The Runner’s Paradox: Could Extreme Endurance Exercise Increase Colon Cancer Risk?

5 minutes read time.

A Friend’s Marathon

There is a man I know, a good friend, whose dedication is beyond impressive. After his wife died of cancer at a tragically young age, leaving him the single father of their 18-month-old son, he took up running. Not just jogging, but marathon running. He now runs a marathon almost every weekend, a grueling and continuous effort to raise money for the cancer research that he hopes will spare other families the grief he has endured. His relentless motion is an act of love, a tribute to his wife’s memory, and a fierce battle against a devastating disease. But now, a new and deeply unsettling line of research raises a terrible, counterintuitive question: could the very act of extreme endurance, the pinnacle of physical health and dedication, be quietly increasing his own risk of cancer?

A Staggering Scientific Signal

This frightening paradox lies at the heart of a groundbreaking new study from a team of oncologists at Inova Schar Cancer in Fairfax, Virginia. The study’s lead researcher, Dr. Timothy Cannon, was inspired to begin his investigation after a disturbing pattern emerged in his practice: several young, lean, and extraordinarily fit ultramarathoners, all under the age of 40, presented with advanced-stage colorectal cancer. Troubled, he launched a prospective clinical study to see if there was a connection.

The study enrolled 100 high-endurance athletes between the ages of 35 and 50, all of whom had run at least five marathons or two ultramarathons and had no family history of the disease. When these athletes underwent screening colonoscopies, the results were, in the words of the researchers, “staggering.” While the expected rate of advanced adenomas—polyps with a high risk of becoming cancerous—in this age group is around 1.2%, the study found them in 15% of the runners. Nearly half of the participants had at least one polyp of some kind. This finding, presented at the 2025 American Society of Clinical Oncology conference, suggests a potential link that the medical community is taking very seriously. “It tells us there’s a signal here,” Dr. David Lieberman, a professor emeritus at Oregon Health and Science University, told the New York Times.

The Inflammation Hypothesis: Why Would Running Pose a Risk?

The central question, of course, is why an activity universally associated with good health could have such a dark side. The leading hypothesis, as explained by Dr. Cannon, centers on chronic inflammation. During extreme, prolonged exercise, the body diverts a massive amount of blood flow away from the internal organs and toward the laboring muscles. This can lead to a condition in the gut known as bowel ischemia, where the cells of the colon are temporarily starved of oxygen. Over time, in a high-mileage runner, these repeated cycles of cell damage and repair can create an environment of recurrent inflammation where “mutagenesis”—the formation of cancer-causing mutations—is more likely to occur.

This underlying biological risk is compounded by a dangerous cultural one. The gastrointestinal distress caused by ischemia is so common among long-distance runners that it has its own colloquial name: “runner’s trots.” Athletes often experience symptoms like abdominal pain, bowel urgency, and even bloody stools, which they learn to dismiss as a normal, benign side effect of their sport. This is a tragic and potentially fatal mistake. As the heartbreaking case of Josh Wadlington, one of the patients who inspired Dr. Cannon’s study, illustrates, these are the very symptoms that can signal colorectal cancer. Wadlington, an ultramarathoner who was vegan, didn’t drink, and was the picture of health, ignored his symptoms for years, believing they were just part of the sport. He was diagnosed with advanced colon cancer in his late 30s and died at 41.

A Call for Awareness, Not Alarm

It is crucial to frame these provocative findings with caution. As experts are quick to point out, the study is preliminary and has not yet undergone the formal peer-review process. More importantly, this research should not scare anyone away from exercise. The many proven health benefits of regular physical activity—which include a lower risk of at least eight other types of cancer—still far outweigh the potential risks for the vast majority of people. This is not an argument for becoming a couch potato.

Rather, the study is a powerful call for a shift in thinking for a very specific, high-risk population. The goal, as Dr. Cannon stated, is to “define a new, at-risk population for earlier colorectal cancer screening.” In a world where colorectal cancer rates are already rising mysteriously among adults under 50, this research provides a vital new clue.

The New Rules of the Road for Endurance Athletes

For the millions of people who find joy, purpose, and health in long-distance running, this news is undoubtedly unsettling. But the takeaway from this research should not be fear; it should be empowerment. The science is a reminder that more is not always better and that no one is invincible, no matter how healthy they seem. The message is clear and actionable:

• Listen to your body. Do not ignore or normalize persistent symptoms like rectal bleeding, changes in bowel habits, or unexplained abdominal pain.
• Talk to your doctor. If you are a high-endurance athlete, especially over the age of 35, have a conversation with your physician about the possibility of earlier colorectal cancer screening. Current guidelines recommend a first colonoscopy at age 45, but for this newly identified at-risk group, that may be too late.

The good news, as Dr. Cannon emphasizes, is that “screening can prevent advanced cancers.” The story of my friend, who channels his grief into a punishing physical challenge for a greater good, is a story of incredible strength. This new research doesn’t diminish that strength; it simply adds a new layer of necessary wisdom. Running strengthens the heart and mind; it’s just time to make sure the colon gets the protection it needs, too.