The Cost of Super Marathon

How does extreme endurance running accelerate red blood cell aging?

In recent years, endurance sports have continued to grow in popularity worldwide. Statistics show that currently, 1.1 to 1.3 million people complete marathons each year globally, with over 800 marathon events held annually around the world.

The number of participants in major events continues to rise. For example, the lottery registration for the 2026 London Marathon exceeded 1.1 million, setting a new global record for marathon registrations.

As participation increases, endurance activities like marathons are gradually shifting from specialized athletic competitions to popular sports accessible to the general public, especially among middle-aged and amateur runners.

Ultra-distance running is an extreme endurance sport. Visual China | Image

However, with more people taking part in extreme endurance events such as marathons, Ironman triathlons, and ultra-trail races, a question has begun to attract scientific attention: as exercise intensity and duration increase, how much load can the human body actually withstand? Do the health benefits of exercise begin to turn into harm beyond a certain threshold?

Endurance Running Damages Red Blood Cells

A recent study offers some answers.

In February 2026, the journal Blood published a study revealing that extreme endurance running may damage red blood cells in multiple ways, affecting their normal function. How long this damage lasts and whether it has long-term health impacts remain unclear.

“Based on current data, we cannot yet advise whether one should participate in these events. But it’s clear that when people engage in such extreme endurance activities, their bodies are under sustained stress, which can affect red blood cells,” said Travis Nemkov, associate professor of biochemistry and molecular genetics at the University of Colorado and the lead author of the study.

Red blood cells are among the most numerous cells in the human body. Their main function is to transport oxygen via hemoglobin and carry metabolic waste products like carbon dioxide back to the lungs for exhalation. To flow through tiny capillaries throughout the body, red blood cells must be highly flexible, capable of constantly changing shape.

In fact, sports medicine has long observed a phenomenon: in ultra-marathons or long-distance events, some athletes experience destruction of red blood cells or even anemia during the race. This phenomenon is sometimes called “runner’s anemia.” However, scientists have long lacked a clear understanding of the specific mechanisms behind it.

To explore this issue, Nemkov and his team conducted systematic studies on changes in red blood cells during endurance events.

They collected blood samples from athletes in two world-class trail races: the Martigny–Combes to Chamonix race (~40 km) and the Ultra-Trail du Mont Blanc (UTMB), approximately 171 km.

In these two events, the team recruited 23 endurance runners, collecting blood samples before and after the races. Using molecular omics techniques, they analyzed thousands of proteins, lipids, metabolites, and trace elements in plasma and red blood cells, creating the most detailed molecular map to date of how endurance exercise affects red blood cells.

Results showed that after long-distance races, athletes’ red blood cells became significantly less flexible, becoming more “stiff.” This change could impair their ability to transport oxygen, nutrients, and waste products within the body.

Further analysis revealed that two mechanisms jointly caused these red blood cell damages. One was mechanical injury caused by changes in fluid pressure during running. During prolonged running, fluid pressure and shear forces in the blood circulation fluctuate, potentially directly damaging the red blood cell membrane structure. The other involved molecular-level damage: endurance events induce significant inflammatory responses and oxidative stress. When the body’s antioxidant capacity is insufficient, reactive oxygen species increase and can damage DNA and cellular structures.

The researchers also noted that these changes resemble molecular features seen in natural red blood cell aging, suggesting that “the process of red blood cell aging and breakdown is accelerated.”

More notably, this damage appears to be related to the distance run. After a 40 km race, clear red blood cell changes could already be detected; in the 171 km ultra-distance race, the damage was even more pronounced.

“There may be a distance threshold between marathons and ultramarathons, beyond which red blood cell damage increases significantly,” he said. “We’ve observed these changes, but we still don’t know how long it takes for the body to recover, or whether these effects are ultimately beneficial or harmful in the long run,” Nemkov added.

The researchers believe that further studies could help sports scientists develop more scientific training and recovery strategies, such as personalized training, nutrition, and recovery plans, to improve performance while reducing potential health risks.

Additionally, understanding how the body responds under extreme endurance conditions could help improve blood storage and processing techniques. During blood bank storage, red blood cells gradually undergo structural and functional changes, known as “storage lesions.” These changes typically become significant after several weeks of storage. According to the U.S. Food and Drug Administration (FDA), blood stored for more than six weeks is no longer suitable for clinical transfusion.

However, the study has limitations, including a small sample size, lack of racial diversity among subjects, and blood samples only collected at two time points (pre- and post-race). Larger, longer-term studies are needed to further validate these findings.

Risks of Extreme Sports

The impact of extreme endurance sports on human health has been a topic of discussion for many years. Numerous studies show that moderate exercise can significantly reduce the risk of cardiovascular disease, diabetes, and other chronic illnesses, and extend lifespan. But as exercise intensity and duration increase, the health benefits tend to diminish.

Previous research has found that extreme endurance activities can impose additional physiological stress. For example, a study published in the European Heart Journal showed that some marathon finishers exhibit elevated levels of cardiac injury markers, indicating that the heart may experience temporary stress during prolonged high-intensity exercise. However, these changes usually normalize within days.

Other studies focus on long-term endurance athletes. Research published in Circulation and other cardiovascular journals suggests that long-term participation in marathons, cycling races, or Ironman triathlons may increase the risk of atrial fibrillation (AF). Many endurance athletes develop AF later in life, and some even during their athletic careers. Researchers believe that sustained high-intensity training can lead to structural adaptations of the heart, such as atrial enlargement or myocardial fibrosis, which increase arrhythmia risk.

It is well known that resting adult heart rate ranges from 60 to 100 beats per minute, but long-term endurance athletes, like marathon runners, often have resting heart rates as low as 30–40 bpm, sometimes even lower during sleep. Recent studies suggest that this “athlete’s bradycardia” is related not only to neural regulation but also to decreased expression of key ion channels like HCN4 (hyperpolarization-activated cyclic nucleotide-gated channel 4) in the sinoatrial node. Animal experiments show that prolonged endurance training reduces HCN4 levels, slowing the heart’s intrinsic pacemaker activity.

Meanwhile, research in exercise immunology indicates that within hours to days after extreme endurance events, the immune system may experience temporary suppression, known as the “open window” phenomenon, during which athletes are more susceptible to respiratory infections and other illnesses.

However, most sports medicine experts emphasize that this does not mean people should avoid endurance sports altogether. For most individuals, regular running, cycling, or swimming remains one of the most effective ways to stay healthy. The real concern is not exercise itself but the long-term effects of excessive, extreme training beyond the body’s capacity.

In recent years, many researchers have proposed that the relationship between exercise volume and health follows a “U-shaped curve”: both too little and too much exercise are detrimental. Insufficient activity increases the risk of chronic diseases, while excessive exercise can impair immune function, cause fatigue, and increase cardiovascular risks. Moderate, consistent exercise over the long term is the most beneficial.

The World Health Organization (WHO) recommends that adults engage in 150–300 minutes of moderate-intensity or 75–150 minutes of high-intensity activity weekly to gain significant health benefits. If these standards are not met, any activity is better than none.

Events like ultra-trail races, Ironman triathlons, or ultra-marathons are more about testing physical limits than promoting health.

Zhejiang University School of Medicine’s Department of Cardiology Director Zhu Jianhua also pointed out: “Competitive sports push the body’s functions to the limit. When exercise exceeds what the body can handle, vital organs may be affected. Long-term intense exercise can cause damage that the body cannot fully repair once a certain threshold is reached.”

Special contributor: Yuan Duanduan, Southern Weekend

Editor: Zhu Liyuan