Scientists may have uncovered a possible link between extreme endurance running and colorectal cancer, after a preliminary study found colon polyps in nearly half of marathon and ultramarathon runners screened.
The early research, which is yet to be peer-reviewed, recruited 100 long-distance runners aged 35 to 50 for colonoscopy screening.
Almost 50 per cent had polyps (growths), while 15 per cent had advanced adenomas – growths with a high chance of progressing into cancer.
By comparison, recent studies suggest that only around 4.5 to 6 per cent of adults in their late 40s in the general population have such advanced adenomas. It should be noted, however, that these studies involved far greater numbers of participants.
“I became very worried when I saw over the course of one year a cluster of ultramarathoners in their 30s with stage four really advanced colorectal cancer,” Dr Timothy Cannon, an oncologist at Inova Schar Cancer Institute in Virginia, who led the study, told BBC Science Focus.
“They were describing bleeding and cramps after they ran, and at least one or two of them had said a doctor advised that was normal after running.”
Such symptoms are sometimes attributed to ischaemic colitis, a condition where blood is diverted from the colon to fuel legs and other muscles, temporarily starving gut tissue of oxygen. This can cause swelling and bleeding, but it usually resolves on its own.
Cannon wondered whether repeated cycles of damage and repair could create the right conditions for cancer.
“There’s no proof that ischaemic colitis causes cancer,” he said. “But it’s not hard to imagine how it would. When you have so many cells dying and then growing back in a disorderly and unregulated way, there are lots of opportunities for DNA replication errors.”
Other explanations are possible. Endurance athletes are known to have different gut microbiomes from non-runners, and their diets often include large amounts of ultra-processed products such as bottled energy drinks.
Cannon’s team now plans to compare the gut bacteria of runners with and without adenomas with those of non-runners.
The findings were presented at the American Society of Clinical Oncology’s annual meeting, but researchers stressed they are only early results and further studies are needed. They also noted the screening study did not include a control group of non-runners.
“This is hypothesis-generating more than proving anything,” Cannon said, stressing that the results should not put people off exercise.
“I definitely don’t want people to come away saying exercise is not good, because it is good. 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.”
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