Thinking of hitting the sidewalk this weekend for a marathon? Hold on before you tie up your running shoes, and read this first.A new study from the Inova Schar Cancer Institute has raised eyebrows in the running world by suggesting that extreme endurance running – like marathons and ultramarathons – may increase the risk of precancerous colon growths in younger adults.The study looked at healthy runners aged 35-50 who had completed many long-distance races and found that many of them had abnormal colon findings, more than what is usually expected. Although this research is early and does not prove cause and effect, it signals that even very fit people might need to pay attention to colon health.
What the study found
Colon cancer is one of the more common cancers worldwide. It’s a type of cancer that develops in the tissues of the colon or rectum. It starts in the large intestine (colon) and often develops from small, noncancerous clumps of cells called polyps. It’s the second leading cause of cancer death in the United States.The aforementioned study was led by Dr. Timothy Cannon and colleagues at Inova Schar Cancer, and the results were presented recently at the 2025 American Society of Clinical Oncology (ASCO) meeting. It involved 100 adult runners, aged 35 to 50, who had done at least five marathons or two ultramarathons, and who had never before had a colonoscopy. Those with known risk factors (hereditary cancer syndromes, inflammatory bowel disease) were excluded.
Key results of the study:
About 41% of participants had at least one adenoma (a polyp in the colon).15% had advanced adenomas. These are larger or more abnormal polyps that are more likely to turn into cancer over time. For this age group and population, the typical expected rate is only about 1-2%.The average age in the study was roughly 42.5 years, which is below the usual age at which many people begin screening.
What these findings mean
First things first – the results of this study are a proof of association, not a testament to causation. The study shows a correlation (link) between extreme long‐distance running and a higher prevalence of precancerous colon growths. But it does not prove that running causes colorectal cancer. Other factors may also be involved.Next comes the limitations due to a small sample size. Only 100 people were included. There was no control group of non-runners under identical conditions, so comparing risks is harder. Also, full histories of diet, genetics, training intensity, recovery, and gastrointestinal symptoms were not all controlled.Furthermore, there’s the matter of precursor lesions vs cancer. Adenomas are polyps that may turn into cancer, but many never do. Detecting and removing them early reduces cancer risk. So finding advanced adenomas early is useful.
Why extreme/marathon running might raise the risk
But why is marathon running under fire?During extreme/long runs, the blood flow is diverted toward muscles and away from the gut. This diversion can temporarily reduce oxygen delivery in the intestines, possibly causing small injuries. And repeated stress or inflammation could lead, over time, to changes in the colon lining. Now, “runner’s colitis” or “runner’s trots” (loose stools, occasional bleeding, inflammation) is a known phenomenon among endurance athletes. And while symptoms are often dismissed as normal, repeated gastrointestinal bleeding or irritation may contribute to risk.Moreover, differences in gut microbiome in long‐distance runners (types and balance of bacteria) might play a role in increasing the risk. Diet, hydration, supplements, or fueling during running (gels, processed foods), and recovery also may matter in that case.
Who’s at risk
Runners in the study were in their mid-30s to 50, a younger group than traditionally screened for colon cancer. So runners in this age group may need to be more alert.People who run frequently, do many marathons/ultramarathons, or have very high training volumes.Anyone with gastrointestinal symptoms like rectal bleeding, changes in stool, persistent diarrhea, or pain after running should take those symptoms seriously.
What you should be careful about
Don’t postpone screening: If you are 45 or older (or even somewhat younger but with a high endurance running history or symptoms), consider talking to your doctor about a colonoscopy or other screening test. Many guidelines recommend starting screening at 45.Monitor gastrointestinal symptoms: Bleeding after running, loose stools, abdominal discomfort, or changes in bowel habits are not always “just runner’s issues.” If symptoms persist or worsen (more than a few days or weeks), seek medical evaluation.Manage training load: Give time for recovery. Avoid repeatedly pushing the gut under stress, especially in extreme distances. Hydrate well, ensure nutrition supports gut health (fiber, balanced diet), avoid excessive processed fueling (gels, bars) without sufficient rest.Discuss risk with your doctor: Be open about your running history, symptoms, and family history of colon cancer. Some doctors may recommend earlier or more frequent screening for endurance athletes.Lifestyle factors still matter: Maintain a healthy weight, limit alcohol, avoid smoking, eat plenty of fruits, vegetables, fiber, and manage stress. All these help reduce colon cancer risk overall.