Over the course of about a year, Timothy Cannon, MD, who co-directs the Gastrointestinal Cancer Program at Inova Schar Cancer Institute, saw three strikingly similar patients. All of them were relatively young. All of them were not just healthy, but “extreme athletes” who ran long distances. And all of them had stage IV colon cancer.
“It was peculiar that these three all had running in common,” Cannon told Health—so he designed a study to learn more.
The results of that study, which were recently presented at an American Society of Clinical Oncology conference, suggest there may indeed be a link between distance running and disease. Out of 100 serious runners included in the study, 15 had a precursor of colon cancer—a finding that suggests “it’s a risk to run extreme distances, even marathons,” Cannon said.
But, experts told Health, you shouldn’t hang up your sneakers based on that finding alone.
“I would hate to deter people from running, or starting to run or even jog, as a result of this study,” said Christina Dieli-Conwright, PhD, MPH, an associate professor of medicine at the Harvard Medical School who researches exercise and cancer at Dana-Farber Cancer Institute and was not involved in the new research. “That would be unfair to running.”
Cannon’s team examined 100 runners ages 35 to 50. These were not casual joggers—they had all completed at least two ultramarathons or five regular marathons.
Everyone in the study got a colonoscopy, during which researchers looked for polyps with a high chance of turning into cancer. In previous studies of adults in their 40s without specific risk factors for colon cancer, this kind of polyp turned up in about 5% of people screened.
In Cannon’s study group, 15% of people had them. “I was really surprised that it was that many,” he said.
There are some important caveats, though. The study, which has not yet been published in a peer-reviewed journal, was small, specific to endurance athletes, and provided limited information about their other possible cancer risk factors. It also did not involve a comparison group of non-runners.
“Would [those polyps] have already been present regardless of their athletic status?” asked Cathy Eng, MD, who leads the Gastrointestinal Cancer Research Program at Vanderbilt University’s Ingram Cancer Center and was not involved in the new research. There’s no way to know from the study, which Eng called “thought-provoking” but “not definitive.”
The research “provides initial evidence that we should be thinking about this further,” Dieli-Conwright said. But far more research is needed to draw any firm conclusions, particularly about more casual athletes—that is, the vast majority of runners.
Again, the new study does not prove that distance running causes colon cancer. But even on a theoretical level, how might the two be linked?
During a long, grueling run, the body temporarily diverts blood flow from the GI tract so it can support muscles powering the activity. (Runners, you can blame this process for the digestive issues that often hit after a workout.)
Extreme runners who spend lots of time in this state may experience physiologic changes that raise cancer risk, Cannon theorized. There may also be differences in the gut microbiomes of runners vs. non-runners, among other possibilities, he said. But more research is needed to determine if there really is a connection between running and colon cancer and, if so, what’s driving it.
In a word: no. Even Cannon, the study’s author and a runner himself, has no plans to stop. “I wouldn’t say that I’ll never run a marathon again,” he said. “I may.”
Despite the new study’s findings, lots of published research suggests that physical activity may actually help prevent cancer, as well as other chronic diseases and health problems.
Running, specifically, can be hard on the joints—especially for those who participate in extreme events, such as ultramarathons—but it’s also known to be great for cardiovascular health.
And it doesn’t take a marathon to make a difference. Compared to non-runners, people who jog for only five to 10 minutes per day are at reduced risk of dying from causes including heart disease, according to one 2014 study.
While his study’s findings may sound alarming, Cannon said he doesn’t want anyone to be discouraged from exercising—or even distance running—because of them, especially given the need for further research. “The bigger problem with our health is we don’t exercise enough,” he said. “People should keep exercising, for sure.”
If you log a lot of miles, he said, just make sure you stay up-to-date on screenings for colon cancer and get thoroughly checked out if you experience unusual symptoms, like blood in your stool.
“The problem is that athletes look so healthy that people presume that there’s no way they could have any risk of cancer,” Eng said. If nothing else, the new study emphasizes that screening is important across the board.
