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 Dr. Cannon, an oncologist with Inova Schar Cancer in Fairfax, Va., launched a study, recruiting 100 marathon and ultramarathon runners aged 35 to 50 to undergo a colonoscopy.
The results were staggering. Almost half the participants had polyps, and 15 percent 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 percent to 6 percent, according to recent studies The figure among extreme runners was even higher than the 12 percent 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 Va., 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.
“Running is typically associated with better body mass and lower stress and lots of other positives — you never hear it’s bad for you,” said Ms. Linville, who took up ultramarathon running for a while during the pandemic.
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 under 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 Dr. 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 Dr. Cannon (who ran the New York City Marathon himself in 2010).
“But I do believe, after what I’ve seen from my patients and what we’ve found here, that extreme exercise may increase the risk of this cancer.”
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 Dr. 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,” Dr. Rubin suggested.
And the endurance runners may have missed telltale signs that can signal colon cancer, like bloody stools, bowel urgency and diarrhea. Runners often develop gastrointestinal symptoms that they dismiss as benign — so-called runner’s trots, for example.
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 body (like 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.
Dr. Rubin noted, however, that there is no evidence people with ischemic colitis are at greater risk for colon cancer. Experts urged runners who experience symptoms like 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, assistantprofessor of oncology at Johns Hopkins School of Medicine, who was not involved with the study.
“If you have symptoms, say something, even if people look up to you as a picture of health,” he said.
“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 Dr. 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, Mr. 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. Mr. Wadlington died in 2021, at 41.
The other two patients who came to see Dr. Cannon, including a woman triathlete, have also died.
“He didn’t drink, he didn’t smoke, he was vegan. He was always fit,” said Mr. Wadlington’s widow, who asked not to be named because she felt the focus should be on her husband.
“We noticed, looking back, how many things we brushed off. But you ignore signs you shouldn’t because you think you’re doing everything right.”
Roni Caryn Rabin is a Times health reporter focused on maternal and child health, racial and economic disparities in health care, and the influence of money on medicine.
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