For many people, running long distances is a symbol of health, discipline, and strength.
But new research is raising questions about whether this form of extreme exercise might carry hidden risks—particularly for the colon.
A recent study suggests that marathon and ultramarathon runners between the ages of 35 and 50 could face a higher chance of developing precancerous growths in their colon, despite appearing to live a model “healthy lifestyle.”
The Study Behind the Concern
Dr. Timothy Cannon, a medical oncologist in Washington, D.C., recruited 100 marathon and ultramarathon runners within this age group to undergo colonoscopies. What they found was surprising:
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41 percent of participants had polyps, growths in the colon that can be a precursor to cancer.
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15 percent of runners had advanced adenomas, benign tumors with the potential to turn cancerous.
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More than half of those with growths reported rectal bleeding—a classic warning sign of colon cancer.
For comparison, only about 4.5 to 6 percent of adults in their 40s in the general population are typically found with advanced adenomas.
Why Running Could Play a Role
So why would long-distance running, considered such a positive lifestyle choice, increase cancer risk? Dr. Cannon explained that when people run for extended periods, blood is redirected to the legs and away from the colon.
This reduced supply can cause tissue damage, cell death, and rapid cell turnover—conditions that may allow harmful mutations to form.
“I’ve seen several patients in their 30s who ran ultramarathons and still developed colon cancer,” he said. “That’s what made us look closer.”
Remembering Patients Who Inspired the Study
The research was carried out in memory of two patients—Josh Wadlington and Spencer “Trigg” Robbins—both of whom were marathon runners who lost their battle with colon cancer at a young age.
Their experiences highlighted the need to investigate whether extreme exercise might be linked to the disease.
Who Took Part in the Research
Participants in the study were marathoners or ultramarathoners who:
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Had completed at least two 50km (31-mile) races, or five marathons of 42km (26.2 miles).
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Ran 20 to 40 miles per week on average.
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Were between 35 and 50 years old, with an average age of 42.
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Did not have inflammatory bowel disease or other known colon cancer risk factors.
The group included 55 percent women, and all underwent colonoscopies between October 2022 and December 2024.
Other Factors at Play
While running itself may be part of the picture, diet could also play a role. Dr. Cannon noted that many distance runners rely on energy bars and gels during training—foods that are highly processed and have previously been linked to colon cancer risk.
He also stressed that exercise overall is still incredibly important: “You never want to give people an excuse not to exercise.
But extreme endurance training may come with risks we’re only starting to understand.”
The Warning Signs Runners Shouldn’t Ignore
One key finding was how often rectal bleeding in runners was brushed off as “runner’s colitis,” a condition thought to be harmless. Dr. Cannon now urges caution.
“If you see blood in your stool—even if you’re younger than the recommended age for screening—it’s worth asking your doctor for a colonoscopy,” he said.
In the study, most people with advanced adenomas had reported this very symptom.
A Growing Cancer Risk Among the Young
Colon cancer is no longer a disease only of the older population.
The American Cancer Society estimates more than 154,000 Americans will be diagnosed with colorectal cancer this year—about 20,000 of them under 50.
What’s even more concerning is the pace of growth:
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Early-onset colon cancer diagnoses in people aged 20 to 34 are projected to rise by 90 percent between 2010 and 2030.
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Teen rates have skyrocketed by 500 percent since the early 2000s.
For people born in the 1980s, the lifetime risk of colon cancer is now double that of their parents’ generation.
Stories Behind the Statistics
The faces behind the numbers are runners and health enthusiasts who never thought cancer could strike them.
Laurie Koshers, a vegetarian who ran every day, was diagnosed at 44. Marathoner John B. Johnson was just 35 when he learned he had the disease.
Their stories echo the same unsettling theme—sometimes even the healthiest lifestyles don’t guarantee protection.
What This Means for Runners
For now, experts say the findings are still early, and more research is needed before firm conclusions can be drawn.
But the message is clear: long-distance runners should pay close attention to warning signs, and doctors may need to take symptoms in these patients more seriously.
Extreme endurance may strengthen the body in many ways—but it might also put hidden strain on areas we don’t expect.
