In the realm of human endurance, ultra-endurance athletes push the boundaries of physical and mental resilience by competing in races that stretch across hundreds of miles and span multiple days. These events challenge not only the athlete’s willpower and muscle stamina but also probe the fundamental limits of human metabolism. A groundbreaking study published in the prestigious journal Current Biology has now shed light on a critical aspect of human physiology: the maximum sustainable metabolic rate during prolonged extreme exertion, known as the “metabolic ceiling.”
The metabolic ceiling is a parameter that represents the uppermost limit at which the human body can burn calories over an extended duration. Previously, it had been hypothesized that humans could transiently reach metabolic rates up to ten times their basal metabolic rate (BMR)—the minimal number of calories expended while at complete rest—for short bursts of intense activity. However, this new research led by anthropologist and endurance athlete Andrew Best systematically investigates whether ultra-endurance athletes can sustain or break through this purported metabolic ceiling over long timescales.
To explore this, the research team recruited fourteen elite-level athletes specializing in ultra-running, cycling, and triathlon disciplines. The athletes were monitored during their training routines and competitions, with the researchers employing a cutting-edge methodology involving stable isotope tracers. Participants ingested water labeled with isotopes deuterium and oxygen-18, heavier analogues of hydrogen and oxygen atoms. By analyzing the isotope concentration in urine samples, the investigators could accurately determine the amount of carbon dioxide the athletes exhaled, thereby quantifying their energy expenditure with remarkable precision.
The findings revealed that during multi-day ultra-endurance races, some athletes achieved extraordinary metabolic intensities, burning calories at rates six to sevenfold times their BMR, roughly translating to a staggering 7,000 to 8,000 calories per day. Despite these peak values during brief periods, when researchers extended their analysis to cover longer durations—measuring energy expenditure over 30 and 52 weeks—the athletes’ caloric burn rates converged closely with the predicted metabolic ceiling. On average, the sustained metabolic intensity was approximately 2.4 times their basal rate.
This sustained ceiling implies a physiological cap that humans, even those with exceptional endurance capabilities, find arduous to exceed over prolonged intervals. Andrew Best emphasizes that while transient metabolic spikes beyond this threshold are feasible, they are unsustainable in the long term. Surpassing the ceiling chronically results in the body’s consumption of its own tissues, leading to muscle mass loss and overall physical decline.
Moreover, the study provides compelling insights into the body’s adaptive strategies to manage energy constraints under extreme endurance stress. The researchers discovered that athletes instinctively conserve energy by reducing non-essential physical activities. This physiological adaptation manifests as decreased spontaneous movements, lower fidgeting rates, and increased tendencies to rest or nap, orchestrated by the brain as a mechanism to preserve calories for vital functions and athletic performance.
The research also highlights the highly individualized nature of metabolic ceilings. While the study’s cohort represents elite athletes, it acknowledges the possibility that outliers with uniquely exceptional metabolic capacities may exist but were not captured in this sample. These nuanced differences underscore the complexity of human metabolism and the array of factors that govern energy utilization, including genetics, training history, and overall physiology.
Importantly, these findings hold profound implications not just for athletes striving to maximize performance but also for broader biomedical research. Understanding the inherent metabolic ceilings in humans can illuminate how energy constraints influence other aspects of human biology, such as immune function, reproduction, and aging, where energy allocation plays a pivotal role.
For the average individual, the concept of reaching a metabolic ceiling may seem abstract, but the realities are grounded in everyday physiology. To sustain a metabolic output 2.5 times one’s basal rate would require extraordinary physical exertion—estimated at about 11 miles of running per day over an entire year. For most people, and even many athletes, injuries and recovery needs will impose limitations long before reaching any metabolic boundary.
This study, supported by Duke University and the Massachusetts College of Liberal Arts Faculty Incentive Award, marks a significant advance in our understanding of the energetic limits that define human endurance. It frames the metabolic ceiling as a cornerstone metric in both sports science and human biology, providing a framework for future studies aiming to unravel the mechanistic underpinnings of energy expenditure constraints.
Looking forward, explorations into this field may uncover strategies to optimize energy use, potentially influencing athletic training methodologies, nutritional interventions, and treatments for metabolic disorders. The delicate balance between pushing the limits of performance and maintaining physiological integrity remains a central theme as researchers delve deeper into the metabolic ceiling’s ramifications.
This pioneering research invites us to reconsider the capacities and limits of the human body. It highlights the remarkable adaptations of ultra-endurance athletes and offers a window into the complex interplay between metabolism, physiology, and survival. The metabolic ceiling not only defines the outer bounds of physical endurance but also reflects a fundamental characteristic of life itself—a ceiling shaped by evolution and the intrinsic constraints of biology.
Subject of Research: People
Article Title: Ultra-endurance athletes and the metabolic ceiling
News Publication Date: 20-Oct-2025
Web References: http://www.cell.com/current-biology
References: Best et al., Current Biology, 10.1016/j.cub.2025.08.063
Image Credits: Howie Stern
Keywords: Physical exercise, Human biology, Human physiology, Metabolic rate
Tags: anthropological studies in sportscalorie burning limitselite endurance competitionsendurance training monitoringextreme exertion in sportshuman metabolic capacitymaximum sustainable metabolic ratemental endurance challengesmetabolic ceiling researchphysical resilience in athletesphysiological limits of athletesultra-endurance athletes
