Thinking of hitting the sidewalk this weekend for a marathon? Hold on before you tie up your running shoes, and listen. A recent study from the Inova Schar Cancer Institute has raised concerns by suggesting that extreme endurance running—such as marathons and ultramarathons—may increase the risk of precancerous colon growths in younger adults. The study was presented at the 2025 American Society of Clinical Oncology (ASCO) Annual Meeting in Chicago Colon canceris one of the most common cancers worldwide and starts as noncancerous polyps in the large intestine. While this study doesn’t prove that marathon running causes colon cancer, it does highlight a possible link that even very fit people should be mindful of regarding their colon health. The study focused on healthy runners aged 35 to 50 who had completed many long-distance races. The researchers found that 41% of these runners had at least one adenoma, a type of colon polyp that can develop intocancer. More alarmingly, 15% had advanced adenomas, which are larger, more abnormal growths that are more likely to turn cancerous over time. These rates are much higher than the typical 1-2% expected in this age group. Why might marathon running increase the risk? During long runs, blood flow is diverted from the gut to the muscles, reducing oxygen supply to the intestines and possibly causing small injuries. This repeated stress, along withinflammation, might lead to changes in the colon lining. Known symptoms like "runner’s colitis" or “runner’s trots” include loose stools and occasional bleeding, which might contribute to risk. ““This is the first prospective study exploring whether exercise-induced intestinal stress from long-distance running might raise cancer risk. While not proving cause, results suggest intense endurance exercise could increase colorectal cancer risk and warrant further research. This work may define a new at-risk group for earlier cancer screening. Young runners with blood in their stool after long runs—often dismissed as normal—should get screened, as early screening can prevent advanced cancers” said Dr. Cannon. Reference:Tim Cannon, MD, Risk of pre-cancerous advanced adenomas of the colon in long distance runners; 2025 ASCO Annual Meeting; 2025 American Society of Clinical Oncology; Doi: 10.1200/JCO.2025.43.16_suppl.3619