Marathon Running and Colon Cancer Risk

If you’re thinking about running a marathon this weekend, you might want to pause and consider a few important findings first.

A recent study from the Inova Schar Cancer Institute has captured attention by suggesting that extreme endurance running—like marathons and ultramarathons—could heighten the risk of precancerous colon growths in younger adults.

The research focused on healthy runners aged 35 to 50 who had finished numerous long-distance races. The results showed a surprising number of participants had abnormal colon findings—much higher than normally expected. While this study doesn’t establish a direct cause-and-effect relationship, it raises a flag about colon health, even for those who are quite fit.

Study Overview

Colon cancer, which develops in the colon or rectum, is one of the most common cancers globally. It’s known to start in the large intestine and often arises from small, noncancerous clumps of cells called polyps. In the U.S., it ranks as the second leading cause of cancer deaths.

This study, led by Dr. Timothy Cannon and his team, was recently presented at the 2025 American Society of Clinical Oncology (ASCO) meeting. It included 100 adult runners aged 35 to 50 who had completed at least five marathons or two ultramarathons—and who had no prior colonoscopy. Participants with known risk factors, like hereditary cancer syndromes or inflammatory bowel disease, were not included.

Key Findings

• Around 41% of participants had at least one adenoma (a type of colon polyp).
• 15% had advanced adenomas, which are larger or more abnormal polyps likely to evolve into cancer. The expected rate for this age group is only about 1-2%.
• The average age of participants was approximately 42.5, which is earlier than the standard age for screening.

Interpreting the Results

Importantly, the findings indicate an association, but that doesn’t mean running causes colorectal cancer. The correlation is evident, but other factors could be impactful as well.

Another thing to consider is the small sample size. With just 100 participants, and no control group of non-runners for comparison, establishing overall risks becomes tricky. Additionally, diet, genetics, training intensity, and recovery patterns were not comprehensively controlled.

Also noteworthy is the discussion around adenomas—while they can become cancerous, many do not. Early detection and removal can decrease cancer risks, making these findings valuable.

Potential Reasons for Increased Risk

So, what’s the deal with marathon running being scrutinized here? When engaged in extreme or long runs, blood flow is redirected to the muscles, diverting it from the gut. This diversion can lead to reduced oxygen levels in the intestines, possibly causing minor injuries. Over time, these repeated stresses might alter the colon lining. “Runner’s colitis” is a known issue among endurance athletes; while often dismissed as normal, ongoing gastrointestinal bleeding or irritation could add to the risk.

Moreover, the gut microbiome in long-distance runners might differ significantly, with varying types and balances of bacteria that could influence this risk. Factors like diet, hydration, and what athletes consume during runs—like gels or processed foods—may also play a part.

Who Should Be Concerned?

The runners in this study are younger than those typically screened for colon cancer, emphasizing the need for awareness among this age group. Frequent runners, marathon or ultramarathon participants, and those with high training volumes should be vigilant. Anyone experiencing gastrointestinal symptoms—like rectal bleeding or changes in stool—should take those seriously.

Recommendations

• Don’t delay screening: If you’re 45 or older (or younger with a high endurance running background), it could be wise to discuss a colonoscopy with a doctor, as many guidelines suggest starting screening at 45.
• Watch for gastrointestinal symptoms: Symptoms like bleeding, loose stools, or abdominal pain after running shouldn’t be ignored. If they persist, seek medical advice.
• Manage your training load: Allow adequate recovery time. Pushing the gut repeatedly during extreme distances may be risky. Stay hydrated and ensure your nutrition supports gut health.
• Discuss risks with your physician: Be open about your running habits and any symptoms. Some doctors may recommend earlier screenings for avid runners.
• Maintain a healthy lifestyle: Keep a healthy weight, limit alcohol, avoid smoking, eat a balanced diet rich in fruits, vegetables, and fiber, and effectively manage stress to lower overall colon cancer risk.