Could your passion for long-distance running secretly be hurting your health?
For decades, regular exercise has been hailed as one of the most powerful weapons against cancer. Countless studies show that staying active lowers the risk of many illnesses, including various forms of cancer. But here's where it gets controversial: new research suggests that pushing the body to its absolute limit—like in marathons and ultramarathons—might actually backfire when it comes to colon health.
The Big Question
Could intense endurance running increase the risk of colon cancer?
The Surprising Verdict
Possibly yes, but only for extreme endurance athletes.
A new study, led by Dr. Timothy Cannon, an oncologist at the Inova Schar Cancer Institute in Virginia, has uncovered a puzzling trend. After noticing that several ultramarathoners were being diagnosed with advanced colorectal cancer at unusually young ages, Dr. Cannon and his team decided to dig deeper.
They recruited 100 serious long-distance runners aged 35 to 50—each of whom had completed at least five marathons or two ultramarathons—and screened them through colonoscopy. To keep the results focused, runners with genetic cancer syndromes or inflammatory bowel disease were excluded.
What They Found
The results were difficult to ignore: nearly half of the participants had polyps, and 15 percent showed advanced adenomas, which are high-risk precancerous growths. By comparison, only about 4.5 to 6 percent of adults in their late 40s from the general public have these same types of adenomas. The prevalence in runners even exceeded that of Alaska Natives, a group known for their higher colon cancer rates.
As Dr. Cannon explained, this study is the first of its kind to explore whether intestinal stress from extreme endurance exercise—specifically intestinal blood restriction, or bowel ischemia—might heighten the risk for colon cancer. While the study can’t confirm cause and effect, the data point to an intriguing and concerning correlation.
Why This Might Happen
During long-distance runs, the body diverts blood away from the intestines to power the legs and heart. This temporary oxygen deprivation can injure gut cells—a condition known as ischemic colitis. Many endurance runners experience this as cramping, diarrhea, or what’s casually called “runner’s trots.”
But could those digestive issues be more serious than they seem? Dr. Cannon raised a provocative idea: “There’s no concrete evidence that ischemic colitis directly causes cancer, but the repeated cycle of tissue injury and regeneration could raise the chance of DNA mutations.” Over time, this process might create the perfect storm for cancerous growths to form.
A Word of Caution
Before runners panic, other experts advise careful interpretation. The study was small, lacked a non-runner control group, and is still awaiting peer review. Dr. Marianna Zeichen from Jackson Medical Group pointed out that while the results show a connection, they do not prove that endurance running causes cancer. Factors such as diet, supplements, or family history might also play a role.
The Bottom Line
This study is not a warning against exercise itself—it’s a reminder about balance. Consistent physical activity is still one of the best ways to ward off disease. However, Dr. Cannon believes there’s likely a threshold beyond which exercise could shift from protective to potentially harmful. His advice? Any runner noticing unexplained rectal bleeding or persistent digestive problems should get a colonoscopy instead of dismissing the symptoms as routine.
The study was unveiled at the 2025 American Society of Clinical Oncology Annual Meeting in Chicago and quickly caught public attention, featuring in outlets like The New York Times and Fox News.
So now the question turns to you: Do these findings make you rethink the idea that “more exercise is always better”? Could there be a point where dedication to fitness crosses into risk? Let’s hear your take—share your thoughts in the comments below.
