Quick Facts

• 15% of ultramarathon runners had advanced adenomas compared to 5-6% in the general population
• Colorectal cancer is now the leading cause of cancer death in young adults, with 1 in 5 diagnoses under age 55
• Screening guidelines dropped from age 50 to 45 in 2021, but athletes may need earlier testing

A groundbreaking study presented at the 2025 American Society of Clinical Oncology Annual Meeting reveals endurance athletes face significantly higher rates of precancerous colon polyps than previously thought.

Dr. Timothy Cannon at Inova Schar Cancer Institute studied 100 runners who completed at least two ultramarathons or five regular marathons. The results were alarming: 15% had advanced adenomas with high cancer potential, nearly triple the 5-6% rate found in the general population of adults in their 40s.

The research began after three young, physically fit endurance athletes under age 40 were diagnosed with advanced colon cancer. One was a triathlete, another completed seven marathons in seven days, and a third regularly ran 100-mile ultramarathons. All three later died from the disease.

“We believe this research may help define a new, at-risk population for earlier colorectal cancer screening,” Cannon said. “If we can identify these cancers earlier in people who otherwise wouldn’t meet current screening criteria, we can save lives.”

The timing is critical. Colorectal cancer rates are surging among young adults. One in five people diagnosed with colorectal cancer is now under age 55. The disease has become the leading cause of cancer-related death among young adults.

In the early 2000s, about 5% to 7% of colon cancer diagnoses were early onset. Now, around 10% occur in people younger than 50. Researchers project 45% of colorectal cancer diagnoses this year will be in individuals younger than 65, up from 27% in 1995.

The U.S. Preventive Services Task Force lowered screening recommendations from age 50 to 45 in 2021. But the new research suggests high-mileage runners may need screening even earlier.

Cannon recommends annual FIT testing starting at age 35 for high-mileage runners. Positive results require colonoscopy. He particularly urges screening for young runners who experience blood in stool after long runs.

“I feel strongly that young runners who have blood in their stool after long runs, a problem that has historically been dismissed as ‘normal,’ should receive screening,” Cannon said.

The proposed mechanism involves blood flow loss to the gut during intensive exercise. This ischemia causes cellular damage and rapid cell turnover, potentially linking extreme exercise to cancer development.

Cannon stressed the study should not discourage exercise. “Let me first say that people should continue exercising,” he said. “We have far more problems from not exercising than from exercising too much. Exercise decreases your risk of cancer overall.”

Athletes should monitor warning signs including persistent blood in stool, changes in bowel habits, unexplained stomach pain, or iron-deficiency anemia. These symptoms should not be dismissed as normal training effects.

The study, while small at 100 athletes with no control group, represents the first research linking extreme endurance exercise to colon cancer risk. Larger controlled studies are needed to confirm the findings.

Read more: Why Even Young & Fit Athletes Can’t Ignore Colon Cancer Screening