For decades, long-distance runners have been celebrated as the picture of perfect health. With their lean builds, low heart rates, and iron-clad endurance, they’ve often been held up as models of fitness. But new research is challenging that perception. It suggests that marathoners and ultramarathoners may actually face a higher risk of developing colon cancer than the general population.
How the Research Began
The investigation started with Dr. Timothy Cannon, an oncologist at the Inova Schar Cancer Institute in Virginia. Cannon began noticing something unusual among his patients: several extremely fit endurance athletes, all under 40, came to him with advanced colon cancer. They didn’t smoke, didn’t drink, and had no known risk factors. One was even vegan.
Puzzled, Cannon launched a study of 100 distance runners between the ages of 35 and 50. He expected to find very little. Instead, the results shocked him. Nearly half of the runners had polyps, and 15 percent had advanced adenomas — the type of growths that can develop into cancer. To put that into perspective, only 4.5 to 6 percent of people in the general population have such growths, and even Alaska Natives, a group with one of the nation’s highest rates of colon cancer, show rates around 12 percent. For these runners, the number was much higher.
Evidence of a Hidden Risk
Many of the runners in the study had no idea anything was wrong. Symptoms like bloody stools or stomach cramps were often brushed off as “runner’s trots” or common side effects of endurance training. Sadly, in some cases, that misjudgment proved fatal.
One man, Josh Wadlington, was a devoted ultramarathoner who sometimes ran multiple extreme races a month. He ignored his gastrointestinal symptoms for years, assuming they were part of the sport. By the time he was diagnosed, it was too late. He died of colon cancer at just 41 years old.
Doctors are still debating whether running itself causes the risk or if it simply hides warning signs. One leading theory centers on blood flow. During intense exercise, the body diverts blood away from the digestive system to power the muscles. This can deprive colon cells of oxygen, leading to temporary inflammation and injury. Over time, repeated cycles of damage and repair may trigger mutations, eventually allowing dangerous growths to form.
Some runners experience ischemic colitis — a condition caused by reduced blood flow to the colon — which can lead to cramps, diarrhea, and bleeding. While this condition often heals on its own, experts suspect that repeated episodes could create the conditions that allow cancer to develop.
What This Means for Runners
The findings are striking but not definitive. The study lacked a control group, and the results have not yet been published in a medical journal. Still, experts believe the data is strong enough to warrant caution and more research.
Dr. Cannon himself emphasized that exercise remains one of the best things people can do for overall health. But he also warned that extreme exercise may carry unexpected risks. Other doctors agree that marathoners shouldn’t quit running altogether but should pay close attention to their bodies. Symptoms such as rectal bleeding, persistent cramps, or changes in bowel habits should not be dismissed as harmless runner’s issues.
How to Lower the Risk
If you are a dedicated distance runner, the lesson is not to stop running but to stay vigilant:
- Get screened early. Current guidelines recommend colonoscopy at 45, but for endurance athletes, talking to a doctor about earlier screening may be wise.
- Listen to your body. Do not ignore signs like blood in the stool, abdominal pain, or diarrhea that doesn’t resolve.
- Stay hydrated. Proper hydration may reduce gastrointestinal stress during long races.
- Balance training with recovery. Avoid overloading the body with excessive mileage without adequate rest.
The study comes at a time when early-onset colon cancer is already on the rise. Nearly 10 percent of new colorectal cancer cases worldwide are now diagnosed in people under 50, a dramatic shift from past decades. Cases among young adults in the United States have doubled in the last 25 years, a trend that remains unexplained.
Dr. David Lieberman, professor emeritus at Oregon Health and Science University, summed it up: “We wouldn’t have expected these rates of high-risk adenomas in such a young group. There’s a signal here, and it deserves further study.”
The message is clear. Running marathons may build resilience in the heart, lungs, and mind, but that doesn’t mean the colon is immune. The safest path forward for athletes is to keep training hard — but also keep screening early, paying attention to warning signs, and protecting gut health as fiercely as they protect their mileage.
