A beloved physical activity endorsed by millions for its health benefits—long-distance running—may be unexpectedly associated with a higher risk of developing precancerous colon growths among young adults, according to a new study presented at the prestigious American Society of Clinical Oncology (ASCO) 2025 conference in Chicago. The study, conducted by researchers based in Washington, DC, challenges the prevailing wisdom that vigorous aerobic exercise is universally protective against all forms of cancer, raising pressing questions for health-minded Thais—especially the country’s growing cohort of marathon and ultramarathon enthusiasts.
The research, presented at the world’s largest gathering of cancer specialists, found an alarmingly high prevalence of colon adenomas—benign tumours that can evolve into malignant cancer—among long-distance runners aged 35 to 50. In examining 100 individuals who had completed at least two ultramarathons (over 50km) or five marathons (over 42km) within a defined period, scientists discovered that 41% had at least one adenoma, and 15% had advanced adenomas—rates noticeably higher than expected in the general population. Notably, over half of those with advanced adenomas reported rectal bleeding, a symptom sometimes overlooked or misattributed to “runner’s colitis,” a benign but common side effect of strenuous exercise (DailyMail.com).
This research is particularly significant for Thai readers, given the exponential rise in interest and participation in marathon and ultra-distance events nationwide over the past decade. Thailand’s annual marathon calendar now features dozens of events from Bangkok and Chiang Mai to Phuket, attracting both local and international runners. Major events routinely attract thousands of participants in the 30-50 age range, the very group identified as at heightened risk in the study.
The study’s lead investigator, a medical oncologist from a respected cancer institute in Washington, DC, explained the working hypothesis: during prolonged periods of running, blood flow is diverted from the digestive tract to the legs, leading to short-term reductions in oxygen and nutrients to the colon. This, in turn, may trigger cellular stress, cause cell death (or “infarcts”), and prompt rapid tissue regrowth—creating more opportunities for mutations that could lead to cancer. “As a colon cancer oncologist, I had seen several patients die of colon cancer in their thirties who were ultramarathon runners,” the investigator told DailyMail.com. “My hope is that, with more awareness, people who run long distances and have bleeding will get a colonoscopy.”
The study, which took place between late 2022 and December 2024, recruited runners with no pre-existing digestive diseases or known risk factors for colon cancer, making the findings even more concerning. Participants—average age 42 and 55% female—typically ran 32 to 64km (20 to 40 miles) per week and reported frequent consumption of energy bars, gels, and other ultra-processed foods. This dietary pattern may further complicate the risk, as recent research has linked high intake of processed foods to increased colorectal cancer rates (BMJ 2022).
While the study is observational and cannot definitively prove that distance running is the cause of increased risk—other lifestyle or dietary factors may play a role—the findings are enough to prompt new scrutiny among athletes and healthcare providers. The researchers stress that any runner experiencing symptoms of rectal bleeding, persistent abdominal pain, unexplained weight loss, or changes in bowel habits—regardless of age—should not dismiss these symptoms as exercise-related minor injuries but should demand thorough medical evaluation, including colonoscopy if necessary.
Thailand, like other middle-income countries, is facing a rising tide of colorectal cancer, now the third most common cancer in the country, with incidence steadily climbing, especially among those under 50 (Thai National Cancer Institute). While traditional risk factors such as Western-style diets, obesity, and sedentary lifestyles are well-known, the Thai running boom has, up to now, been seen as a countermeasure—further evidence that nuanced health messaging is required. The study’s results echo earlier international trends: the American Cancer Society has documented a 90% increase in early-onset colon cancer between 2010 and 2030 in the United States, with some of the steepest jumps among people in their 20s and 30s (American Cancer Society).
In interviews and case studies, several fit, health-oriented young runners described their shock at being diagnosed with advanced colon cancer—despite doing “everything right” in terms of exercise and, in some cases, diet. While individual cases, including a vegetarian daily runner diagnosed at 44 and a marathoner at 35, cannot be generalized to the entire population, such stories underscore the unpredictable nature of cancer risk and the limitations of relying solely on lifestyle to prevent disease.
Thai doctors contacted for comment have urged caution and careful interpretation of these findings. A gastroenterologist at a leading Bangkok private hospital remarked that “While the absolute risk may remain low for most, these data suggest we should not automatically dismiss rectal bleeding in young, active adults. It is essential we look beyond stereotypes of who might be at risk for colorectal cancer, especially as Thai lifestyles evolve.” An advisor at a national sports science institute echoed this, reminding runners not to skip regular health checks and to be transparent with their personal and family medical history when seeking care.
Cultural and historical context is also important. Thai society maintains a deep respect for athletic achievement and sees running as both a spiritual and physical pursuit, with long-distance temple runs and royal marathons promoting values of perseverance and self-discipline. However, the increasing reliance on processed sports supplements and high-sugar energy products—heavily marketed at Thai races—mirrors trends cited in the study and may be an underappreciated risk factor requiring further scrutiny.
Looking to the future, researchers plan deeper analysis of other potential contributors, such as diet composition and gut microbiome changes in long-distance runners. More robust experimental or longitudinal studies are still needed to separate correlation from causation. If further studies confirm a link, it could have significant ramifications for cancer screening policy in Thailand, where current guidelines generally recommend colonoscopy only for those 50 and older or with specific risk factors. Updating these protocols to include symptomatic younger adults—regardless of athletic status—could be life-saving.
For now, Thai runners and exercise enthusiasts are advised not to abandon healthy activity but to pay close attention to their bodies and avoid dismissing warning signs. For those with symptoms, prompt medical evaluation is vital, and discussing lifestyle, dietary habits, and even routine colon cancer screening with a healthcare provider may be warranted, even before the age of 50. Dietary moderation—especially in regard to processed energy products—may also reduce risk, echoing traditional Thai values of balanced eating with abundant fresh vegetables and fruits.
As new findings challenge assumptions about health, the crucial lesson for Thai society is that “healthy” lifestyles remain only one line of defence against diseases like cancer. Seeking out medical advice, embracing innovation in early detection, and supporting better research into the unique health patterns of active Thais will enable runners and non-runners alike to make informed choices and safeguard their well-being in a rapidly changing world.
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