A study suggests that running ultramarathons over mountainous terrain leads to greater age-related damage in red blood cells compared to shorter races. Researchers from the University of Colorado Anschutz examined blood samples from participants in both a 40-kilometre trail race and a 170-kilometre ultramarathon. The findings highlight potential cellular impacts of extreme endurance exercise.
Exercise benefits health, but extreme forms like ultramarathons could harm blood cells. Scientists led by Angelo D’Alessandro at the University of Colorado Anschutz analyzed blood from 11 adults, averaging 36 years old, shortly before and after a 40-kilometre trail race over mountainous terrain. A separate group of 12 similar-aged participants underwent the same analysis after a 170-kilometre ultramarathon on comparable ground.
Both events increased damage to red blood cells from reactive oxygen species, molecules that rise when cells deliver more oxygen during exertion. However, the ultramarathon caused substantially more such age-related damage. D’Alessandro noted, “Anecdotally, the blood after an ultramarathon looks like the blood of somebody who’s just been hit by a car.” The cells in ultramarathon runners also shifted faster from a disc shape—ideal for navigating blood vessels—to a spherical form associated with ageing.
Team member Travis Nemkov explained, “This spherical shape means they get stuck in the spleen and eaten up by immune cells.” He attributed the damage to exercise-induced inflammation and the forceful circulation during intense activity. Ultramarathon participants saw a roughly 10 per cent drop in red blood cell counts post-race, though Nemkov said this is too minor to cause anaemia and the body likely recovers quickly.
Previous research has connected long-distance running to issues like temporary immune suppression and anaemia. The team plans to investigate effects one day after races and whether these changes influence performance. Nemkov added, “This could just be what the damage signals look like to make the body more resilient to endurance running, or it could have a negative impact.”
The study appears in Blood Red Cells & Iron (DOI: 10.1016/j.brci.2026.100055).
