Running at the limit: What extreme sports do to the body

Legs feel heavy as lead, knees ache, and sore muscles: Many marathon runners still feel the effects days after the race. However, some effects may go unnoticed. A study recently published in the journal Nature Metabolism revealed surprising changes in the brains of marathon participants—and rekindles the age-old question of how healthy such extreme performances actually are for amateur athletes.

For the study, a team led by neurologist Carlos Matute from the University of the Basque Country in Leioa examined the brains of ten experienced amateur marathon runners before and within 48 hours of the race. The eight men and two women were placed in a magnetic resonance imaging (MRI) scanner. The research group discovered changes in the white matter of the brain, which surrounds the extensions of nerve cells (axons) and is called myelin . Myelin enables the rapid and efficient transmission of electrical signals in the nervous system.

Immediately after the marathon, the study participants showed a decrease in myelin of up to 28 percent in certain regions of the brain, specifically those that play a role in the control of motor skills and coordination, as well as in sensory and emotional processing. However, the majority of the myelin was unaffected—and the changes were no longer detectable two months after the marathon. The scientists also don't know whether they have an impact on neurophysiological and cognitive functions in these regions.

The brain also has to provide energy reserves when running

However, they have an explanation for the observed loss: myelin consists primarily of fat. During extreme exercise like a marathon, the body relies on its energy reserves. These primarily include carbohydrates stored in the muscles, but later also increasingly fat when the glycogen in the muscles is depleted. Under extreme metabolic conditions, myelin could therefore serve as an extra energy reserve. Does this mean that endurance sports are unhealthy for the brain? Quite the opposite, but it depends on the degree of exertion, the researchers say. "Regular, moderately intense physical activity is a key factor in maintaining brain health throughout life."

It's been proven that exercise is generally beneficial for the brain and health. However, the research is less clear when sports are taken to extremes—as in marathons, ultramarathons , or Ironman-length triathlons . Such extreme competitions are trending. This is demonstrated, for example, by the finishes at the Berlin Marathon, the most popular race of its kind in Germany: While there were just over 28,000 finishers in 2004, this number has almost doubled 20 years later. In London, more than 56,000 runners crossed the finish line this year—a world record.

More and more people are also signing up for even longer distances, although by no means on a comparable scale. Runner's World magazine reports that in 2000, a good 3,000 German runners completed an ultra. In 2019, this number rose to more than 11,000, but this still corresponds to a share of ultra runners of just 0.06 percent of all runners. Such races usually cover 100 kilometers (the most common variant). But there are also races lasting several days and covering thousands of kilometers.

The longest race in the world goes around a city block 5649 times

The Self-Transcendence Race, the world's longest ultramarathon , involves 4,989 kilometers—5,649 laps around a New York City block. The course record is 40 days. There are other incredible feats, for example, in 24-hour races . The men's world record has been held since 2022 by Lithuanian Aleksandr Sorokin, with more than 319 kilometers. That's almost the distance from Berlin to Kap Arkona in a single day. In the women's race , American Camille Herron managed a good 270 kilometers in 24 hours in 2019.

The niche nature of such distances is also reflected in the relatively few studies on the subject. However, as early as 2009, a study planned by neurologist and radiologist Wolfgang Freund of Ulm University Hospital examined the brains of ultrarunners in the 4,487-kilometer Trans Europe Foot Race . In the journal BMC Sports Science, Medicine and Rehabilitation, the research group reported temporary changes in brain structure. Specifically, the volume of gray matter in several brain regions—including areas for language and spatial processing—decreased by up to 6.1 percent during the race. After eight months, however, these changes had completely reversed.

The study, which included twelve runners and a control group, also showed that there were no differences in brain volume between athletes and non-athletes before the start. The researchers suspect that these temporary changes are part of an adaptation process in which the brain deliberately downregulates energy-intensive networks during extreme physical stress. No chronic damage from long-term training was observed.

