A groundbreaking study presented at the 2025 American Society of Clinical Oncology Annual Meeting has revealed a concerning connection between ultramarathon running and colorectal cancer in younger adults.
Dr. Tim Cannon, an oncologist at Inova Schar Cancer in Fairfax, initiated the research after noticing multiple ultramarathoners under 40 presenting with advanced-stage colon cancer. The prospective study enrolled 100 healthy participants aged 35–50 who had completed at least five marathons or two ultramarathons and had never undergone a colonoscopy.
The findings were striking: 15 percent of participants had advanced adenomas—precancerous lesions—compared to just 1–2 percent expected in average-risk individuals their age. Additionally, 41 percent had at least one adenoma, despite an average age of 42.5 years.
Dr. Cannon hypothesizes that intense physical stress from endurance training may cause bowel ischemia (a condition where the intestines do not receive enough blood flow) and recurrent inflammation, potentially triggering mutagenesis. Many runners experience bleeding after running, a symptom often dismissed as normal.
While the results don’t prove causation, they suggest extreme endurance exercise could be a meaningful risk factor warranting earlier screening for high-performance athletes. The research, conducted from October 2022–December 2024, may help identify a new at-risk population for life-saving early detection.
This article originally appeared in the February 2026 issue.
