• Posted February 19, 2026

Ultra-Marathon Running Depletes Athletes' Red Blood Cells, Study Finds

Ultra-marathon runners must be incredibly fit to endure races that can extend more than 100 miles, but their bodies pay a price for their athleticism, a new study says.

Extreme endurance runners experience a breakdown of their normal red blood cells during a race, putting them at increased risk of anemia, researchers reported Feb. 18 in the journal Blood: Red Cells & Iron.

Essentially, the damage done by such long-distance running causes accelerated aging and breakdown of red blood cells, and this destruction mounts as the miles stack up, researchers found.

“Based on these data, we don’t have guidance as to whether people should or should not participate in these types of events,” said lead researcher Travis Nemkov, an associate professor of biochemistry and molecular genetics at the University of Colorado Anschutz.

“What we can say is, when they do, that persistent stress is damaging the most abundant cell in the body,” Nemkov said in a news release.

For the study, researchers compared blood samples drawn before and after athletes participated in two French world-class running events: the 25-mile Martigny-Combes à Chamonix race and the 106-mile Ultra Trail de Mont Blanc race.

By comparison, a typical marathon is 26.2 miles long.

Samples were taken from 11 runners in the shorter race and 12 in the longer event.

The team specifically looked at changes in the runners’ red blood cells, which transport oxygen, nutrients and waste products throughout the body.

These cells must be flexible enough to squeeze through small blood vessels, but results showed that the cells become less flexible over the course of an endurance race.

Blood pressure changes, inflammation and oxidative stress all caused damage to red blood cells during long-distance running, researchers found.

This pattern of damage was clearly seen after the 25-mile race and was amplified in the 106-mile race, researchers found.

“At some point between marathon and ultra-marathon distances, the damage really starts to take hold,” Nemkov said.

As the length of a run increases, athletes can expect to lose more red blood cells and accumulate more damage to those cells remaining in circulation, researchers said.

However, researchers can’t say how this depletion of red blood cells will affect athletes’ short-term or long-term health.

“We’ve observed this damage happening, but we don’t know how long it takes for the body to repair that damage, if that damage has a long-term impact, and whether that impact is good or bad,” Nemkov said.

Further study could help not only endurance athletes but also everyday folks needing a blood transfusion, researchers noted.

Stored blood starts to break down after a few weeks of storage, and becomes unusable for transfusions after six weeks. Insights gained from endurance athletes might reveal better ways to handle and preserve blood donations, researchers said.

“Red blood cells are remarkably resilient, but they are also exquisitely sensitive to mechanical and oxidative stress,” said senior researcher Angelo D’Alessandro, a professor at the University of Colorado Anschutz.

“This study shows that extreme endurance exercise pushes red blood cells toward accelerated aging through mechanisms that mirror what we observe during blood storage,” D’Alesandro said in a news release. “Understanding these shared pathways gives us a unique opportunity to learn how to better protect blood cell function both in athletes and in transfusion medicine.”

Researchers plan to repeat this study using more participants and additional blood samples taken during and after races.

More information

The Cleveland Clinic has more on heart risks from extreme exercise.

SOURCE: American Society of Hematology, news release, Feb. 18, 2026