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A "groundbreaking" new study led by Dr. Tim Cannon, a medical oncologist at Inova Schar Cancer in Fairfax, has revealed a potential link between extreme endurance exercise and increased risk of advanced colon cancer in younger adults, raising new questions about screening for high-performance athletes, Inova announced Tuesday.
The study was presented at the 2025 American Society of Clinical Oncology (ASCO) Annual Meeting in Chicago.
Background
Several years ago, Cannon noticed a disturbing trend: multiple ultramarathoners under age 40 were presenting at Inova with advanced-stage colorectal cancer. Troubled by the pattern, he launched a prospective clinical study to investigate whether long-distance running could be associated with an elevated risk of developing colon cancer. Recruitment material was made with the assistance of the wives of two of the deceased runners.
“The participant athletes were otherwise healthy and had no known genetic predisposition or inflammatory conditions,” said Cannon. “Given that many runners describe bleeding after running, and runner’s colitis is understood to be related to colonic ischemia 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.”
The study enrolled 100 participants between the ages of 35 and 50 who had completed at least five marathons or two ultramarathons and had never undergone a colonoscopy. Importantly, individuals with hereditary cancer syndromes or inflammatory bowel disease were excluded to isolate the potential impact of endurance exercise alone.
Each participant underwent a screening colonoscopy as part of the study. Prior to the procedure, each subject also completed a questionnaire covering dietary habits, bowel history, and long-distance running patterns. All polyps discovered during colonoscopy were reviewed by a panel of gastroenterologists, pathologists, and oncologists to determine if they met the criteria for advanced adenomas.
The findings show:
- 15% of participants had advanced adenomas (pre-cancerous lesions), compared to the expected 1–2% in average-risk individuals in this age range.
- 41% of participants had at least one adenoma.
- The average age of participants was 42.5 years old – two and a half years below the recommended age for a first screening colonoscopy.
“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 Cannon. “While the results don’t prove causation, they strongly suggest that intense endurance exercise could be a meaningful risk factor for colorectal cancer and warrants further study.”
The study was conducted from October 2022 through December 2024 and received Institutional Review Board (IRB) approval. The team now hopes these findings will spark discussion about earlier screening for high-risk athletes and lead to life-saving detection strategies.
“We believe this research may help define a new, at-risk population for earlier colorectal cancer screening,” said Cannon. “If we can identify these cancers earlier in people who otherwise wouldn’t meet current screening criteria, we can save lives. 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.”
Further research is underway to analyze dietary factors, training regimens, and other lifestyle habits from the study group to better understand what’s driving this trend. Inova Schar Cancer continues to lead national efforts in early detection, personalized cancer care, and clinical research that shapes the future of cancer prevention.
