated by a panel of experts. The results were striking:
- 15% had advanced adenomas—precancerous lesions that are significantly higher than the typical 1–2% expected in average-risk individuals of the same age.
- A larger group—41%—had at least one adenoma.
“It was a surprise to me—it was that many,” Cannon added, referencing the unusually high prevalence of adenomas in the cohort.”
Experts stress that this initial study is not definitive for various reasons:
- The study was small, lacked a control group of non-runners, and remains unpublished in a peer-reviewed journal
- Dr. Cathy Eng noted uncertainty: “Would [those polyps] have already been present regardless of their athletic status?”
- Dr. Christina Dieli-Conwright emphasized, “I would hate to deter people from running … That would be unfair to running.” She described the findings as “thought-provoking” but in need of further research.
Until more research is done on non-runners, researchers propose a plausible—but unproven—mechanism: during prolonged intense exercise, blood is diverted away from the gastrointestinal tract, potentially leading to repeated intestinal ischemia (low blood flow), injury, and inflammation, which may foster precancerous changes.
Dr. Cannon underscores the importance of not discouraging exercise: “The bigger problem with our health is we don’t exercise enough. People should keep exercising, for sure.”
Yet he also urges vigilance: “I feel strongly that young runners who have blood in their stool after long runs … should receive screening. The good news is that screening can prevent advanced cancers.”
