Ultramarathons May Be Aging Your Blood Cells Faster Than You Think

Ultramarathons might push red blood cells past their limits—accelerating their aging in the process.

Running extreme distances may take a toll on the body at the cellular level. A study published today (February 18) in the American Society of Hematology’s journal Blood Red Cells & Iron reports that ultra-endurance events can damage red blood cells in ways that may interfere with how well they work. While researchers do not yet know how long the damage lasts or what it means for long-term health, the findings add to growing evidence that pushing exercise to extremes may sometimes do more harm than good.

Earlier research had shown that ultramarathon runners often experience a breakdown of normal red blood cells during races, which can potentially lead to anemia. Until now, scientists did not fully understand why this happens. The new study found that after long races, red blood cells become less flexible. Because these cells must bend and squeeze through tiny blood vessels to deliver oxygen and remove waste, reduced flexibility could limit their ability to do their job efficiently. The researchers also created the most detailed molecular profile to date showing how endurance racing alters red blood cells.

“Participating in events like these can cause general inflammation in the body and damage red blood cells,” said the study’s lead author, Travis Nemkov, PhD, associate professor in the department of biochemistry and molecular genetics at the University of Colorado Anschutz. “Based on these data, we don’t have guidance as to whether people should or should not participate in these types of events; what we can say is, when they do, that persistent stress is damaging the most abundant cell in the body.”

What Happens to Red Blood Cells During Ultramarathons

To investigate, scientists measured indicators of red blood cell health before and after athletes competed in two demanding events: the Martigny-Combes à Chamonix race (40 kilometers or about 25 miles long) and the Ultra Trail de Mont Blanc race (171 kilometers or 106 miles long). Red blood cells play a critical role in carrying oxygen throughout the body and transporting waste products away from tissues. Their flexibility is essential for navigating narrow blood vessels.

The research team collected blood samples from 23 runners immediately before and after their races. They examined thousands of proteins, lipids, metabolites, and trace elements in both plasma and red blood cells. The results showed consistent signs of injury caused by both mechanical (physical) and molecular forces. Mechanical damage likely stemmed from repeated shifts in fluid pressure as blood circulates during intense running. Molecular damage appeared to be driven by inflammation and oxidative stress (when the body has low levels of antioxidants, which fight off molecules that damage DNA and other components within cells).

Longer Races, Greater Cellular Stress

Signs of accelerated aging and breakdown in red blood cells were visible even after the 40-kilometer race and became more pronounced in runners who completed the 171-kilometer event. The researchers suggest that as race distance increases, athletes may lose more red blood cells and accumulate greater damage in those that remain in circulation.

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

Implications for Athletes and Blood Storage

Although more research is needed, the team believes these findings could eventually help shape training, nutrition, and recovery strategies aimed at protecting athletes from potential negative effects of extreme endurance exercise. The work may also have implications beyond sports. Stored blood used for transfusions begins to deteriorate after several weeks and cannot be used after six weeks under U.S. Food and Drug Administration regulations. Understanding how stress affects red blood cells during races could provide insights into preserving blood for medical use.

“Red blood cells are remarkably resilient, but they are also exquisitely sensitive to mechanical and oxidative stress,” said study co-author Angelo D’Alessandro, PhD, professor at the University of Colorado Anschutz and member of the Hall of Fame of the Association for the Advancement of Blood and Biotherapies. “This study shows that extreme endurance exercise pushes red blood cells toward accelerated aging through mechanisms that mirror what we observe during blood storage. 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.”

Study Limitations and Next Steps

The study included a relatively small group of participants and lacked racial diversity. In addition, blood samples were collected at only two time points. The researchers plan to conduct larger studies with more frequent sampling after races to better understand how long the effects last. They also intend to continue exploring ways to extend the shelf life of stored blood.

Reference: “Long-Distance Trail Running Induces Inflammatory-Associated Protein, Lipid, and Purine Oxidation in Red Blood Cells” by Travis Nemkov, Emeric Stauffer, Francesca Cendali, Daniel Stephenson, Elie Nader, Mélanie Robert, Sarah Skinner, Monika Dzieciatkowska, Kirk C. Hansen, Paul Robach, Guillaume Y Millet, Philippe Connes and Angelo D’Alessandro, 18 February 2026, Blood Red Cells & Iron.
DOI: 10.1016/j.brci.2026.100055

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