It’s an activity loved by the health-conscious, but long distance running might raise the risk of colon cancer in young people.

A study has shown that individuals aged 35 to 50 who frequently participate in marathons have a higher likelihood of developing precancerous tumors in their colon, which can be early signs of the disease.

Dr Timothy Cannon, a medical oncologist, recruited 100 marathon and ultramarathon runners in Washington, DC ages 35 to 50 to undergo a colonoscopy. 

Results showed 41 percent had polyps, with 15 percent having advanced adenomas, benign tumors that could develop into cancer.

Of these people, more than half reported suffering from rectal bleeding, a warning sign of cancer.

According to Dr. Cannon, who spoke to DailyMail.com, the findings were more significant than anticipated for the general population since advanced adenomas are typically found in approximately 4.5 to six percent of adults in their 40s.

This research, which was shared in June at the American Society of Clinical Oncology, is observational and not yet published in a medical journal. Consequently, more studies are necessary to determine whether distance running increases the risk or if other lifestyle aspects like diet may contribute.

But Dr Cannon said the exercise could increase risk because running for long periods causes blood to be directed away from the colon to the legs, raising the risk of cell death and damaging mutations.

Shown above is colon cancer patient Phil Decker in January 2025, pictured running a marathon with Alecia Allen. He was getting ready to run another marathon when he was diagnosed with stage four colon cancer 

He said: ‘As a colon cancer oncologist, I had seen several patients die of colon cancer that were in their 30s that ran ultramarathons, which were 50km or higher runs.

‘We did this study because we were concerned that these runners were/are diverting all their blood flow to their legs during runs.

‘And [then are having] infarcts in their colon, and a lot of cell turnover; the cells die and then turn over very quickly [meaning] there may be more risk of cancer causing mutations.’

Infarcts are area of dead tissue caused by a loss of blood supply. When blood flow is blocked or severely reduced —usually by a clot, embolism, or narrowed vessels — the affected tissue doesn’t get the oxygen and nutrients it needs, and it dies. 

It is just the latest explanation being put forward by scientists to explain the surge in colon cancer cases among those under 50 years old, with those born in the 1980s now having double the risk of being diagnosed with the cancer compared to their parents.

Many of these patients have been shocked by their diagnosis, saying they are young, healthy and – they thought – were doing everything right. 

Patients include Laurie Koshers, a vegetarian who ran every day, but was diagnosed with colon cancer at 44, and marathon runner John B. Johnson, who was diagnosed at 35 years old.

The latest study was carried out in memory of two patients – Josh Wadlington and Spencer ‘Trigg’ Robbins – who both battled colon cancer despite running marathons.

The research was presented at ASCO 2025 in Chicago, Illinois, the world’s largest gathering of cancer researchers.

Radwah Oda never thought she would get cancer as she was in ‘excellent health’ at the time of her diagnosis at age 33, working out five times a week

Oda said she wishes she advocated for herself more with doctors and pushed for further tests

Patients aged 35 to 50 years old who were ultramarathon runners were recruited for the study. 

This was defined as people who had completed at least two marathons of 50 kilometers (31 miles) or more, or five marathons of at least 42km (26.2miles) or longer.

People who had inflammatory bowel disease or other conditions linked to colon cancer were excluded from the analysis.

Overall, participants had an average age of 42 years, 55 percent were female and they reported running around 32 to 64km (20 to 40 miles) every week.

Between October 2022 and December 2024, patients underwent a colonoscopy – a screening for colon cancer where a flexible tube with a small camera on the end is placed into the colon.

Dr Cannon said they were still investigating other factors that may have put them at risk of developing pre-cancerous polyps.

He also told The New York Times that he doesn’t want these results to sway people from exercising. 

He said: ‘You never want to give people an excuse not to exercise, because by and large, we have bigger problems from people not exercising enough.

‘But I do believe, after what I’ve seen from my patients and what we’ve found here, that extreme exercise may increase the risk of this cancer.’

Preliminary results had shown that runners ate at least five energy bars and also consumed gels daily. These products are considered highly processed foods, which have previously been linked to a higher risk of colon cancer. 

Rylie Toomey (right), pictured here with her fiancé, was diagnosed with stage four colon cancer at 27. She told TODAY.com she regularly plays lacrosse, runs and cycles and sticks to a healthy diet

Toomey is one of the thousands of healthy Americans under 50 to be struck by the disease

Dr Cannon added: ‘I think that I would get a colonoscopy if you have blood from your stool, even if you are not at the age yet where colonoscopies are approved – or at least I would ask your doctor for one.

‘I think bleeding from the rectum in runners is dismissed because it is a known entity, runners colitis is known, and it hasn’t been known to be associated with cancer, so it is often dismissed by doctors.

‘But my hope is that, with more awareness, people who run long distance and have bleeding will go get a colonoscopy.

‘In our study, most of the people who were having advanced adenomas were having blood in their stools.’ 

The American Cancer Society estimates over 154,000 Americans will be stuck by colorectal cancer this year, including about 20,000 under 50. 

While this is roughly in line with two decades ago, the disease is rising sharply in younger groups.  

According to the latest data, early-onset colon cancer diagnoses in the US are expected to rise by 90 percent in people 20 to 34 years old between 2010 and 2030. 

In teens, rates have surged 500 percent since the early 2000s.