A recent study published in Current Biology sheds new light on a long-standing question in human physiology: Is there a fundamental limit to how much energy the human body can expend over extended periods of intense physical activity? By closely monitoring elite ultramarathon runners over the course of a year, researchers have provided compelling evidence suggesting that even the fittest endurance athletes encounter a hard metabolic ceiling that constrains their sustained energy expenditure.
For decades, scientists have been interested in understanding the upper boundaries of human metabolism—essentially, how many calories a person can burn while maintaining necessary bodily functions and physical exertion. Early research dating back to the 1980s and 1990s, including foundational studies during the grueling 23-day Tour de France cycling race, estimated that humans could sustain energy expenditures up to four to five times their basal metabolic rate (BMR). The BMR is the amount of energy the body uses at complete rest to maintain essential functions such as breathing, circulation, and cellular processes.
However, more recent studies involving shorter but intense endurance events revealed that athletes could temporarily exceed this figure. For instance, during 11-hour Ironman triathlons, athletes have been documented to burn up to 9.4 times their BMR, and 25-hour ultramarathons see metabolic rates around 8.5 times BMR. These observations prompted researchers to hypothesize that the maximum sustainable metabolic rate depends heavily on the duration of exertion. Specifically, while the body can tolerate extremely high caloric burn rates during short bursts of intense activity, over longer periods—such as weeks or months of continuous training and competition—the average metabolic rate tends to stabilize around 2.5 times the basal rate.
The current study aimed to put this hypothesis to the test by focusing on a select group of individuals uniquely suited to challenge it: elite ultramarathon runners. Twelve male and two female ultraendurance athletes, many of whom were professional competitors in ultramarathons, Ironman triathlons, multiday cycling races, and other prolonged endurance events, were followed closely for up to a year. This represents the longest continuous metabolic monitoring of athletes in such demanding sports to date.
To accurately measure their energy expenditure, the researchers employed a sophisticated technique involving “doubly labeled water.” This method involves having participants consume water enriched with stable (non-radioactive) isotopes of hydrogen and oxygen. By tracking how quickly these isotopes are eliminated from the body through urine, scientists can precisely estimate the total amount of carbon dioxide produced and thus calculate the total calories burned over time—without interrupting the athletes’ daily routines or training.
The results were striking. During competitive events, ultramarathoners burned as many as 11,000 calories in a single day, reaching metabolic rates up to seven times their basal level. This confirms just how extraordinarily demanding these races are on the human body. Yet, despite these remarkable bursts of energy expenditure, the athletes’ average metabolic rates over longer periods never surpassed about 2.5 times their BMR. Even the most dedicated runners, including one individual who logged approximately 4,500 miles annually on rugged trails, maintained energy expenditures well within this proposed ceiling.
This finding supports the idea that there is a biological constraint preventing humans from sustaining higher rates of calorie burning indefinitely. But what costs do athletes pay to approach this limit? Amanda McGrosky, an evolutionary anthropologist not involved in the study, explains that the body appears to make trade-offs when operating near the metabolic ceiling. Although the precise mechanisms remain unclear, early evidence suggests that the body may compensate by slowing digestion, weakening immune responses, and even temporarily reducing brain volume. Moreover, energy devoted to sexual arousal and reproductive functions appears to diminish during and after prolonged exertion. These physiological sacrifices highlight how taxing extreme endurance activity can be—even for those with exceptional fitness.
While the study’s insights are robust, experts caution that the sample size was relatively small, and more research is needed to confirm these findings across broader populations. Bryce Carlson, a former anthropology professor and world-record-holding ultramarathoner who was not involved in the study, emphasizes the potential practical benefits of understanding this metabolic ceiling. If athletes can learn how close they are to their individual limits during training and competition, they might optimize performance and recovery strategies accordingly.
Carlson also points to a critical unanswered question about what determines the metabolic ceiling. One leading hypothesis is that the limit is tied to the body’s capacity to digest and