Human Endurance Has a Restrict, Examine Finds

They run a whole lot of miles over days, push by ache and defy fatigue; nonetheless, even elite ultra-endurance athletes can’t escape the physique’s pure limits.

Researchers from the Massachusetts College of Liberal Arts and Duke University tracked ultra-athletes by months of coaching, discovering that even probably the most excessive endurance competitors can’t surpass an average metabolic ceiling of ~2.5 occasions their basal metabolic price (BMR) – irrespective of how laborious they push.

Ultra-athletes take a look at the boundaries of human endurance

Ultra-endurance occasions corresponding to ultramarathons, Ironman triathlons and multi-day biking races push human stamina to its outer limits. For the athletes who participate, the query turns into: how a lot vitality can the physique burn and maintain over lengthy intervals, and what organic components set that restrict?

Researchers have lengthy used the idea of a metabolic ceiling – the utmost sustainable vitality output an individual can keep, often expressed as a a number of of their BMR. Earlier research discovered that in brief, intense efforts, people can burn as much as ~10 occasions their BMR. However, when exercise lasts for weeks or months, the restrict seems to drop – one evaluation discovered that the ceiling asymptotically settled around 2.5 × BMR.

Despite this theoretical mannequin, direct measurements of elite ultra-endurance athletes over prolonged timeframes have been uncommon. Most work has captured temporary snapshots of competitors, not steady monitoring over months.

“Every living thing has a metabolic ceiling, but exactly what that number is, and what constrains it, is the question,” stated lead writer Dr. Andrew Best, an assistant professor on the Massachusetts College of Liberal Arts, who can also be an endurance athlete. 

“To find out, we asked, if we get a group of really competitive ultra-athletes, can they break this proposed metabolic ceiling?”  

With that in thoughts, the brand new research got down to take a look at this ceiling utilizing direct physiological monitoring of ultra-endurance athletes throughout each brief occasions and long-term coaching.

Researchers measured ultra-athlete vitality use

Best and the workforce recruited 14 elite ultra-endurance athletes (12 males and a pair of ladies, common age ~37 years), together with ultramarathoners, triathletes and cyclists. They measured vitality use with the doubly labeled water technique, the place members drink water containing secure isotopes that enable scientists to trace how briskly the physique burns energy.

The athletes additionally offered detailed coaching logs (from platforms corresponding to Strava and direct athlete data). This allowed researchers to estimate vitality expenditure not solely throughout brief occasions (24 hours to ~13 days) but additionally over extended intervals as much as 1 12 months (30–52 weeks).

During brief, intense occasions, among the athletes burned as much as 6–7 × BMR, roughly 7000–8000 kcal per day. However, when averaged over the long run, expenditure dropped and converged close to 2.4 × BMR, aligning with the expected ceiling.

A couple of people briefly exceeded ~2.7 × BMR, however most remained at or beneath the edge all year long.

Mathematical evaluation confirmed that as actions lasted longer, the athletes’ vitality use progressively leveled off, simply as concept predicted.

Additionally, as workloads elevated, non-exercise vitality use (corresponding to fidgeting, spontaneous motion, thermoregulation) sharply declined, suggesting the physique actively reallocates vitality from on a regular basis features when pushed laborious.

“Your brain has a really powerful influence on how much you fidget, how much you want to move and how encouraged you are to take a nap. All these fatigues we feel save calories,” stated Best. 

The authors counsel that this vitality saving may lengthen past motion, probably affecting hormone steadiness and tissue restore.

Why ultra-athletes can’t exceed their metabolic ceiling

Even in elite ultra-endurance athletes, vitality use appears to plateau on the anticipated 2.5 × BMR. This ceiling doubtless displays the boundaries of nutrient absorption and the physique’s potential to course of and distribute vitality with out breaking down its personal tissues.

“If you go over the ceiling for short periods, that’s fine. You can make up for it later. But long term, it’s unsustainable because your body will start to break down its tissue, and you’ll shrink,” stated Best.

Even knowledge from world-class athletes, corresponding to Kilian Jornet and Kristian Blummenfelt, counsel their year-round coaching suits inside the identical 2.5 × BMR restrict.

The outcomes additionally present how the physique adapts when pushed to its metabolic edge. Under heavy workloads, vitality is conserved by dialing down non-essential features, and this inner budgeting could also be why even the fittest athletes ultimately hit the identical higher restrict.

Beyond athletics, these findings supply a framework for understanding how people steadiness vitality calls for throughout laborious bodily labor, long-term sickness or excessive environments. They can also counsel how our species developed endurance capabilities that depend on managing, not exceeding, vitality limits.

However, the research had a small cohort of 14 athletes and long-term vitality estimates had been reconstructed from fashions. Since most members had been ultramarathoners, the findings could mirror endurance operating greater than different disciplines. Future work with bigger and extra numerous teams – and year-round direct measurements – may refine the two.5 × BMR threshold and take a look at whether or not prime professionals with precision vitamin can edge greater.

“For most of us, we’re never going to reach this metabolic ceiling,” stated Best. “It takes running 11 miles on average a day for a year to achieve 2.5 times BMR. Most people, including me, would get injured before any sort of energetic limit comes into play.”

Reference: Best A, Sadhir S, Hyatt E, Pontzer H. Ultra-endurance athletes and the metabolic ceiling. Curr Biol. 2025. doi: 10.1016/j.cub.2025.08.063

This article is a rework of a press launch issued by Cell Press. Material has been edited for size and content material.