Nearly half of extreme endurance runners who took part in a small study were found to have precancerous colon polyps, raising concerns about a possible link between intense long-distance running and gut health. The research, involving 94 athletes aged 35 to 50 who had each completed at least five marathons or two ultramarathons, also found that 15 percent had advanced adenomas – polyps with a higher risk of progressing to cancer.
The study was prompted by an observation made in 2019 by Dr Timothy Cannon, a medical oncologist and co-director of the gastrointestinal cancer program at Inova Schar Cancer Institute in Fairfax, Virginia. Within six months, he had treated three unusually young patients – two in their 30s and one in their 40s – with advanced colon cancer. All three were “extremely prolific runners” who regularly competed in ultramarathons or multiple marathons, yet lacked the typical risk factors such as a family history of colorectal cancer, poor diet, or obesity.
Dr Cannon then led a study in which participants underwent colonoscopies. The findings were striking: nearly half had polyps, and a significant proportion had advanced adenomas. However, the study did not include a non-running control group, leaving the central question unresolved: did the researchers accidentally sample an unusually high-risk group of people who also happen to be extreme runners, or could the intense physical demands of the sport itself be driving the higher polyp rate? “We don’t know at this point,” Dr Cannon said. “But I do think there’s a signal here” related to the effects of extreme running, “and it’s worth paying attention to.”
Potential biological mechanisms
Researchers are exploring several theories about how extreme endurance exercise could affect the colon. Dr Cannon pointed to the phenomenon of “blood shunting” – during prolonged exercise, blood flow is diverted away from the gut and towards working muscles. This reduced circulation could starve intestinal tissues of oxygen and nutrients, potentially causing damage. Over time, repeated episodes of such stress could contribute to changes in the lining of the bowel.
Another line of thinking centres on chronic inflammation and mechanical irritation. Endurance athletes frequently experience gastrointestinal symptoms such as nausea, cramping, and rectal bleeding – problems many in the sport accept as normal. Eric McGlinchey, a study participant and extreme runner, put it succinctly: “We’ve all been there.” He reported having multiple non-advanced polyps himself. Dr Cannon suggested that the repeated pattern of injury and repair in the gut wall could, in theory, increase the risk of mutations that lead to polyps, though he stressed this mechanism remains unproven.
Beyond direct physical effects, researchers are looking at the gut microbiome. Chronic stress – a known companion of intense training – has been shown to disrupt the balance of bacteria in the digestive tract. Studies in mice have demonstrated that stress reduces beneficial Lactobacillus bacteria, which can help target tumour cells, leading to faster tumour development. Supplementing with Lactobacillus in those animal studies slowed tumour progression, suggesting a potential link between training-induced stress, microbial imbalance, and cancer risk. Chronic inflammation itself is a well-recognised risk factor for colorectal cancer, and tumours often show higher levels of inflammation along with a reduced ability to resolve it.
Expert caution and the need for perspective
Despite the intriguing findings, experts have urged caution. Dr Amy S. Oxentenko, a professor of medicine and gastroenterologist at Mayo Clinic, described the work as “super interesting” but “quite preliminary.” She warned: “Runners shouldn’t panic.” The study is “only hypothesis-generating,” she added, and the small sample size limits any firm conclusions.
Dr Sara Campbell, who is affiliated with Rutgers University, echoed that sentiment. While she acknowledged that symptoms such as rectal bleeding should always be evaluated, she said “the worst outcome would be for people to start thinking that running is bad for you.” She added: “Running is good for you and, most of the time, good for your gut.” The benefits of moderate exercise for colorectal cancer prevention are well established. A large 2016 study of nearly 1.5 million people found that regular exercisers were up to 20 percent less likely to develop colorectal cancer – and 12 other major cancers – compared with sedentary individuals. More recently, a groundbreaking 2025 trial showed that a structured exercise program for colon cancer survivors reduced the risk of recurrence and death by 28 percent and 37 percent respectively.
Dr Cannon is careful to distinguish between moderate and extreme exercise. He urged athletes not to ignore persistent symptoms. “Don’t let anyone tell you that rectal bleeding is normal for runners. Get it checked,” Dr Oxentenko added. The study’s authors are planning a larger follow-up involving around 300 runners and a non-running control group. That research will investigate genetics, diet, the gut microbiome, and other endurance sports to determine whether the effect is specific to running or applies to intense long-term exercise more broadly.
The findings come against a backdrop of rising early-onset colorectal cancer rates in the UK. Bowel cancer is the fourth most common cancer in the country, with around 48,200 new cases each year. While more than 90 percent of cases occur in people over 50, rates among adults aged 25 to 49 have increased by 22 percent between the early 1990s and 2018. An estimated 54 percent of bowel cancer cases in the UK are considered preventable, and lifestyle factors such as sedentary behaviour, obesity, poor diet, and alcohol use are being investigated as contributors. However, the current study raises the possibility that for a small subset of highly active individuals, exercise intensity – not lack of it – may be a factor worth exploring. Until more data is available, the message from experts is clear: keep running, but take gut symptoms seriously. “Don’t let anyone tell you that rectal bleeding is normal for runners. Get it checked.”
