Ripped, lean — and at risk?
A shocking new study is raising concerns about a possible link between an extreme form of exercise and colorectal cancer, after researchers found an unusually high number of precancerous growths in the colons of young, super-fit endurance athletes.
“If we can identify these cancers earlier in people who otherwise wouldn’t meet current screening criteria, we can save lives,” Dr. Timothy Cannon, an oncologist with Inova Schar Cancer and lead researcher, said in a statement.
The study examined 100 people aged 35 to 50 who’d completed at least two ultramarathons or five standard marathons. None had a family history or other clear risk factors for colorectal cancer.
Between October 2022 and December 2024, each participant underwent a colonoscopy, where doctors threaded a long, flexible, camera-tipped tube through the rectum and into the colon to check for abnormalities.
They were specifically hunting for advanced adenomas, a type of large or unusual polyp that, while benign, carries a higher risk of turning into cancer.
The results were jaw-dropping: nearly half the participants had polyps, and 15% had advanced adenomas — far above the 1%–2% normally seen in average-risk adults their age.
“I was surprised — you would think running is super healthy,” Laura Linville, 47, longtime marathon runner who participated in the study, told the New York Times. She had seven polyps, including some so large she needed additional procedures.
“Running is typically associated with better body mass and lower stress and lots of other positives — you never hear it’s bad for you,” she continued.
While independent experts told the outlet that most young colon cancer patients they see aren’t marathon runners, they agreed the findings are notable and deserve further study.
The reason behind the potential link is still being explored, but Cannon has a theory.
A few years ago, several ultramarathoners under 40 began showing up at his office with advanced colorectal cancer.
Troubled by the pattern, Cannon dug into the research and found that intense exercise can sometimes cause the body to temporarily redirect blood flow from the gut to the leg muscles — potentially leading to cellular damage.
For some athletes, this triggers gastrointestinal distress, like the infamous “runner’s diarrhea,” common in long-distance races.
Researchers speculate that repeated blood flow loss and cell damage may cause chronic inflammation in the stomach, intestines and nearby organs, which could increase cancer risk.
“While the results don’t prove causation, they strongly suggest that intense endurance exercise could be a meaningful risk factor for colorectal cancer and warrants further study,” Cannon said.
Notably, the average participant was just 42.5 years old — two and a half years younger than the recommended age for a first screening colonoscopy.
Outside experts told the New York Times the findings are notable but limited. For example, the study lacked a control group of non-runners and hasn’t yet undergone formal peer review.
Future research will aim to confirm these findings and untangle the causes behind the possible link. Cannon and his team are already analyzing dietary habits, training routines and other lifestyle factors to understand what might be driving the trend.
For now, experts stress that marathoners shouldn’t hang up their sneakers — but they should listen closely to their bodies. Symptoms of runner’s diarrhea, like changes in stool, rectal bleeding and abdominal pain, can also be warning signs of colon cancer that are easily dismissed.
This study comes amid a startling rise in colorectal cancer among young Americans, baffling doctors and upending the typical patient profile.
Cases in people under 55 jumped from 11% in 1995 to 20% in 2019, while death rates in that age group have climbed about 1% per year since the mid-2000s, according to the American Cancer Society.
“This is the first prospective study we know of to explore whether exercise-induced intestinal stress — specifically bowel ischemia from long-distance running — might increase cancer risk,” Cannon said.
“We believe this research may help define a new, at-risk population for earlier colorectal cancer screening,” he added.
