Can too much running cause colon cancer? New study raises questions about extreme training

After seeing three marathoners with colon cancer in a span of about six months, oncologist Dr Timothy Cannon could not help but wonder if it was more than a coincidence. Could there be a connection between the cancer and the gastrointestinal (GI) problems many runners experience during long events? “Runner’s ischaemia is well described,” says Cannon, a researcher from the Inova Schar Cancer Institute in the US state of Virginia, referring to a condition that develops during prolonged endurance exercise, when blood is diverted away from the digestive system and towards the working muscles. This can leave the gut temporarily short of blood supply, leading to cramps, nausea, diarrhoea and, in some cases, bleeding. “A lot of [endurance runners] have had GI issues after running. I wondered whether there could be a connection [with colon cancer] if you’re having so much cell turnover from cell death after these long runs?” How was the connection explored? A study Cannon led to explore the idea found unusually high rates of precancerous colon polyps in endurance runners, raising questions about the long-term effects of extreme training on gut health. The study tracked 94 endurance runners aged 35 to 50 who had completed either two ultra-marathons of at least 50km (31 miles) or five standard marathons (42.2km each). All underwent a screening colonoscopy. The results were startling: researchers found adenomas – benign growths that can become cancerous – in 41.5 per cent of participants. More strikingly, 15 per cent had advanced adenomas, which carry a higher risk due to their size or cellular traits. No cancers were detected. For comparison, Cannon said advanced adenomas are typically found in just 2 to 7 per cent of the general population in that age bracket. Cannon’s theory is that years of repeated episodes of ischaemia could potentially create conditions that favour the development of adenomas. “If you’re getting recurrent ischaemia, you’ve got cells dying and new ones growing in a relatively rapid and disorderly fashion, and so you’re more likely to get all types of mutations,” he says. “That would seem to include the type that predisposes you to colon cancer and precancerous polyps.” Does this mean there is causation? Cannon was keen to stress that this remains only a hypothesis. “I don’t know for sure what the mechanism is,” he says. “It could be diet, microplastics, stress and other potential factors. We don’t know why there’s this predisposition.” The study was a preliminary look rather than proof of causation. Without a non-running control group, researchers cannot confirm if running triggered the polyps. “There are definitely some problems with the methodology,” Cannon acknowledges. “The patients self-selected themselves and for all we know they could have been people who were already worried about cancer.” Still, he believes the results justify a much larger trial. As someone who enjoys long-distance running himself, he says the research “feels personal”. The research team is now seeking funding for a controlled study involving more than 500 participants, comparing endurance runners with people who have never raced further than 10km. What else can be investigated? Alongside colonoscopies, researchers hope to investigate diet, lifestyle, gut bacteria and sports nutrition habits to understand any potential differences between the groups. Amy Oxentenko, a consultant gastroenterologist at the Mayo Clinic in the US state of Minnesota, describes the study as “an interesting hypothesis-generating paper” that raises important questions. But along with Cannon, she stresses that the research does not prove that endurance running causes adenomas. “We have known that regular exercise can decrease the risk for colorectal neoplasia [abnormal, uncontrolled growth of cells in the large intestine], so this study raises important questions on whether extreme endurance running may actually increase colorectal neoplasia risk,” she says. Oxentenko notes that the detected polyp rates are higher than normal for this age group, adding that Cannon’s theory linking reduced gut blood flow to cell stress is “plausible”. “One could imagine that in cases of prolonged periods of colonic ischaemia, which can be seen in extreme runners, the cycle of ischaemia – inflammation, cell turnover and healing – that is being repeated consistently could lead to cellular neoplasia.” One of the most notable findings from the study was that runners who reported rectal bleeding after races appeared more likely to have advanced adenomas. For Cannon, that was the most important practical message. “If you have bleeding, or you get a lot of cramping and discomfort after you run, it would be a good idea to get that checked out,” he says. “No matter what age you are, if you’re having bleeding after you run, you should be evaluated.” The importance of exercise Since the study was published, Cannon says several runners have contacted him after arranging colonoscopies of their own. “I’ve got several letters from people who went and got their colonoscopy and had cancer,” he says. At the same time, he stressed the importance of exercise. “We know exercise protects you from colon cancer,” he says. “The idea that there’s a dose of running that may take it the other direction into the risk category, we’re in the exploratory phase of that hypothesis.” For now, the message for runners remains unchanged. Keep running, pay attention to persistent gut symptoms and bleeding, and do not skip recommended screening when you become eligible. Like what you read? Follow SCMP Lifestyle on Instagram. You can also sign up for our eNewsletter here.

Can too much running cause colon cancer? New study raises questions about extreme training

.css-1c6uqr6{color:inherit;font-weight:inherit;font-size:inherit;font-family:inherit;line-height:inherit;overflow-wrap:break-word;}Study finds unusually high rates of precancerous colon polyps in endurance runners, raising concerns over marathoning’s impact on gut health

How was the connection explored?

Study finds unusually high rates of precancerous colon polyps in endurance runners, raising concerns over marathoning’s impact on gut health