Why Elite Athletes Are Showing Up With Colon Cancer at 35

The ultramarathoner was 38, impossibly fit, and dying of stage 4 colon cancer. He'd run the London Marathon in under three hours, completed multiple 100-mile races, and maintained a body fat percentage most people dream of. Yet sitting in Dr Tim Cannon's oncology office at Inova Schar Cancer Institute, he was facing a diagnosis that shouldn't exist. Young, healthy endurance athletes don't get colon cancer. Except increasingly, they do.

What Dr Cannon discovered next has sent shockwaves through the running community and medical establishment alike. After noticing multiple young ultramarathoners presenting with advanced colorectal cancer, he launched a prospective study that revealed something extraordinary: extreme endurance athletes showed precancerous colon growths at rates typically seen in people decades older. The findings, presented at the 2025 American Society of Clinical Oncology Annual Meeting, challenge everything we thought we knew about exercise and cancer prevention.

The numbers are stark. Among 100 marathon and ultramarathon runners aged 35-50, 15% had advanced adenomas, the precancerous lesions that develop into colon cancer. In the general population of this age group, that figure sits at 1-2%. Even more alarming, 41% of runners had at least one adenoma, compared to expected rates below 10%. These weren't weekend joggers or casual 5K participants. These were elite athletes who'd completed at least five marathons or two ultramarathons, people whose fitness levels put them in the top percentile of cardiovascular health.

The Study That Changed Everything

Dr Cannon's research began with tragedy. Two ultramarathoners under 40, both presenting with advanced colon cancer, both dying within months of diagnosis. Their wives helped recruit participants for the study, determined that their husbands' deaths might prevent others. The recruitment posters appeared at marathon expos and running clubs across Virginia: "Have you run 5+ marathons? Never had a colonoscopy? You may be at risk."

The study design was meticulous. Researchers excluded anyone with family history of colorectal cancer, inflammatory bowel disease, or genetic predispositions. They wanted to isolate the impact of extreme endurance exercise alone. Participants averaged 42.5 years old, below the standard screening age of 45. All underwent comprehensive colonoscopies with expert panel review of any findings.

What emerged wasn't just statistical noise or coincidence. The pattern was clear: the more extreme the running history, the higher the likelihood of advanced adenomas. Ultramarathoners showed higher rates than marathon runners. Those running over 50 miles weekly had more polyps than those running 30-40 miles. The dose-response relationship suggested causation, not correlation.

The running community's reaction ranged from disbelief to panic. Forums exploded with runners sharing symptoms they'd previously dismissed: blood after long runs, chronic digestive issues, unexplained anaemia. Stories emerged of doctors who'd waved away rectal bleeding as "runner's colitis," a benign and expected consequence of distance running. How many cases had been missed? How many runners were harbouring precancerous growths while logging their miles?

The Biological Mechanism

Understanding why running might increase colon cancer risk requires examining what happens to the gut during extreme endurance exercise. When you run for hours, your body makes brutal prioritisation decisions. Blood flow diverts from the intestines to working muscles, dropping gut perfusion by up to 80%. The intestinal lining, starved of oxygen, begins to break down.

This exercise-induced ischemia isn't subtle. Studies using capsule endoscopy show visible damage to the intestinal lining after marathons: erosions, bleeding, inflammation. The gut barrier becomes permeable, allowing bacteria and toxins to enter the bloodstream. Inflammation markers spike. The intestinal lining must then repair itself, a process involving rapid cell division that, when repeated hundreds of times, may accumulate mutations.

Runner's colitis, long considered a badge of honour in endurance circles, might actually signal precancerous changes. The symptoms, rectal bleeding, cramping, urgent bowel movements after long runs, indicate significant intestinal trauma. Dr Cannon's hypothesis: decades of this repeated injury-repair cycle creates the perfect environment for adenoma development.

The timing matters too. Most runners fuel with simple sugars during races: gels, sports drinks, energy chews. These high-glycemic foods, consumed when the gut is already compromised, may feed harmful bacteria and promote inflammation. The combination of mechanical stress, ischemia, and dietary factors creates what researchers call a "perfect storm" for polyp formation.

