The Bowel Symptoms Runners Should Not Write Off as Runner’s Gut

When Ciara Mageean died on Nov. 1 at the age of 34, she had been living with stage four bowel cancer for about 18 months. In June she described how long it had taken her to take the early symptoms seriously, and her explanation will sound familiar to a lot of runners.

“As an athlete, you’re used to having an unsettled bowel,” she said, as we reported at the time. What changed was the pattern. She found herself needing the toilet four times in a morning and feeling crampy, and she had a reason ready for each episode. “I kept finding logical reasons for it, which many people would.” It was her partner, Thomas Moran, who pushed her to get checked. By the time she went for what she expected to be a routine scope on May 20, 2025, the cancer had already spread to her liver and lungs.

Why Runners Are Easy to Fool

Gut trouble is part of the sport. A 2014 review in Sports Medicine found that “studies suggest that 30-50% of athletes experience such complaints,” with endurance athletes especially affected. The reasons are well understood: during hard exercise, blood flow to the gut can fall by as much as 80 percent, and the repeated jolting of running shakes the intestines with every stride. The same review lists urgency, diarrhea and even bleeding among the problems that running alone can cause.

Those are also some of the classic warning signs of bowel cancer, which is the difficulty. A runner who has spent years planning routes around toilets, or who has seen blood after a long run before, has a ready explanation for symptoms that would send someone else to a doctor.

The Signs That Need Checking

The NHS lists the main symptoms of bowel cancer as:

• a change in your poo that is not usual for you, such as looser stools, diarrhea or constipation
• going more or less often than usual
• blood in your poo, which can look red or black, or bleeding from your bottom
• often feeling the need to go, even straight after you have been
• stomach pain, bloating or a lump in your stomach
• losing weight without trying
• feeling unusually tired or short of breath, which can be a sign of anemia

The phrase that matters most for runners is “not usual for you.” Mageean’s symptoms were a change from what her body normally did, not the same race-day trouble she had always lived with. Two items on the list deserve particular attention from anyone in heavy training, because unexplained weight loss and constant tiredness are easy to put down to the mileage.

The NHS advice is to see a doctor about any of these symptoms, to get an urgent appointment for black or dark red stools or bloody diarrhea, and to go to an emergency department for heavy or non-stop bleeding. The Prevent Cancer Foundation, a U.S. charity, puts its threshold at changes that last “more than a few days.” Neither treats the symptoms themselves as a diagnosis. “Having the symptoms does not definitely mean you have bowel cancer,” the NHS says.

Young and Fit Is Not a Shield

Mageean was 33 when she was diagnosed, and she is far from an outlier. The American Cancer Society’s 2026 colorectal cancer statistics report found that rates among adults aged 20 to 49 are rising by about 3 percent a year, even as they fall in people over 65. The report projects 24,640 new cases in Americans under 50 this year, and about three in four younger patients are diagnosed once the disease is already advanced. Routine screening in the United States begins at 45.

Whether endurance running itself plays any part is an open question. Last year Dr. Timothy Cannon, an oncologist at Inova Schar Cancer Institute in Virginia, presented a pilot study of 100 marathon and ultramarathon runners aged 35 to 50, and found advanced precancerous growths in 15 percent of them. Inova compared that with the 1 to 2 percent it expects in average-risk people of that age. As we reported, the study had no control group and has not been published in a peer-reviewed journal, and Cannon himself said the results “don’t prove causation.” Either way, a high training load is no reason to assume a gut problem is harmless.

For runners, the practical point is simple enough. Knowing your own normal is part of training, and a change in it is worth a conversation with a doctor, however good the explanation seems. As the NHS puts it, “it’s important to get checked by a GP.”