Scientists found the metabolic ceiling that limits human endurance

When ultrarunners lace up for races that last days and span hundreds of miles, they aren’t just testing grit and quads – they’re running controlled experiments on human physiology. A new study finds a stark result: even the fittest endurance athletes can’t outrun a long-term “metabolic ceiling.”

The limit is about 2.5 times their basal metabolic rate (BMR) – the energy the body spends at rest just to keep you alive.

“Every living thing has a metabolic ceiling, but exactly what that number is – and what constrains it – is the question,” said lead author Andrew Best, an anthropologist at the Massachusetts College of Liberal Arts, and an endurance athlete himself.

The team set out to see whether the world’s most driven competitors could push past that line. “To find out, we asked, if we get a group of really competitive ultra-athletes, can they break this proposed metabolic ceiling?”

What the metabolic ceiling means

Think of BMR as your engine idling in neutral. The metabolic ceiling is how fast that engine can safely rev over weeks and months.

For short bursts – a hard interval, a late-race surge – humans can shoot well above that line. But averaged across long spans, the body settles back to roughly 2.5× BMR. The new data show that exceeding that limit for too long is physiologically unsustainable.

“If you go over the ceiling for short periods, that’s fine. You can make up for it later,” Best said. “But long term, it’s unsustainable because your body will start to break down its tissue, and you’ll shrink.”

Measuring burn beyond the lab

To test the ceiling in the “wild” rather than a lab, the researchers followed 14 elite competitors – ultrarunners, cyclists, and triathletes – through training blocks and multi-day events.

Instead of relying on heart rate estimates or treadmill readouts, the team used the gold standard labeled water technique.

Participants drank water enriched with harmless stable isotopes (deuterium and oxygen-18). By tracking how quickly those markers appeared in urine, the team calculated carbon dioxide production and therefore true total energy expenditure.

During the most brutal stretches, some athletes briefly torched energy at six to seven times BMR – roughly 7,000 to 8,000 calories per day. But as the window widened to 30 and 52 weeks, the math snapped back to the same answer: on average, about 2.4× BMR.

The takeaway is blunt. Even among the outliers of human endurance, the long-term burn rate obeys a shared cap.

Energy savings inside the body

One of the study’s most intriguing insights is how the body defends that ceiling: it steals from other budgets.

As athletes pour calories into forward motion, the brain subtly trims energy elsewhere – less fidgeting, more napping, fewer unconscious movements, a general dampening of spontaneous activity.

“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,” Best said. “All these fatigues we feel save calories.”

It’s an economy of micro-savings that lets the body keep moving while keeping the long-term ledger in balance.

The bigger picture of endurance

A hard energy cap helps explain why performances flatten over stage races and expeditions and why even hardened adventurers come home gaunt.

It also reframes weight-loss expectations: you can’t simply exercise your way to unlimited calorie burn, because over time the body compensates and total daily energy expenditure plateaus.

There are broader health angles, too. If total energy is capped, lavish spending in one domain – weeks of heavy training, for instance – may force austerity in others. Those could include immune function, wound healing, or reproductive health.

That trade-off lens could help clinicians think about recovery from chronic illness and guide athletes and coaches in pacing training blocks to avoid overuse and burnout.

Limits most of us will never touch

For almost everyone, this ceiling is theoretical. “For most of us, we’re never going to reach this metabolic ceiling,” Best said.

“It takes running about 11 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.”

Short-term spikes above the line are routine; sustained months-long averages near it are rare.

The sample was small – 14 athletes – and true outliers might exist. But the convergence across sports and timeframes suggests a robust human constant. The authors also emphasize that individual physiology matters.

Genetics, body composition, and training history likely shape how close any one person can hover to the ceiling, and for how long, before compensations kick in.

Training smarter within limits

For endurance athletes, the message is both sobering and useful. You can flirt with the redline, but across months your body will drag you back to cruising speed. That implies strategic periodization.

Schedule short pushes above the ceiling, then plan genuine recovery blocks so the long-term average remains sustainable.

It also supports the quiet habits coaches have long championed – sleep, rest days, and caloric adequacy are not luxuries but the levers your brain uses to keep the system in balance.

For non-athletes, the study explains a common frustration: adding more exercise doesn’t always yield ever-higher daily burn, especially over time.

The body adapts by trimming non-exercise activity thermogenesis (NEAT) – all the little movements and fidgets you don’t notice – so the total number stops climbing.

That doesn’t make exercise pointless; it means expectations should factor in compensation. Exercise still delivers cardiovascular, metabolic, and mental health benefits that go far beyond calories burned.

Biology behind the metabolic ceiling

What enforces the cap? The paper doesn’t claim a single mechanism but points to a coordinated suite of constraints.

These include digestive throughput (how much energy you can absorb and process), tissue maintenance costs, endocrine signals, and the brain’s central regulation of fatigue and motivation.

Across species, similar ceilings have emerged, hinting at deep evolutionary roots. Humans can spike to 10× BMR for short sprints and survive ultra-events at 6–7× BMR for days, but the long game is governed by that ~2.5× average.

In the end, the conclusion is almost reassuring: our physiology is built to protect itself. You can hammer the accelerator now and then, but the engine has a governor. The smartest play is to work with that biology, not against it.

The study is published in the journal Current Biology.

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