Ultra-endurance sports turn the human body into a live experiment in energy management. The new work reported by Cell Press and published in Current Biology, led by Andrew Best with collaborators Srishti Sadhir, Emily Hyatt, and Herman Pontzer, tests a simple question with big implications: how hard can humans burn fuel, and for how long, before biology pushes back. Earlier studies from Duke researchers, including Pontzer, John Speakman, and Caitlin Thurber, point to the same answer and provide context. This article is a synthesis of those sources and their coverage, not original research.
The world of ultra-endurance
Ultra-endurance covers ultramarathons of 50 to 250 miles, Ironman triathlons, multi-day cycling events, and long trail records that stretch across weeks. In the short term, elite athletes can burn six to seven times their basal metabolic rate, often 7,000 to 8,000 calories in a day. For very brief efforts, earlier work suggests humans can reach about ten times basal metabolic rate. Yet when the clock moves from days to months, the average settles to a predictable limit.
Best describes the idea plainly: “Every living thing has a metabolic ceiling, but exactly what that number is, and what constrains it, is the question.” His team set out to see if world-class performers could break that limit. “If we get a group of really competitive ultra-athletes,” he asked, “can they break this proposed metabolic ceiling?”
Measuring the body’s true burn rate
The researchers recruited fourteen elite ultra-endurance athletes, twelve men and two women with an average age near thirty-seven, including ultrarunners, a cyclist, and triathletes. They used the gold standard for tracking real-world calorie burn: the doubly labeled water method. Athletes drank water enriched with deuterium and oxygen-18. By tracking how quickly those harmless tracers left the body in urine, the researchers calculated carbon dioxide production and then total energy burned.
Training logs and performance data were paired with these measurements to reconstruct the athletes’ “metabolic scope” over periods of one week to an entire year. Their data covered 24-hour events, 13-day races, and full-season training cycles.
What the numbers revealed
Short efforts reached astonishing highs. In one 23.5-hour run on the Appalachian Trail, a runner hit a metabolic scope of 7.08. Across multi-day races, several athletes reached values above four, and some even six or seven times their basal metabolic rate. But over longer time frames, those numbers fell back toward a stable limit. The average energy expenditure across thirty weeks was about 2.43 times basal metabolic rate, and 2.39 times at fifty-two weeks. A few athletes briefly reached 2.7, but no one sustained higher rates for long.
“If you go over the ceiling for short periods, that’s fine,” Best explained. “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.”
How the body conserves energy
The body manages its energy budget like a strict accountant. As workloads rise, non-exercise activity falls. Fidgeting, casual movement, and the desire to stay active outside of training all decline. “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,” said Best. “All these fatigues we feel save calories.”
During light training weeks, athletes spent roughly nine hundred calories per day on non-running activity. During race periods, that spending nearly disappeared as the body redirected energy to vital muscle function and endurance effort. The data suggest that other processes, like hormone balance and tissue repair, may also adjust downward under prolonged stress.
The biological ceiling
Why does this limit exist? Duke’s Herman Pontzer calls it “the realm of what’s possible for humans.” He and his colleagues point to digestion as a likely bottleneck. No matter how much food an athlete eats, there is a cap on how many calories the gut can absorb and process. “There’s just a limit to how many calories our guts can effectively absorb per day,” Pontzer said. Thurber’s work supports this view. In a transcontinental run, athletes burned six hundred fewer calories a day than expected, evidence that metabolism “downshifts” to stay within sustainable bounds.
The pattern appears across all endurance efforts. The data trace an L-shaped curve: energy output spikes early, then falls and flattens near 2.5 times basal metabolic rate for months-long efforts. That curve holds whether the subject is a triathlete, a cyclist, or even a pregnant woman sustaining high energy output over many weeks.
Outliers and possible exceptions
Individual differences remain. Some world-class athletes, such as Kilian Jornet or Kristian Blummenfelt, may sustain slightly higher levels based on training volume. Historic records from Pat Farmer’s run around Australia or Serge Girard’s yearly mileage suggest brief excursions beyond the ceiling. Yet these figures rely on estimates rather than direct measures, and researchers caution that assumptions about terrain and gait make exact comparisons unreliable.
While the metabolic ceiling itself seems fixed, efficiency at that ceiling can improve. Professional coaching, nutrition, and strategic rest help elite athletes operate closer to their personal maximum. For most people, though, the barrier is injury and recovery rather than energy capacity. As Best put it, “It takes running about eleven 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.”
Beyond sport: what the ceiling means
The findings reach beyond athletics. The same energy rules that cap ultramarathon performance also shape how humans handle physical labor, illness, or pregnancy. Our bodies balance survival and endurance by rationing energy rather than defying physics.
Best and his coauthors conclude that the metabolic ceiling defines human endurance itself. You can sprint beyond it briefly, but you cannot live above it. As Pontzer observed, the ceiling “defines the realm of what’s possible for humans.” For all the grit and training of the world’s toughest athletes, biology still holds the stopwatch.
