A recent study has provided compelling evidence that even the world’s most elite endurance athletes face a fundamental biological limit to how much energy their bodies can sustainably expend. This research, published in the journal *Current Biology*, tracked ultramarathon runners and other ultraendurance athletes over the course of an entire year, revealing that despite their extraordinary physical feats, these athletes cannot exceed a certain metabolic ceiling for prolonged periods without consequences.
For many years, scientists have speculated about the existence of a maximum sustained metabolic rate in humans — essentially, the highest rate at which our bodies can burn calories over a long duration. Early research dating back to the 1980s and 1990s, largely based on data from the grueling 23-day Tour de France cycling race, suggested that this metabolic ceiling was roughly four to five times an individual’s basal metabolic rate (BMR). BMR represents the amount of energy required to maintain basic physiological functions while at rest.
As endurance sports evolved, subsequent studies examining shorter endurance events revealed that athletes could push well beyond this initial estimate during intense but shorter efforts. For example, Ironman triathletes competing for about 11 hours have been observed to burn calories at rates up to 9.4 times their BMR, and ultramarathoners in 25-hour races reached about 8.5 times their resting metabolic rate. This led scientists in 2019 to propose a nuanced view: the metabolic ceiling is not a fixed number but depends heavily on the duration of exertion. While short bursts of extreme calorie expenditure are possible, over longer periods—such as many weeks or months of training and competition—the metabolic rate appears to average out to about 2.5 times BMR.
This 2.5-times-BMR threshold equates roughly to burning around 3,750 calories per day for a lean person weighing about 150 pounds. For most people, including casual exercisers and hobby runners, this limit remains far beyond what they approach in daily life. The new study aimed to test whether even ultraendurance athletes—who arguably push human limits more than anyone else—could break through this metabolic ceiling, or if it truly represents a hard biological boundary.
The research team, led by biological anthropologists including Andrew Best from the Massachusetts College of Liberal Arts, followed 14 ultraendurance athletes (12 men and 2 women) over the span of an entire year, marking the longest such study ever conducted. Most participants were professional athletes specializing in ultramarathons, Ironman triathlons, multiday cycling races, and other demanding endurance events. The team employed a sophisticated metabolic measurement technique involving doubly labeled water—a form of water containing heavier isotopes of hydrogen and oxygen—which allows precise tracking of energy expenditure through analysis of urine samples.
During competitions, these athletes exhibited astonishingly high calorie burns, reaching up to seven times their basal metabolic rates. In some cases, this translated to roughly 11,000 calories burned in a single day, an energy demand that dwarfs typical daily requirements. However, the study’s key finding was that such extreme metabolic rates were not sustainable across the long term. After about 30 weeks, the athletes’ average metabolic rates dropped to approximately 2.5 times BMR or lower, aligning closely with the previously hypothesized ceiling.
This pattern suggests that even the most highly trained and motivated endurance athletes cannot maintain ultra-high energy expenditures indefinitely. Instead, the body appears to impose a metabolic limit, possibly as a protective mechanism to prevent physiological damage or depletion of critical resources.
The existence of this metabolic ceiling raises questions about what physiological trade-offs the body makes when operating near its limit. Amanda McGrosky, an evolutionary anthropologist at Elon University who was not involved in the study, notes that sustaining exercise at such intense levels likely forces the body to compromise other functions. Early research indicates the human body may slow digestion, suppress immune responses, and even temporarily reduce brain tissue volume during prolonged extreme exertion. Additionally, there is some evidence that sexual arousal and reproductive functions receive less energy investment following major endurance events, highlighting the body’s prioritization of immediate survival and performance over other biological processes.
While the study’s results are compelling, experts caution that the sample size was limited, with only 14 athletes followed. Nevertheless, the findings have practical implications for athletes and coaches aiming to optimize training and competition strategies. Bryce Carlson, a former anthropology assistant professor and world-record-holding ultraendurance athlete who also