Is Ultrarunning Bad for Your Body?

The Short Answer

For most healthy runners, ultrarunning is not bad for the body over the long run. During and just after a race, muscle damage, gut trouble, and a temporary dip in heart function are normal; the what happens to your body answer lays those out hour by hour. This page is about what lasts. Two long-term concerns have real evidence behind them, heart rhythm and repeated kidney injury. Two common fears, worn-out knees and a shorter life, have no evidence behind them.

Long-Term Evidence at a Glance

Heart

The acute effect is well measured. Across 17 studies in a 2024 meta-analysis by Lima and colleagues, left-ventricular ejection fraction was 3.7 percentage points lower and heart rate 18 beats per minute higher immediately after an ultramarathon, and right-ventricular function was also reduced.

Over years, the concern with the best evidence is heart rhythm. In Newman and colleagues' meta-analysis, athletes had 2.46 times the odds of atrial fibrillation of non-athletes, with higher risk in athletes under 55 and in mixed sports than in pure endurance sports. Those were athletes in general, not ultrarunners. Scheer and colleagues' 2022 review of ultra-endurance running calls the cardiovascular system one of the clearest long-term effects, naming right-ventricular dysfunction and a possible higher risk of arrhythmias and high blood pressure. On the other side, in 71 runners aged 26 to 69 measured before the Western States 100, 86% had arteries less stiff than predicted for their age, although the authors note that the long-term changes from ultramarathon training have not yet been determined. If you have exertional chest pain, palpitations, or fainting, see who should be screened before racing.

Kidneys

Short-term kidney stress is common. Rojas-Valverde and colleagues pooled 43 studies of 813 endurance athletes: 43.5% met the threshold for exertional rhabdomyolysis (creatine kinase above 5,000 U/L), 16.4% had rhabdomyolysis with acute kidney injury, and 96.9% of those combined cases were ultrarunners. Muscle and kidney markers returned to baseline after a mean of 5.86 days.

The long-term question is repetition. The same review states that it is unknown whether repeated episodes predispose to chronic kidney disease, while Scheer's review cites evidence that rare cases of acute kidney injury in ultra-endurance running could lead to progressive renal scarring and chronic kidney disease. The one risk you control is NSAIDs. Up to 75% of ultrarunners take them while racing, and in Lipman's randomized trial, 52% of runners on ibuprofen developed acute kidney injury against 34% on placebo. The trial was small and that gap was not statistically conclusive, but it works out to one extra case for every 5.5 runners who took the drug. The hydration and sodium guide covers the fluid side of kidney protection.

Knees and Joints

The claim that running wears out knees is not supported. Burfield and colleagues' meta-analysis of 9 studies and 12,273 people, 1,272 of them runners, found no difference in knee osteoarthritis between runners and controls (odds ratio 0.97, 95% confidence interval 0.56 to 1.68). Runners covering more than 48 km a week had an odds ratio of 0.62, which was not statistically significant. Two limits apply: most of the studies had a high or very high risk of bias, and the top band was simply "more than 48 km a week," so the data cannot separate a 50-km week from a 150-km week.

Bones and Energy Availability

In the ULTRA Study of 1,212 active ultrarunners, 5.5% reported a stress fracture in the previous year, most often in the foot (44.5%), and stress fractures were more common in women than in men. Scheer's review adds that data specific to female athletes are limited and that women may be at greater risk of some long-term problems because of interactions between energy availability and sex hormones. Eating enough to cover training is part of bone health, not only performance.

Immune System, Lungs, and Liver

In the two weeks after the 160-km Western States Endurance Run, 24% of 106 finishers reported an upper respiratory infection lasting at least two days. Across the group, secretion of salivary IgA, an antibody that defends the mouth and throat, fell 46% from before the race to the finish. Knechtle and Nikolaidis's review of ultramarathon physiology also notes that liver enzymes can rise considerably during a race and that such biomarker changes are usually temporary. In the ULTRA Study, allergies or hay fever (25.1%) and exercise-induced asthma (13.0%) were the most common chronic conditions, more common than in the general population.

Overall Health and Lifespan

No study has measured how long ultrarunners live. What is established is that physical activity in general reduces all-cause mortality, which is where Scheer's review starts. The ULTRA Study baseline found serious disease to be uncommon among active ultrarunners: coronary artery disease 0.7%, diabetes 0.7%, and cancers 4.5%. The median runner missed 0 days of work or school to illness or injury in the previous year, even though 64.6% had an exercise-related injury that cost a median of 14 training days. These were self-reported, one-time answers from runners healthy enough to still be racing, so they show who keeps running, not what running does. Later ULTRA Study papers have covered foot and ankle injuries, COVID-19, and motivation, but none has compared chronic disease or mortality over time.