The Young Onset Crisis

This discovery arrives amid a broader colorectal cancer crisis. Rates in under-50s have doubled since the 1990s, with researchers scrambling to understand why. Millennials face twice the colon cancer risk of baby boomers at the same age. By 2030, colorectal cancer will be the leading cancer killer in people under 50.

The usual suspects don't fully explain this epidemic. Yes, obesity rates have risen, but lean young adults are developing cancer too. Processed food consumption has increased, but health-conscious individuals aren't immune. Sedentary behaviour is blamed, yet here we have ultra-fit athletes showing elevated risk. The Inova study suggests we've been missing a piece of the puzzle.

Consider the modern fitness culture transformation. Marathon participation has increased 300% since 2000. Ultramarathons, once fringe events for eccentrics, now sell out within hours. Strava culture celebrates suffering. Instagram glorifies extreme challenges. The weekend warrior has been replaced by the everyday extremist, with young professionals treating Ironman training like a part-time job.

This intensity escalation coincides perfectly with rising young-onset colorectal cancer rates. While correlation doesn't prove causation, the timeline is suggestive. The generation experiencing unprecedented cancer rates is the same one revolutionising endurance sports participation.

Redefining Risk Categories

Current screening guidelines recommend colonoscopy at 45 for average-risk individuals, earlier only for those with family history or genetic syndromes. The Inova findings suggest extreme athletes represent a previously unrecognised high-risk group requiring different protocols.

Dr Cannon advocates screening any runner experiencing post-run bleeding, regardless of age. This represents a radical departure from current practice, where runner's colitis is typically dismissed as benign. He argues that waiting until 45 could mean missing the window for prevention in high-mileage athletes.

The challenge lies in defining "extreme" exercise. Is it five marathons lifetime? Fifty miles weekly? Two hours daily? The study couldn't establish clear thresholds. Risk likely exists on a spectrum, with occasional 10K runners at minimal additional risk and hundred-mile-week athletes at significant elevation.

Insurance coverage presents another obstacle. Screening colonoscopies before 45 require documented risk factors. "I run ultramarathons" doesn't currently qualify, meaning athletes must pay privately or wait for symptoms. By then, prevention opportunities may be lost.

The Screening Revolution

For runners accepting they need early screening, options extend beyond traditional colonoscopy. FIT (Faecal Immunochemical Test) detects microscopic blood in stool, catching 79% of colorectal cancers and 30% of advanced adenomas. While less sensitive than colonoscopy, annual FIT testing offers non-invasive monitoring.

Cologuard combines FIT with DNA markers, improving sensitivity to 92% for cancer and 42% for advanced adenomas. The test requires no preparation or time off training. However, positive results still necessitate colonoscopy for confirmation and polyp removal.

Virtual colonoscopy uses CT scanning to visualise the colon without scope insertion. Athletes appreciate avoiding sedation and immediate return to training. However, it can't remove polyps, meaning abnormalities require traditional colonoscopy anyway. Radiation exposure also concerns younger patients needing repeated screening.

Blood-based tests represent the future. Guardant's Shield test, recently FDA-approved, detects colorectal cancer through circulating tumour DNA. While currently less sensitive than stool tests, the technology improves rapidly. Within five years, annual blood draws might replace more invasive screening.

Living With the Knowledge

For the running community, processing this information proves challenging. Running defines identity for many athletes. Morning miles provide mental health benefits that no medication matches. Racing offers community, purpose, achievement. How do you balance these profound benefits against potential cancer risk?

The answer isn't stopping running. The health benefits of regular exercise remain overwhelming: 25% reduction in all-cause mortality, 30% lower cardiovascular disease risk, decreased rates of thirteen cancer types (though perhaps not colorectal), improved mental health, and enhanced longevity. Even if extreme running increases colon cancer risk, the net health impact likely remains positive.

Instead, the solution involves risk mitigation. Smart training periodisation with recovery phases allows intestinal healing. Proper fueling strategies minimise gut stress. Attention to warning signs enables early detection. Most importantly, appropriate screening catches problems before they become life-threatening.

