Ultra endurance races test the absolute limits of human stamina. Events that stretch beyond 100 miles demand months or even years of disciplined training, mental resilience, and peak physical conditioning. However, emerging research suggests that pushing the body to such extremes may come at a hidden physiological cost.
A new study published in the journal American Society of Hematology journal Blood: Red Cells & Iron reports that ultra marathon running can significantly damage red blood cells, potentially increasing the risk of anemia in endurance athletes.
Researchers found that athletes participating in ultra distance races experienced accelerated breakdown and aging of their red blood cells. These cells are essential because they transport oxygen, nutrients, and waste products throughout the body.
The research examined blood samples from runners before and after competing in two world class French races:
For comparison, a traditional marathon measures 26.2 miles. While marathons are already physically demanding, ultra marathons push well beyond that threshold.
Blood samples were taken from 11 runners in the shorter race and 12 runners in the longer event. Scientists specifically focused on how red blood cells responded to prolonged physical stress.
The findings were clear. The longer the race, the greater the damage to red blood cells.
Red blood cells must remain flexible to pass through tiny blood vessels. During extreme endurance races, several stressors interfere with that flexibility, including:
Over time, these stressors cause red blood cells to stiffen and break down more quickly than normal. This process resembles accelerated cellular aging.
The damage was visible even after the 25 mile race. However, it was significantly amplified in runners who completed the 106 mile event. Researchers noted that somewhere between traditional marathon distance and ultra marathon distance, the cellular damage becomes more pronounced.
As mileage increases, athletes lose more red blood cells and the remaining cells show greater structural damage.
Anemia occurs when the body does not have enough healthy red blood cells to carry adequate oxygen to tissues. Symptoms may include fatigue, weakness, shortness of breath, and dizziness.
While the study confirms red blood cell depletion during ultra marathon events, researchers say it is still unclear whether this temporary breakdown translates into long term anemia or lasting health consequences.
Scientists do not yet know:
Further studies are needed to determine both the short term and long term implications for endurance athletes.
Interestingly, the findings could extend beyond sports medicine.
Stored blood used for transfusions begins to degrade after several weeks. Most donated blood becomes unsuitable for transfusion after about six weeks due to similar cellular breakdown processes.
Researchers observed that the stress mechanisms affecting ultra marathon runners closely mirror what happens during blood storage. Understanding how red blood cells respond to mechanical and oxidative stress in athletes may help scientists develop better strategies for preserving blood used in transfusions.
This crossover between endurance physiology and transfusion medicine could lead to improvements in blood storage methods, ultimately benefiting patients who rely on blood donations.
Red blood cells are both remarkably durable and highly sensitive. They endure constant circulation through the body, squeezing through microscopic capillaries thousands of times per day. However, they are also susceptible to mechanical strain and oxidative damage.
Extreme endurance exercise appears to push these cells toward accelerated aging. While trained athletes often adapt to high levels of stress, there may be biological thresholds beyond which cellular damage becomes unavoidable.
The research highlights an important balance between athletic performance and physiological recovery.
The researchers emphasize that they are not advising against participation in ultra endurance events. Instead, they aim to provide insight into how the body responds to extreme physical stress.
At present, there is no official guidance suggesting that individuals should avoid ultra marathons due to red blood cell breakdown alone. However, athletes may benefit from:
Sports medicine specialists may also use these findings to refine training strategies and recovery protocols.
Ultra marathon participation has increased significantly worldwide. Events such as the Ultra Trail de Mont Blanc attract elite runners and amateur endurance enthusiasts alike.
These races often span mountainous terrain, extreme elevations, and variable weather conditions. The combination of prolonged exertion and environmental stress compounds the physical toll on the body.
As more athletes pursue ultra distance achievements, understanding the physiological effects becomes increasingly important for both safety and performance optimization.
The study authors plan to conduct larger trials with more participants and additional blood sampling during and after races. This expanded research may help determine:
Greater insight could benefit not only endurance athletes but also individuals receiving blood transfusions.
Here are the main conclusions from the research:
For athletes, awareness is essential. Monitoring biomarkers, including hemoglobin and iron levels, may become part of standard care for ultra endurance competitors.
For medical researchers, the study opens new avenues in transfusion science.
American Society of Hematology. News release, Feb. 18, 2026. Published findings in Blood: Red Cells & Iron.
This article is for informational and educational purposes only. Statistical data presented in medical research reflects general trends and does not apply to every individual. Personal health outcomes can vary significantly based on genetics, medical history, nutrition, training intensity, and other factors. Always consult a qualified healthcare professional for personalized medical advice, diagnosis, or treatment decisions.
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