A new study finds a conclusive result. Even the most trained endurance athletes cannot surpass their long-term “metabolic ceiling“.
The limit is around 2.5 times their basal metabolic rate (BMR). This is the energy the body expends at rest, just to keep us alive.
“Every living being has a metabolic ceiling, but the question is exactly what it is and what limits it,” commented lead author Andrew Best, anthropologist at Massachusetts College of Liberal Arts and an endurance athlete himself.
The team set out to see if the most motivated athletes in the world could break this limit.
“To find out, we asked whether, if we gathered a group of true ultra-endurance athletes, could they surpass this presumed metabolic ceiling?”
What Metabolic Ceiling Means
We can imagine BMR as the energy expenditure of an engine idling in neutral. The metabolic ceiling is how fast that engine can safely run for weeks and months.
For short bursts – a strong sprint, a final push at the end of a race – humans can go well above this limit. But averaged over long periods, the body returns to approximately 2.5×BMR. New data show that exceeding this threshold for too long is physiologically harmful.
“If you go above the ceiling for short periods, that’s fine. You can compensate later,” explains Best. “But in the long term, it’s unsustainable, because your body will start breaking down its own tissues and crash.”
Measuring Energy Burn Outside the Lab
To test the ceiling “in the wild,” rather than in a lab, researchers tracked 14 elite endurance athletes – ultramarathoners, cyclists, and triathletes – through training blocks and multi-day events.
Instead of relying on heart rate estimates or treadmill readings, the team gave participants water enriched with two rare isotopes: deuterium and oxygen-18.
By analyzing the concentration of these isotopes in urine samples from each participant, researchers could calculate the daily rate of carbon dioxide production in their bodies and thus their daily energy expenditure. This approach, known as the Doubly Labeled Water (DLW) method, is the gold standard in public health for measuring calories burned each day during normal daily life. It is simple, completely safe, and accurate.
During the most intense exercise periods, some athletes briefly burned energy at six to seven times their basal metabolic rate (BMR) – roughly 7000 to 8000 calories per day. But when averaged over 30 to 52 weeks, the result was the same: around 2.4×BMR.
The conclusion is clear: even among the most exceptional examples of human endurance, long-term energy expenditure follows a universal limit.
Doubly Labeled Water Method
The Doubly Labeled Water (DLW) method is a non-invasive, gold-standard technique for measuring total energy expenditure in free-living conditions. It works by administering a dose of water labeled with stable isotopes of hydrogen (²H) and oxygen (¹⁸O).
The method tracks how quickly these isotopes are eliminated from the body through fluids such as urine or saliva. Over a period of 5 to 20 days, both isotopes are excreted. Deuterium (²H) leaves only as water, while oxygen-18 (¹⁸O) leaves as both water and carbon dioxide (CO₂). Samples are collected periodically, and the difference between the elimination rates is used to calculate CO₂ production, which, along with the known or estimated respiratory quotient (RQ), provides total energy expenditure.
The Body Saves Energy
One of the most intriguing findings of the study is how the body protects this ceiling: it steals from other energy budgets.
While athletes channel calories into forward motion, the brain subtly reduces energy elsewhere. Fewer unconscious movements, more naps, overall decreased spontaneous activity.
“Our brain has a strong influence over how many unconscious movements we make, how much it wants us to move or nap,” says Best. “All that fatigue we feel actually saves calories.”
This is an economy of micro-savings that allows the body to move while maintaining long-term balance.
The Bigger Picture of Endurance
The strict energy ceiling explains why performance declines during stage races and expeditions, and why even hardened adventurers return home thinner.
It also reframes our expectations for weight loss. You cannot simply exercise endlessly to achieve unlimited calorie burn, because over time, the body compensates and total daily energy expenditure stabilizes.
There are also broader health implications. If total energy is limited, spending generously in one area – for example, weeks of intense training – can force strict cutbacks in others, such as immune function, wound healing, or reproductive health.
This trade-off perspective could help clinicians consider recovery from chronic illness and guide athletes and coaches in structuring training blocks to avoid overload and burnout.
Limits Most of Us Will Never Reach
For almost everyone, this ceiling is theoretical.
“Most of us will never reach this metabolic ceiling,” notes Best.
“You’d have to run an average of about 20 km a day for a year to hit 2.5×BMR. Most people, myself included, would get injured long before hitting any metabolic limit.”
Short-term spikes above the line are routine; sustained averages near it lasting months are rare.
The sample size was small – 14 athletes – and real variations may exist. But the convergence across sports and time frames suggests a stable human constant. The authors also emphasize that individual physiology matters.
Genetics, body composition, and training history likely determine how close someone can stay to the ceiling and for how long before compensations appear.
Train Smarter Within the Limits
For endurance athletes, the message is both sobering and useful. You can attack the red line, but over months, your body will pull you back toward its normal pace. This suggests strategic training periodization.
Plan short bursts above the ceiling, then schedule true recovery blocks, so that the long-term average remains sustainable.
Moreover, the study supports what coaches have long emphasized – sleep, rest days, and adequate nutrition are not luxuries but tools the brain uses to keep the system in balance.
For non-athletes, the study explains a common frustration: adding more exercise does not always lead to higher daily calorie burn, especially over time.
The body adapts by reducing non-exercise activity thermogenesis (NEAT) – all the small, unnoticed movements and fidgeting – so the total number stops increasing.
This doesn’t make exercise pointless; it means expectations must account for compensation. Exercise still provides cardiovascular, metabolic, and mental health benefits far beyond calorie burn.
The Biology Behind the Metabolic Ceiling
What enforces the limit? The article does not name a single mechanism but rather a coordinated set of constraints.
These include digestive capacity (how much energy you can absorb and process), tissue maintenance costs, endocrine signaling, and central brain regulation of fatigue and motivation.
Similar limits have evolved across species, suggesting deep evolutionary roots. Humans can reach 10×BMR for short sprints and survive ultra-events at 6–7×BMR for days, but long-term, the average remains about 2.5×BMR.
Ultimately, the conclusion is almost reassuring: our physiology is built to protect itself. We can push the gas pedal at times, but the engine has a governor. The smartest move is to work with this biology, not against it.
