Early study links long-distance running to higher rate of precancerous colon growths in younger adults

A preliminary study presented at the 2025 American Society of Clinical Oncology (ASCO) Annual Meeting found a higher-than-expected prevalence of precancerous colon growths among long-distance runners aged 35 to 50, prompting calls for further research and for runners to seek medical attention for rectal bleeding.

Researchers at the Inova Schar Cancer Institute in Virginia examined 100 endurance athletes who had never undergone a colonoscopy but met study criteria of completing at least five marathons or two ultramarathons. The team reported that 15 percent of participants had advanced adenomas — growths considered more likely to progress to colorectal cancer — and 41 percent had at least one adenoma. Investigators said the average age of the cohort was 42.5, below the usual screening threshold of 45.

The study's lead investigator, Dr. Timothy Cannon of the Inova Schar Cancer Institute, told reporters that the findings support the hypothesis that extreme endurance exercise may be associated with increased risk of precancerous polyps. Each participant underwent a colonoscopy as part of the study and any growths discovered were reviewed by a panel of gastroenterologists, pathologists and oncologists to classify adenomas and determine which met criteria for being advanced.

Investigators excluded individuals with known hereditary cancer syndromes and those with inflammatory bowel disease to focus on the potential effects of prolonged endurance training. Participants also completed surveys about dietary habits, bowel history and running patterns.

Cannon described the results as surprising given the participants' ages. "I was surprised by just how many of the runners had polyps," he said. "I thought they may have more adenomas and advanced adenomas than expected for that age group… but I did not imagine that 15% would have advanced adenoma."

The research team offered a potential physiological explanation grounded in established exercise physiology: during prolonged endurance activity, blood flow is redirected away from the intestines to support active muscles, which can cause repeated episodes of intestinal ischemia and reperfusion. Investigators said this repeated "gut stress" could produce inflammation or damage that, over time, might contribute to mutations and polyp formation. The mechanism remains theoretical and was not directly measured in the study.

Experts and the study authors were careful to note the research is preliminary and has not been peer reviewed. The study did not include a control group of non-runners for direct comparison, and the investigators acknowledged potential selection and regional biases: most participants were drawn from the Washington, D.C., area and the study did not capture comprehensive multigenerational family histories that could affect colorectal cancer risk.

Those limitations mean the reported rates cannot be reliably generalized to all long-distance runners or used to establish a causal link between endurance running and adenoma formation, clinicians said. "We did not have a control group, so this raises the question as to whether or not there is some bias," Cannon said.

Colorectal cancer screening guidelines have shifted in recent years in response to increases in some age groups, and the typical age to begin average-risk screening in the United States remains a point of professional guidance and discussion. The study's participants were below the commonly used starting age for routine screening, which underscores the investigators' concern that symptoms or risk may be overlooked in younger athletes.

Cannon emphasized that the findings should not discourage regular exercise. "Let me first say that people should continue exercising," he said, adding that physical activity reduces overall cancer risk and that the well-established harms of inactivity far outweigh potential risks of exercise. He advised endurance athletes to monitor gastrointestinal symptoms closely and to seek medical evaluation for any rectal bleeding, persistent diarrhea or new abdominal pain.

The study authors and external gastroenterology specialists noted anecdotal reports from some patients that post-exertional rectal bleeding is sometimes dismissed as benign. Cannon said bleeding after running should not be assumed normal and warrants medical attention. Participants in the study who had lesions detected received appropriate follow-up and management according to standard clinical practice.

Further research is needed to verify the initial observations, including larger studies with control groups, broader geographic sampling and more detailed accounting of family history, diet and other risk factors. Such studies would be required to determine whether a causal relationship exists between long-distance endurance running and increased rates of adenomas, and if so, which mechanisms are responsible and what screening or preventive measures might be warranted for high-volume athletes.

Until more definitive evidence is available, clinicians advise balanced interpretation: regular physical activity remains a cornerstone of public health, but clinicians and athletes should remain attentive to gastrointestinal symptoms and consider appropriate screening based on individual risk factors and evolving professional recommendations.