Humans like to believe that there’s no limit to how hard and far we can go. We can race in sensory deprivation for 24 hours or spend months racing through the mountains of France, but as scientists report in Science Advances on Wednesday, there truly is a limit to how much energy the body can burn. It just took 140 days of running back-to back-marathons across the United States for scientists to finally find it.

That absolute limit to human endurance, according to the study, is 2.5 times a person’s resting metabolic rate — which is the amount of calories burned at rest. For a typical person, this would be around 4,000 calories per day.

"The limits of endurance appear to be non-negotiable."

That metric is an “alimentary limit,” explains study author Herman Pontzer, Ph.D., an evolutionary anthropologist at Duke University, meaning it is rooted in nourishment. Beyond that limit, the body isn’t capable of restoring the carbohydrate and fat stores lost during extreme exercise, and eventually, that endless endurance race would come to a grinding halt. Pontzer and his colleagues identified this limit in data collected from athletes who competed in the Race Across the USA, a 140-day race from Huntington Beach, California, to Washington, D.C.

“The limits of endurance appear to be non-negotiable,” Pontzer tells Inverse. “They impose a hard limit on what humans can do.”

Athletes running during the RAUSA event, a race from California to Washington, D.C., that lasts 140 days. Bryce Carlson

The idea that we can put a hard limit on human endurance is a bit complicated. To picture that limit, imagine running a race with no finish line or running endlessly on a treadmill that never stopped. To begin that never-ending race, you would have to think about how to best use your body as an engine. Like any engine, our body requires fuel — in this case, carbohydrates and fats.

If you’re expending less than 2.5 times your resting metabolic rate, you could probably eat enough calories to get that engine going, explains Pontzer. But anything over that, and it would become impossible to eat enough to sustain the work without dipping into the body’s energy reserves, which too, over time, would waste away.

“At expenditures above 2.5 times resting metabolic rate, the body can’t bring energy in fast enough to replenish its stores. You lose weight as you burn more than you take in,” he says. “If [athletes] tried to keep at that pace indefinitely, their bodies would shut down. We see this in, for example, arctic trekkers who hit their limits and have to slow the pace way down.”

Humans actually brush up against this metabolic ceiling fairly often. As Pontzer notes, cyclists competing in the Tour de France, who race for 21 days, can expend up to 7,000 calories during a punishing mountain stage. As part of the paper, the team also analyzed the metabolic expenditure during a lot of other grueling activities, like arctic trekking, which had a documented metabolic expenditure of around 6.6 times resting metabolic rate.

But a race or arctic exploration doesn’t last forever, which has led us to overestimate this metabolic ceiling in the past. Earlier estimates based on Tour de France cyclists put the previous limit to human endurance around 4 or 5 times resting metabolic rate. This new work suggests that athletes are essentially running their engines in overdrive for a short period of time, but if they had to keep it up, eventually it wouldn’t be possible.

Pontzer and his co-authors found this true metabolic ceiling in data collected from Race Across the USA competitors, who averaged about a marathon each day, six days a week, for almost five months.

During the race, scientists took metabolic measurements of the athletes' energy expenditures and found that they plateaued around 2.5 times their resting metabolic rate. Bryce Carlson

These athletes began by expending energy at a far higher rate than 2.5 times their resting metabolic rate, but over time, that rate of energy expenditure began to change — plateauing around 2.5 times their metabolic rate. Their bodies also seemed to make additional sacrifices to keep their engines burning at that rate.

“It’s notable that these racers’ bodies responded to their immense workloads by finding energy savings elsewhere — at the start of the race they were burning exactly as many calories as we’d expect for their body size and workload; by the end they were burning 600 kilocalories per day less,” says Pontzer.

But overall, the energy expenditure plateaued, suggesting that this was truly the upper limit, beyond which the human body simply couldn’t work any more efficiently. To be fair, it’s still a pretty impressive rate of calorie burning, suggesting that our engines are equipped to undertake lengthy athletic challenges, like a race across a continent. But one day would it be possible to transcend that limit to go farther and faster? Maybe so, now that we know what our metabolic ceiling is, says Pontzer.

“We think this work points toward the key mechanism in humans,” he points out. “Knowing that key mechanism advances our understanding of the human body and could potentially point the way toward expanding those limits.”

For now, the RAUSA athletes can take pride in the fact that they’ve looked one of humanity’s hard biological limits square in the face.

Abstract: The limits on maximum sustained energy expenditure are unclear but are of interest because they constrain reproduction, thermoregulation, and physical activity. Here, we show that sustained expenditure in humans, measured as maximum sustained metabolic scope (SusMS), is a function of event duration. We compiled measurements of total energy expenditure (TEE) and basal metabolic rate (BMR) from human endurance events and added new data from adults running ~250 km/week for 20 weeks in a transcontinental race. For events lasting 0.5 to 250+ days, SusMS decreases curvilinearly with event duration, plateauing below 3× BMR. This relationship differs from that of shorter events (e.g., marathons). Incorporating data from overfeeding studies, we find evidence for an alimentary energy supply limit in humans of ~2.5× BMR; greater expenditure requires drawing down the body’s energy stores. Transcontinental race data suggest that humans can partially reduce TEE during long events to extend endurance.