For many people, long-distance running offers several health benefits, from supporting cardiovascular fitness to weight loss advantages. But what if pushing your body to extreme limits, like running marathons and ultramarathons, could quietly increase your risk of colon cancer?
That’s exactly what a new study has suggested, raising an eyebrow in the running community.
What Does The Study Suggest?
The research presented at the 2025 American Society of Clinical Oncology (ASCO) conference in Chicago highlighted a surprising trend: several ultramarathon runners were showing up with advanced colorectal cancer. To dig deeper, they launched a prospective study to see whether long-distance running might be tied to precancerous growths in the colon.
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The study included 100 runners aged 35–50 who had either:
- Completed two ultramarathons (50 km or longer)
- Run at least five marathons (26.2 miles).
When these runners underwent colonoscopies, the results were striking:
- 15% had advanced adenomas (precancerous colon polyps).
- By comparison, only about 1.2% of average-risk people in their 40s are expected to have them.
- Nearly 40% of the runners had at least one polyp.
In simple terms, these endurance athletes had a much higher rate of potentially dangerous colon growths than people their age normally would.
Why Would Running Pose A Risk?
When you first hear of it, it may sound strange. After all, regular exercise is known to protect against many diseases, including cancer. But in the case of extreme endurance running, things might be different.
During long races, the body prioritises sending blood to the muscles and skin. That means the gut gets less blood flow, a condition known as bowel ischaemia. This can damage the intestinal lining. Over time, repeated injury and inflammation may create an environment where abnormal cells can grow, eventually turning into advanced adenomas.
“The participant athletes were otherwise healthy and had no known genetic predisposition or inflammatory conditions,” said Dr Tim Cannon, a medical oncologist at Inova Schar Cancer in Fairfax. “Given that many runners describe bleeding after running, and runner’s colitis is understood to be related to colonic ischaemia 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.”
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What Are Advanced Adenomas?
Advanced adenomas are precancerous colorectal polyps that have a higher risk of developing into colorectal cancer. They are characterised by their size (1 centimetre or larger), the presence of villous features (a specific growth pattern), or high-grade dysplasia (abnormal cell growth).
Detecting and removing advanced adenomas through colonoscopies can significantly reduce the risk of colorectal cancer.
What This Means If You’re A Runner
If you’re training for marathons or ultramarathons, don’t panic. Running is still one of the best things you can do for your overall health.
But this research is a reminder that more isn’t always better and that the gut deserves attention too.
Here’s what you should keep in mind if you are an endurance athlete:
- Talk to your doctor about early screening; current guidelines recommend colonoscopy starting at age 45, but if you’re a long-distance runner, it may be worth starting earlier.
- Pay attention to symptoms. Don’t ignore rectal bleeding, unexplained abdominal pain, or changes in bowel habits.
- Stay hydrated.
- Listen to your body, especially during and after long runs.
Conclusion
Running marathons and ultramarathons improves long-term fitness and endurance. However, it may also put strain on the gut that we’re only beginning to understand. The above-mentioned study found that 15% of long-distance runners under 50 had advanced colon polyps, far higher than the general population. That doesn’t mean runners should stop doing what they love. Instead, it’s a call to be proactive with screening and gut health. Running strengthens the heart and mind; now it’s time to make sure the colon gets some protection too.