A preliminary study has raised concerns about a potential link between extreme endurance running and colorectal cancer, after nearly half of the marathon and ultramarathon runners screened were found to have colon polyps. Conducted by Dr Timothy Cannon, an oncologist at the Inova Schar Cancer Institute in Virginia, the research involved 100 runners aged 35 to 50 undergoing colonoscopy screening. Of these, 50 per cent had polyps, and 15 per cent had advanced adenomas – growths that carry a high risk of developing into cancer.
These figures are significantly higher than the 4.5 to 6 per cent prevalence of advanced adenomas typically found in the general population of similar age. However, experts caution that the study had a small sample size and lacked a control group of non-runners, making it difficult to draw definitive conclusions.
Dr Cannon was prompted to investigate after observing a cluster of ultramarathon runners in their 30s diagnosed with stage four colorectal cancer within a year. These athletes reported symptoms like bleeding and cramps post-run, which were sometimes dismissed as normal or attributed to ischaemic colitis – a condition where blood is redirected from the colon to muscles during intense activity, depriving gut tissue of oxygen.
While there is no direct evidence linking ischaemic colitis to cancer, Cannon speculated that repeated cycles of tissue damage and repair could increase the likelihood of DNA replication errors, potentially leading to malignancy. Other contributing factors might include the altered gut microbiomes of endurance athletes and their consumption of ultra-processed foods, such as energy drinks.
Cannon’s team intends to further investigate by comparing the gut bacteria of runners with and without adenomas to those of non-runners. The findings were presented at the American Society of Clinical Oncology’s annual meeting, but researchers emphasised that the results are preliminary and should not deter people from exercising.
“This is hypothesis-generating more than proving anything,” said Cannon. “I definitely don’t want people to come away saying exercise is not good, because it is. The question is whether there is a dose of exercise that is so high that it actually increases your risk of colon cancer. And I believe there is.”
