Excessive physical activity, and marathon running in particular, may be linked to colon cancer

Illustrative photo: Nasha Niva

As The Washington Post writes, the research story began in 2019 when Dr. Timothy Cannon, co-director of the Gastrointestinal Cancer Program at Inova Schar Cancer Institute (Fairfax, Virginia, USA), encountered a troubling anomaly. Within six months, three young patients with advanced colon cancer came to him.

These were people in ideal physical shape, aged 30-40, and they had none of the traditional risk factors. The only thing they had in common was an obsession with ultra-long-distance running. These runners regularly participated in 100-mile (160 km) ultramarathons or ran several full marathons and half-marathons annually.

Cannon wondered: is there any link between their extreme passion for running and colon cancer? Initially, this seemed unlikely, as numerous studies suggest that moderate physical activity, on the contrary, protects the body from colorectal cancer.

For example, a large-scale analysis of data from almost 1.5 million people in the USA and Europe, published in 2016 in JAMA Internal Medicine, showed that regular exercise reduces the risk of colorectal cancer by 20%. Moreover, according to a 2025 study, even after recovering from the disease, an active lifestyle helps prevent relapses.

Nevertheless, Cannon suspected that in the case of ultramarathoners, the loads were of such a magnitude that they altered the very biology of the intestine.

To test this hypothesis, a team of scientists conducted a study involving 94 experienced runners aged 35 to 50. Each participant had completed at least five marathons or two ultramarathons (distances longer than the standard 42.2 km). All volunteers underwent a colonoscopy.

The results were unexpected: polyps (adenomas) were found in almost half of the participants, and large, advanced formations, highly likely to develop into cancer, were found in 15%. No colon cancer was detected at the time of the examination. These figures significantly exceed the statistical norm for this age group, where the frequency of polyps usually ranges from 1.2 to 6%.

What is the reason?

The biological mechanism of this phenomenon may lie in the specific functioning of the body during many hours of exertion. When a person runs an ultramarathon, their body redirects the main blood flow from the digestive organs to the working leg muscles. As a result, intestinal cells suffer from a lack of oxygen and nutrients, which can lead to their death and disruption of the integrity of the mucous membrane.

This usually provokes inflammation and irritation. Many marathoners and ultrarunners complain of nausea, vomiting, cramps, diarrhea, or rectal bleeding during and after long distances. Later, during tissue repair, intestinal cells begin to divide very intensely and rapidly. This increases the likelihood of mutations that may lead to the appearance of polyps and cancer in the future.

Павялічыць

Polyps in the colon can be asymptomatic for years, but over time, some of them can develop into cancer. Photo: thomsonmedical.com

It's too early to panic

Despite the alarming data, the scientific community calls for prudence. This study is still considered small and preliminary, as it lacked a control group of non-runners.

Experts emphasize: the results obtained are a reason for further study, not for panic. Running still remains beneficial for health, as it accelerates metabolism and positively affects the gut microbiome in most people. The worst consequence of this news could be a complete rejection of physical activity out of fear of getting sick.

It is important to understand the context: in recent years, the number of colon cancer cases among people under 50 has been rapidly increasing — regardless of whether they run or not. According to a study published in early 2026 in the journal JAMA, this is now the leading cause of cancer death in this age group, although just one generation ago this disease ranked only fifth. In this regard, Dr. Cannon and his colleagues strongly advise extreme athletes not to ignore their body's signals.

Any prolonged gastrointestinal disturbances or instances of bleeding should not be perceived as a "normal part of a runner's life." Timely colonoscopy can save lives.

Cannon and his colleagues have a larger study ahead, involving 300 runners, as well as a control group. Scientists want to study genetics, nutrition, and the state of the microbiome to understand how these factors interact with running.

Researchers also want to include triathletes and professional cyclists in the program. This will help clarify whether running itself is the cause, or if any extreme exertion predisposes the body to polyp formation. And of course, they will monitor the participants to see if any of them develop cancer.

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