Distance running has long been celebrated as the pinnacle of endurance and cardiovascular health. But a growing body of research suggests that the physiological trade-offs of pounding the pavement for hours may not be as benign as once believed.
Emerging evidence shows that male endurance athletes consistently exhibit lower testosterone levels than their sedentary peers—by as much as 40%. Now, new findings presented at the 2025 American Society of Clinical Oncology (ASCO) meeting suggest that marathoners and ultramarathoners may also face unexpectedly elevated rates of precancerous growths in the colon.
Testosterone Suppression in Male Runners
Testosterone is essential not just for muscle mass and strength, but also for bone density, metabolic regulation, and sexual health. A landmark study found that men engaged in chronic endurance exercise had significantly reduced resting testosterone levels, sometimes by nearly half compared to non-athletes (PMID: 12665985).
Another investigation confirmed that this suppression was not a transient effect, but a long-term adaptive response in dedicated endurance runners (PMID: 10751104).
The prevailing explanation is that the hypothalamic–pituitary–gonadal (HPG) axis adapts to the constant metabolic stress of endurance training by downregulating testosterone production. While this may serve as an evolutionary trade-off to prioritize energy efficiency over reproduction, it raises concerns about long-term health outcomes, particularly as male athletes age.
Cancer Risks in Long-Distance Runners
Even more concerning are fresh data linking high-mileage running to precancerous growths in the colon. At ASCO 2025, researchers from the Inova Schar Cancer Institute presented findings from a prospective trial (NCT 05419531) involving 100 marathon and ultramarathon athletes aged 35–50.
The results were striking:
- 15% of runners had advanced adenomas (precancerous lesions that can progress to bowel cancer).
- 41% had at least one adenoma of any kind.
To put these numbers into perspective, the expected prevalence of advanced adenomas in average-risk individuals of the same age group is just 1.2%.
The mechanism remains unclear, but researchers hypothesize that repeated episodes of gastrointestinal ischemia during prolonged endurance events could play a role. During long-distance runs, blood flow is shunted away from the gut to fuel working muscles. Over years of training, this ischemic stress may contribute to pathological changes in the intestinal lining, potentially setting the stage for carcinogenesis.
Rethinking the “Marathon Ideal”
While moderate exercise remains one of the most effective interventions for preventing chronic disease, these findings complicate the narrative that “more is always better.” Long-distance running appears to carry hidden risks, particularly for men, who may be sacrificing testosterone health while simultaneously facing higher-than-expected cancer risks.
Experts emphasize that these results should not discourage exercise altogether, but they do highlight the importance of balance and screening. For runners logging extreme mileage, colonoscopic evaluation before the age of 50 may soon become a medical recommendation rather than an outlier precaution.
Scientific References (PMID):
Hackney, A.C. (2003). “Endurance training and testosterone levels in men: A review.” Sports Medicine, 33(8): 575–584. PMID: [12665985].
Wheeler, G.D., et al. (1999). “Reduced serum testosterone and prolactin levels in male distance runners.” Clinical Journal of Sport Medicine, 9(4): 246–252. PMID: [10751104].
American Society of Clinical Oncology (ASCO) 2025 Annual Meeting. Inova Schar Cancer Institute. Abstract: NCT 05419531.