Increased risk of muscle and tendon injuries

A review article in the journal Frontiers in Physiology examines the physiological adaptations and potential risks of ultramarathon running in greater detail. Beat Knechtle of the University of Zurich, himself an ultramarathon athlete, and the Greek physiologist Pantelis Nikolaidis analyzed how extreme long-term exertion affects various body systems—from muscles and heart to kidneys and the immune system. It became clear that while the body possesses remarkable adaptive mechanisms, it can also reach its limits.

The strain on the musculoskeletal system is particularly striking: According to the study, ultrarunners are at an increased risk of muscle and tendon injuries, as the continuous strain can lead to microtrauma. Furthermore, studies have shown that kidney function can be temporarily impaired during such extreme exertion, manifesting in elevated biomarkers for kidney damage. These changes are usually reversible, but can become problematic if recovery is inadequate. The immune system also reacts to the extreme stress: Temporary immunosuppression is often observed after ultramarathons, which increases the risk of infections.

So, based on their results, do the authors advise against ultramarathons? It's not that clear. Rather, they write: "There's no doubt that completing an ultramarathon has no immediate health benefits." However, most of the damage is generally reversible. And: With regard to the length of telomeres – the protective end pieces of chromosomes whose shortening is associated with biological aging – ultramarathon training could have a positive effect on endurance athletes. "Regular endurance training, in particular, appears to have a protective effect on telomere length and should slow the aging process," the two authors conclude.

Heart attacks during competitions repeatedly make headlines

The findings support what many sports physicians emphasize: Thoughtful training for an ultramarathon or a marathon can be healthy – but the race itself is extremely stressful for the body, especially if the appropriate foundation hasn't been laid in the months leading up to it. Incidents such as heart attacks during such races repeatedly make headlines. At the 2024 Berlin Marathon alone, with more than 50,000 participants, the fire department reportedly had to resuscitate five people who had suffered cardiac arrest.

A recent study published in the journal Jama analyzed data from more than 29 million marathon and half-marathon runners in the United States between 2010 and 2023. A total of 176 runners suffered sudden cardiac arrest, two-thirds of whom were successfully resuscitated. The study found that the incidence of cardiac arrest during these races remained relatively constant at 0.60 cases per 100,000 participants.

Encouragingly, it was also observed that the mortality rate in such incidents fell significantly: from 71 percent in the years 2000 to 2009 to 34 percent in the period 2010 to 2023. The authors attribute this decline primarily to improved emergency care, the result of a change in awareness among the organizers of extremely long races.

Extreme doesn't always have to refer to distance or duration: Some athletes demonstrate exceptional performance well into old age—which makes them particularly exciting for research. If we could understand what keeps these people active for so long, it might be possible to derive insights into healthy aging.

Important: Find an optimal level of movement between the extremes

One example is former Australian marathon professional Derek Clayton. The 82-year-old was the first person to run a sub-2:10 marathon in Fukuoka in 1967. Also remarkable is how active he remained in old age: In a study published in the Journal of Applied Physiology in 2024, researchers report that Clayton trained up to 480 kilometers per week during his active years—and even at age 77, he still completed ten to 15 hours of endurance exercise per week.

A stroke of luck for science: Data on Clayton's fitness was collected over 50 years. What's striking is that his VO₂max—his maximum oxygen uptake—remained virtually constant from age 27 to 49. At 77, it was still 240 percent higher than the average for his age group. Furthermore, while his maximum heart rate had decreased by 22 beats per minute due to his age, his cardiac output while cycling was equivalent to that of a healthy 23-year-old.

At the same time, Clayton's fitness tests revealed atrial fibrillation —a cardiac arrhythmia that required treatment. Studies, such as those published in the European Heart Journal , show that endurance athletes may have a slightly increased risk of this condition due to enlarged atria. Nevertheless, they benefit overall from a healthier cardiovascular system and lower mortality.

When it comes to healthy aging, it seems crucial to find an optimal level of exercise between the extremes – and to stay active in general. Clayton also showed an age-related decline in certain factors, such as maximum heart rate. Yet, even as a senior, he achieved exceptional values ​​– not only due to genetic luck, but also as a result of lifelong training. (dpa/fwt, BLZ)