Are marathons and extreme running linked to colon cancer?

The three patients should have been portraits of health. They were young, lean and physically active. Unusually active, in fact: Two regularly ran 100-mile ultramarathons, and one had completed 13 half-marathons in a single year.

By the time they came to see Dr. Timothy Cannon, all three had advanced colon cancer. He was mystified; the oldest of them was 40, and none had any known risk factors. The doctor couldn’t help wondering if extreme running might have played a role.

So Cannon, an oncologist with Inova Schar Cancer in Fairfax, Virginia, launched a study, recruiting 100 marathon and ultramarathon runners ages 35 to 50 to undergo a colonoscopy.

The results were staggering. Almost half the participants had polyps, and 15% had advanced adenomas likely to become cancerous.

The rate of advanced adenomas was much higher than that seen among adults in their late 40s in the general population, which ranges from 4.5% to 6%, according to recent studies. The figure among extreme runners was even higher than the 12% rate among Alaska Natives, who are unusually prone to colon cancer.

The research was presented at an American Society of Clinical Oncology conference but has not yet been published in a medical journal.

“I was surprised — you would think running is super healthy,” said Laura Linville, 47, of Alexandria, Virginia, a longtime marathon runner who participated in the study. She learned she had seven polyps, including some so large that she had to undergo additional procedures.

She has no intention of giving up marathons but will get checked often for signs of colon cancer, she said.

The new study comes amid heightened concerns about a rise in colon and rectal cancer rates among adults younger than 50, a population that historically has had a low risk of cancer.

Older adults still make up the vast majority of those diagnosed with colon cancer. But the increase in so-called early-onset colorectal cancer led in 2021 to a change in screening recommendations. The age for a first colonoscopy was lowered to 45, from 50, for those at average risk.

The rise in colorectal cancer among younger adults has mystified experts. Physical inactivity and rising rates of obesity are frequently blamed — one reason his very fit, lean patients struck Cannon as worth investigating.

“You never want to give people an excuse not to exercise, because by and large, we have bigger problems from people not exercising enough,” said Cannon (who ran the New York City Marathon in 2010).

Several doctors interviewed for this story went out of their way to emphasize that most of the young colon cancer patients they see are not marathon runners. But experts said Cannon’s work was provocative and called for more research.

“It tells us there’s a signal here,” said Dr. David Lieberman, professor emeritus at Oregon Health and Science University. “We wouldn’t have expected these rates of high-risk adenomas, which are cancer precursor lesions, in an age group like this.”

Dr. David Rubin, chief of gastroenterology and director of the Inflammatory Bowel Disease Center at the University of Chicago, said the study was important but limited.

It lacked a control arm consisting of similar young adults who were not long-distance runners, he noted, and the family histories of colon cancer among the marathoners were not entirely known.

“It’s possible exercising didn’t cause the problem but was in fact the reason they became long-distance runners; because someone dear to them had cancer,” Rubin suggested.

The symptoms can be caused by ischemic colitis, a condition that develops when blood flow to the colon is temporarily reduced as it is redirected to muscles in other parts of the body (such as a runner’s legs).

The cells in the colon don’t get enough oxygen, which can cause damage and swelling. But it is often transient and improves on its own.

One hypothesis is that chronic inflammation in a runner’s body leads to repeated cycles of cell damage and repair. This may allow mutations to creep in during faulty cell divisions, precipitating cancer.

Rubin noted, however, that there is no evidence that people with ischemic colitis are at greater risk for colon cancer. Experts urged runners who experience symptoms such as stomach cramps or loose or bloody stools to see a physician for an evaluation.

“I wouldn’t necessarily tell people to stop running; I would tell them to run. But this reinforces the importance of listening to your body,” said Dr. Eric Christenson, assistant professor of oncology at Johns Hopkins School of Medicine, who was not involved with the study.

“This doesn’t fit our stereotype, but there may be an underlying trade-off when it comes to these extreme measures of endurance.”

One of the patients who inspired Cannon’s study was Josh Wadlington, a geographer who ran two or three ultramarathons a month. (Any race longer than a standard 26.2 mile marathon is an “ultramarathon.”)

One year, Wadlington participated in a weeklong event called the Savage Seven, running seven marathons in seven days.

He had often had bloody stools and occasionally abnormal blood test results, but “didn’t think anything of it,” according to his wife, who said he dismissed the symptoms as a benign side effect of endurance running.

He was diagnosed with colon cancer in 2018, when he was in his late 30s. Wadlington died in 2021, at 41.

“He didn’t drink; he didn’t smoke; he was vegan. He was always fit,” said Wadlington’s widow, who asked not to be named because she felt the focus should be on her husband.