Once reserved for a small group of passionate runners, ultra-trail races, defined as events longer than the standard 42 km marathon, have attracted a growing number of participants in recent years. In this expanding field, the Tor des Géants stands out as a true behemoth; spanning 330 km with 24,000 meters of elevation gain, it is often cited as the world’s most challenging mountain ultra-marathon. Featuring extreme conditions such as high altitude, severe temperature fluctuations and sleep deprivation, this race places runners under intense physiological stress, pushing the limits of human physiology.
Unsurprisingly, the potential health effects of competing for such extreme distances and in such conditions have become a major focus of scientific inquiry. Over the past several years, researchers have explored a wide range of physiological parameters, including neuromuscular fatigue, cardiac fatigue and muscle damage, to better understand how the human body responds to this type of effort. However, hematological adaptations, which are known to be a critical component of endurance performance, have yet to be investigated in the context of such ultra-long distances.
Our study, published in the May 2025 issue of Medicine & Science in Sports & Exercise®, aimed to fill that gap. To do so, we measured total hemoglobin mass and plasma volume (via CO-rebreathing), blood viscosity and erythropoietic markers in 22 runners the day before the start and repeated these measurements at the finish line. Only 13 (59%) of the original cohort completed the race. While this dropout rate is typical for events of this magnitude, it highlights how demanding the Tor des Géants truly is.
One of the most striking findings was a marked plasma volume increase at the end of the race. Previously observed in shorter ultra-trails such as the 170 km Ultra-Trail du Mont Blanc, this response appears to be a hallmark of ultra-long distances. This contrasts with the hemoconcentration commonly observed during shorter endurance events. Interestingly, despite expectations of red blood cell loss due to marked hemolysis, no decrease in the total hemoglobin mass was observed at the finish line. In fact, a nonsignificant increase was even reported.
Although insufficient to reduce the total hemoglobin mass, the decline in normalized blood viscosity suggests the selective removal of older, less deformable red blood cells. Simultaneously, erythropoiesis seems to be activated, likely triggered by a combination of factors such as systemic inflammation, chronic sympathetic activation and prolonged exposure to moderate altitude. Overall, while this physiological process warrants further investigation, our findings suggest an accelerated red blood cell turnover in response to an extreme mountain ultramarathon.
While the potential health risks of such extreme distances are often a concern, our results indicate that the hematological deleterious consequences are limited. These findings add a key piece to the puzzle in understanding physiological impacts of ultra-distance competition. Nevertheless, many training adaptations remain unclear in these environments, and future research initiatives are of need in the coming years.
About the Authors
Bastien Krumm, MSc, is a PhD candidate at the Institute of Sport Sciences at the University of Lausanne in Switzerland. His research focuses on hematological adaptations in different contexts such as prolonged exercise, altitude training or heat exposure. As a member of the Center of Research and Expertise in Anti-Doping Sciences, his work also focuses on anti-doping research, particularly on the development of the athlete biological passport and the detection of blood doping.
Antoine Raberin, PhD, is a postdoctoral fellow at the Institute of Sport Sciences at the University of Lausanne. Since completing his PhD, his research has focused on extreme environments, with a particular emphasis on hypoxia. He also studies endurance athletes, with a specific interest in pulmonary limitations to endurance performance, especially in elite athletes and under specific environmental conditions.
