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Ultra-Marathon Runners Face Significant Blood Loss, Study Finds

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  Published in Health and Fitness on Saturday, February 21st 2026 at 23:35 GMT by U.S. News & World Report
      Locales: Colorado, California, Utah, UNITED STATES

Emerging Research Highlights Significant Blood Loss in Ultra-Marathon Runners

Ultra-marathon running, a test of human endurance pushing athletes to their absolute limits, is increasingly popular. However, a newly published study is shedding light on a potentially significant and previously underestimated health risk associated with these grueling events: substantial depletion of red blood cells. Published Thursday in Frontiers in Sports and Active Living, the research provides compelling evidence that ultra-marathons don't just challenge cardiovascular fitness and muscular endurance, but also inflict considerable physiological stress on the circulatory system.

The study, led by Dr. Steve Faulkner of Northumbria University in the United Kingdom, analyzed blood samples from 27 runners participating in a challenging 50-mile race. The findings revealed a startling reduction in red blood cell counts - approximately 16% - and a 23% decrease in hemoglobin, the vital oxygen-carrying protein within those cells. This isn't merely a transient dip during exertion; researchers observed that full recovery of blood cell counts could take up to 12 days post-race, raising concerns about cumulative effects over a runner's career.

The Mechanics of Blood Cell Damage

So, how does this substantial blood loss occur? The researchers posit that the repetitive impact and jarring motion experienced during ultra-marathons - races that can last 10 to 20 hours - cause microscopic damage to red blood cells. These fragile cells, responsible for oxygen transport throughout the body, are essentially 'bouncing around' within the circulatory system for an extended period. This constant physical stress leads to cellular breakdown, resulting in hemolysis (the destruction of red blood cells) and subsequent loss. It's not necessarily about visible bleeding; the damage is occurring at a microscopic level, making it difficult for runners to self-assess the severity of the issue.

Beyond Fatigue: Potential Health Risks

The implications of prolonged and repeated red blood cell depletion are far-reaching. While immediate symptoms might include fatigue and shortness of breath - common experiences for ultra-marathoners even without blood loss - the long-term consequences are more concerning. Dr. Faulkner emphasizes that significant reductions in red blood cells can increase the risk of blood clots, potentially leading to serious cardiovascular events. Furthermore, chronic anemia, resulting from consistently low red blood cell counts, can compromise overall health and athletic performance.

Iron Deficiency and the Ultra-Marathoner

The study also highlights the potential link between ultra-marathon running and iron deficiency. Hemoglobin contains iron, and the loss of red blood cells directly translates to a loss of iron. While the study didn't specifically focus on iron levels, it's logical to assume that ultra-marathoners are at a higher risk of developing iron deficiency anemia, exacerbating the effects of red blood cell depletion. This emphasizes the need for careful monitoring of iron status and potentially supplementation under the guidance of a healthcare professional.

Recommendations for Athletes and Future Research

Dr. Faulkner and his team urge ultra-marathon runners to be aware of these risks and proactively prioritize recovery strategies. This includes adequate hydration, proper nutrition (with attention to iron intake), and sufficient rest. Runners should also be mindful of any unusual symptoms following a race, such as persistent fatigue, dizziness, or unexplained bruising, and consult a doctor if necessary.

The researchers also call for further investigation into the long-term health effects of red blood cell loss in ultra-marathon runners. This includes larger-scale studies to determine the prevalence of this issue, as well as research into potential interventions to mitigate the damage. Specifically, studies could explore the effectiveness of various recovery protocols, including nutritional strategies and potentially even pre-race interventions to bolster red blood cell resilience. Understanding the cumulative impact of repeated blood cell depletion over years of ultra-marathon training is crucial for ensuring the long-term health and well-being of these dedicated athletes. The future of endurance sports needs to address not just performance enhancement, but also proactive health management to protect the athletes who push the boundaries of human capability.

This story was originally published February 19, 2026.