Running ultra‑long distances does more than tire the legs; it can also strain the blood that carries oxygen to every cell. A recent study in the journal Blood Red Cells & Iron shows that extreme endurance events can damage red blood cells, making them less flexible and potentially compromising their function.
Previous investigations noted that ultramarathon participants sometimes develop a drop in healthy red blood cells, a condition that can lead to anemia. The new work goes a step further by demonstrating that the cells become noticeably stiffer after prolonged racing, which may limit their ability to navigate the tiniest capillaries.
What the Researchers Measured
The team followed 23 athletes who tackled two very different races: a 40‑kilometer (about 25‑mile) mountain run and a 171‑kilometer (106‑mile) ultra‑trail across the Mont Blanc range. Blood was drawn right before the start and immediately after crossing the finish line. Thousands of proteins, lipids, metabolites and trace elements were analyzed in both plasma and red blood cells.
Findings pointed to a double‑hit injury. Mechanical stress—caused by rapid shifts in blood pressure during intense running—physically bruised the cells. At the same time, molecular stress linked to inflammation and oxidative damage further eroded cell integrity.
Longer Races, Bigger Cellular Toll
Both events triggered signs of accelerated cellular aging, but the 171‑kilometer ultra‑trail produced a markedly stronger response. In other words, the longer the race, the greater the loss of healthy red blood cells and the more severe the damage to those that remain in circulation.
"At a certain point between marathon and ultra‑marathon distances, the injury really begins to pile up," said one of the investigators. "We still don’t know how quickly the body can repair this damage or whether it has lasting consequences."
Why This Matters for Athletes and Medicine
Understanding how extreme physical stress alters red blood cells could help coaches and athletes fine‑tune training, nutrition and recovery plans to protect blood health while still pursuing peak performance. The insights may also inform blood‑bank practices, since stored blood undergoes similar degradative processes over time.
Study Limits and Next Steps
The current work involved a modest, mostly homogenous sample and captured blood at only two moments. Future studies aim to enroll a larger, more diverse group, add multiple sampling points, and explore ways to extend the shelf life of donated blood.