Unexpected Colon Cancer Risk in Marathon Runners: A DOSE Warning from ASCO 2025

A striking study presented at ASCO 2025 suggests that “extreme endurance” sports, such as marathons and ultramarathons, may pose a paradoxical risk to intestinal health in some people. The question is therefore: Is exercise always beneficial, or is the “dose” critical?

Study group: Aged 35–50, having completed at least 5 marathons (42 km) or 2 ultramarathons (50 km+): 100 endurance athletes
Assessment: Colonoscopy findings

Why Are “Advanced Adenomas” Important?
A substantial proportion of colon cancers develop through the “adenoma → cancer” pathway. An advanced adenoma is a high-risk link in this chain. The 15% rate is therefore not an “ordinary finding,” but may be a sign that we need to rethink screening and awareness .

1) Ischemic Injury and “Runner's Colitis”

During prolonged, intense exercise, the body “draws” blood away from the digestive system and directs it to the muscles. Blood flow to the intestines decreases during this process (ischemia). The return of blood flow after the run (reperfusion) may increase oxidative stress and inflammation.

Hypothesis: Repeated cycles of “injury–repair” may promote the development of polyps by increasing cell proliferation in the intestinal mucosa.

2) Hidden Risks in Sports Nutrition

Because energy requirements are high during endurance sports, athletes often turn to convenient products such as energy gels, bars, carbohydrate powders . Some of these products fall into the category of ultra-processed foods and may adversely affect the intestinal microbiota.

In addition, consuming large amounts of fluid from plastic bottles during intense training may increase microplastic exposure (the evidence in this area is still developing, but the issue is noteworthy).

3) Warning Signs Dismissed as “Just Part of the Sport”

The study's most clinically important warning concerns the athletes: 30% experienced post-exercise rectal bleeding , according to their reports.
Among those with advanced adenomas , this proportion rises to %53.

Many people normalize this bleeding by attributing it to “friction,” “hemorrhoids,” or “runner's diarrhea.” Yet in some individuals, this finding may be a “warning flare” from lesions inside the bowel.

The study's correct message is:
A particular group of people who participate in “extreme endurance” sports may need to take symptoms more seriously and to personalize their screening strategies .

• Listen to symptoms: If you have rectal bleeding, abdominal pain, or changes in bowel habits after long runs, do not dismiss them as “just from exercise.” Seek a gastroenterology assessment .
• Do Not Wait Until Screening Age: If you are an active marathon/ultramarathon runner, particularly if you have symptoms, do not wait until age 45–50 before discussing the timing of colonoscopy with your doctor.
• Fine-Tune Your Nutrition: Instead of relying excessively on energy gels and ultra-processed bars, plan your nutrition, as far as possible, around natural carbohydrate sources .
• Do Not Forget the Concept of “Dose”: Exercise is a very valuable medicine, but excessive doses may carry different biological costs for certain organ systems, particularly the intestines.