Can too much exercise actually fuel colon cancer?

By all conventional measures, they were the picture of health: young, lean, and fiercely active. One woman had completed 13 half-marathons in a single year. Two men regularly ran 100-mile ultramarathons. Yet all three ended up in the office of oncologist Timothy Cannon, MD, with advanced colon cancer. ^[]

A surprising study

To investigate, Dr. Cannon recruited 100 ultramarathon and marathon runners between the ages of 35 and 50 to undergo colonoscopy.

The results were eye-opening: Nearly half had polyps, and 15% had advanced adenomas—lesions with a high risk of progressing to cancer.

By comparison, the rate of advanced adenomas in the general population of adults in their late 40s is only 4.5% to 6%. Even among populations unusually prone to colon cancer, like Alaska Natives, it’s around 12%.

When 'healthy' isn’t protective

Laura Linville, 47, a longtime marathoner from Alexandria, Virginia, was one of the participants. She discovered seven polyps, several large enough to require additional procedures.

“I was surprised—you would think running is super healthy,” she said. “Running is typically associated with better body mass and lower stress and lots of other positives—you never hear it’s bad for you." ^[]

The study’s findings come at a time of rising concern over early-onset colorectal cancer.

While the majority of cases still occur in older adults, rates in those under 50 have been increasing, prompting the US Preventive Services Task Force to lower the recommended age for a first colonoscopy from 50 to 45 for average-risk adults.

Exercise and risk: A complex relationship

Experts caution that the new data do not prove causation. David Rubin, MD, chief of gastroenterology and director of the Inflammatory Bowel Disease Center at the University of Chicago, notes the study lacked a non-running control group and had incomplete information on family history.

“It’s possible exercising didn’t cause the problem but was in fact the reason they became long-distance runners; because someone dear to them had cancer,” Dr. Rubin said. ^[]

Runners may also overlook warning signs. Chronic gastrointestinal issues such as loose stools, urgency, and intermittent bleeding are often attributed to so-called “runner’s trots,” transient symptoms caused by reduced blood flow to the colon during intense exercise.

The link is especially complex considering the recent findings from the CHALLENGE trial, which demonstrated that structured, supervised post-treatment exercise significantly improved both disease-free and overall survival in stage II/III colon cancer patients.

Together, the exercisce/cancer link findings underscore the deeply complex relationship between exercise and colon cancer: While extreme, unchecked efforts may raise red flags, carefully guided physical activity can serve as a powerful ally in strengthening outcomes and prolonging lives.

Listening to the body matters

“I wouldn’t necessarily tell people to stop running; I would tell them to run. But this reinforces the importance of listening to your body,” said Eric Christenson, MD, assistant professor of oncology at Johns Hopkins School of Medicine. ^[]

The cases that sparked Dr. Cannon’s investigation were particularly sobering. For instance, Josh Wadlington, a geographer and ultramarathoner, ran multiple long-distance races each month, including a grueling weeklong challenge called the Savage Seven.

He had occasional bloody stools and abnormal lab results but dismissed them as part of endurance training. Diagnosed in his late 30s, he passed away at 41.

Key takeaways for clinicians

While extreme endurance exercise remains a cornerstone of cardiovascular and metabolic health, Dr. Cannon’s findings suggest a nuanced picture.

For clinicians, the takeaway may be simple but critical: Even highly fit patients are not immune to colon cancer, and persistent gastrointestinal symptoms—regardless of lifestyle—warrant careful evaluation.

Consider maintaining a lower threshold for screening and diagnostic evaluation in young, highly active adults presenting with gastrointestinal symptoms, especially those with a family history of colorectal cancer. Patient education should emphasize that extreme fitness does not replace vigilance.