Some athletes are modifying their approach. The investment banker who ran eight marathons yearly now caps at three. The surgeon who did weekly 20-milers now alternates with cycling. The lawyer training for her fifth Ironman added swimming-focused events. They're not abandoning endurance sports, just approaching them more strategically.

The Nutrition Factor

While the Inova study focused on exercise itself, nutrition likely modifies risk. Endurance athletes often consume diets that, despite being "healthy," might promote adenoma formation. High carbohydrate intake, even from whole grains, feeds certain gut bacteria linked to polyp development. Frequent gel and sports drink consumption bathes the colon in simple sugars.

Protective dietary strategies emerge from broader colorectal cancer research. Fiber intake above 30g daily reduces risk by 10% per 10g increase. Calcium supplementation shows protective effects. Vitamin D levels above 75 nmol/L correlate with lower adenoma rates. Limiting red meat and processed foods remains important even for athletes with high caloric needs.

The timing of nutrition matters. Consuming anti-inflammatory foods (omega-3s, polyphenols, probiotics) in recovery windows might counteract exercise-induced gut damage. Avoiding simple sugars outside training reduces bacterial fermentation. Strategic use of supplements like glutamine and colostrum may support gut barrier function.

Alcohol presents particular concern. Many running clubs celebrate with beers. Marathons often feature finish-line drinks. Yet alcohol significantly increases colorectal cancer risk, with the combination of extreme exercise and regular drinking potentially synergistic.

Getting Tested Now

If you're a serious runner reading this with growing concern, immediate action beats anxiety. Start with honest symptom assessment. Any rectal bleeding, even if only after long runs, warrants investigation. Changes in bowel habits, unexplained anaemia, or persistent abdominal discomfort deserve evaluation regardless of fitness level.

Document your running history comprehensively: lifetime marathon count, weekly mileage, years of training, ultramarathon participation. This information helps clinicians assess risk and justify early screening to insurance providers. Include any history of runner's colitis or exercise-induced GI symptoms.

Blood tests provide initial insights. Complete blood count catches anaemia suggesting chronic bleeding. Inflammatory markers (CRP, calprotectin) indicate gut inflammation. CEA levels, while not diagnostic, can suggest further investigation needs. These tests won't detect polyps but identify warning signs.

FIT testing offers non-invasive screening you can do at home. Annual testing starting at 35 for high-mileage runners seems prudent based on current evidence. Positive results mandate colonoscopy, negative results provide reassurance while maintaining vigilance.

The Professional Response

At The Online GP, we're seeing increasing numbers of young athletes seeking screening after the Inova study's publication. Our approach combines comprehensive risk assessment with appropriate testing pathways. We understand the unique concerns of endurance athletes and provide evidence-based guidance balancing cancer prevention with athletic goals.

Initial consultation involves detailed history including training volume, racing history, GI symptoms, and dietary patterns. We perform thorough examination and arrange immediate FIT testing. Blood tests screen for anaemia, inflammation, and nutritional deficiencies common in runners. Based on findings, we coordinate colonoscopy referrals when indicated.

For athletes with polyps, we develop surveillance strategies considering both medical guidelines and athletic goals. This might mean scheduling colonoscopies during off-seasons or adjusting training to minimise risk between screenings. We work with sports nutritionists optimising diets for both performance and cancer prevention.

Don't let concern become paralysis. If you're a serious runner, particularly with any GI symptoms, professional assessment provides clarity and peace of mind. WhatsApp us on 07399323620 for immediate consultation. We offer same-day appointments, including weekends when you're not training.

The Inova study doesn't mean abandoning your passion for running. It means approaching extreme endurance sports with awareness and appropriate medical surveillance. Early detection transforms outcomes. Polyps removed during screening never become cancer. Knowledge truly is power, especially when that knowledge might save your life.

The Online GP provides comprehensive health screening for endurance athletes at 49 Marylebone High St, London. Expert assessment, immediate testing, and colonoscopy coordination available seven days a week. Book via WhatsApp: 07399323620