A widely celebrated form of exercise, long-distance running, may be linked to an unexpected risk among certain young adults. A study presented at ASCO 2025 in Chicago found a higher-than-expected rate of colon adenomas—precancerous growths—in adults aged 35 to 50 who regularly run ultramarathons or multiple marathons. The results prompt careful discussion for health-minded Thai runners, where endurance events have surged in popularity in recent years.
The investigation examined 100 runners who completed at least two ultramarathons (over 50 km) or five standard marathons (42 km) within a defined period. Among participants, 41% had at least one adenoma and 15% had advanced adenomas. These rates appeared higher than typical figures for the general population. About half of those with advanced lesions reported rectal bleeding, a symptom sometimes mistaken for benign exercise-related issues such as runner’s colitis.
Thai runners may particularly relate to these findings given the country’s vibrant marathon and ultramarathon scene. Bangkok, Chiang Mai, Phuket, and other hubs host events that draw thousands in the 30–50 age bracket—the same demographic highlighted by the study.
The study’s lead investigator, an oncologist in the United States, described a possible physiological mechanism: during prolonged running, blood flow is diverted from the digestive tract to leg muscles, causing temporary reductions in oxygen and nutrients to the colon. This stress could contribute to tissue changes over time. The investigator stressed the importance of medical evaluation for runners who experience warning signs, including rectal bleeding or persistent abdominal pain, rather than assuming these symptoms are merely exercise-related.
Data were collected from runners without prior digestive diseases or known colon cancer risk factors, strengthening concerns about potential undetected risk factors associated with extreme endurance training. Typical weekly training volumes in the study ranged from 32 to 64 km, with diets frequently featuring energy bars and gels. Emerging evidence links high consumption of processed foods with various cancer risks, underscoring the importance of balanced nutrition for endurance athletes.
While the study is observational and cannot prove causation, it highlights the need for heightened awareness among athletes and clinicians. Thai gastroenterologists and public health experts emphasize cautious interpretation. They advise not dismissing rectal bleeding in young, active individuals and encourage transparent discussion of personal and family medical history when seeking care. Runners should maintain regular health checkups and discuss any concerns with their healthcare providers.
Colorectal cancer incidence is rising in Thailand, now among the country’s leading cancer types, with a growing share affecting individuals under 50. This trend mirrors global patterns of increasing early-onset colorectal cancer, underscoring the need for nuanced health messaging. Thai health authorities stress that lifestyle remains a key factor, but not the sole determinant of cancer risk. The study’s findings invite further investigation into diet, gut health, and other potential contributors to cancer risk among active Thais.
Stories from fit, health-conscious runners who were diagnosed with advanced colon cancer illustrate the complexity of cancer risk. While individual experiences should not be generalized, they highlight the unpredictable nature of cancer and the limits of lifestyle alone in prevention.
Looking ahead, researchers plan to explore how diet composition and gut microbiome changes might interact with endurance training. More robust, longitudinal studies are needed to separate correlation from causation. If future research confirms a link, it could influence cancer screening policies in Thailand, potentially expanding considerations for younger adults presenting concerning symptoms to include earlier evaluation.
Meanwhile, practical guidance for Thai runners includes remaining active while staying attentive to body signals. Those experiencing rectal bleeding, ongoing abdominal pain, unintended weight loss, or changes in bowel habits should seek prompt medical assessment, including colonoscopy if indicated. Dietary moderation—especially limiting highly processed energy products—complements traditional Thai dietary emphasis on fresh vegetables, fruit, and balanced meals.
As health science evolves, the broader lesson for Thai society is clear: healthy living is vital but not sufficient alone to prevent cancer. Early detection, informed medical advice, and ongoing research into the health patterns of active Thais will empower runners and non-runners alike to protect their well-being in a changing landscape.
For more information, consult reputable health authorities and national cancer initiatives, and discuss concerns with a healthcare professional.