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When you’re working out and think you just can’t push any further, that’s likely just your mind trying to shelter itself from all the physical pain and discomfort. Any professional athlete will tell you that the biggest obstacle that stands between you and your goals is your own mind. But even the world’s fittest athletes eventually hit a brick wall — the physical limitation of our bodies. A new study sheds light on exactly what this limit is: according to the findings, humans can only burn calories at 2.5 times their resting metabolic rate.

Limitless mind but limited gut

According to Herman Pontzer, an associate professor of evolutionary anthropology at Duke University, “this defines the realm of what’s possible for humans.” He adds that from his experience and studies of the world’s foremost athletes, no one has been able to cross this limit.

“So I guess it’s a challenge to elite endurance athletes,” Pontzer said. “Maybe someone will break through that ceiling someday and show us what we’re missing.”

To find the limit of human endurance, the researchers analyzed data from some of the toughest competitions in the world, such as the 2015 Race Across the USA, a grueling track stretching 3,000 miles from California to Washington, D.C. where competitors run six marathons per week for five months. They also looked at other feats of endurance, such as 100-mile trail races, Arctic expeditions, the Tour de France, and also pregnancy.

There’s a limit to how many calories our guts can absorb effectively per day, and this defines how much exertion humans are capable of. However, the researchers found that the mega-marathoners burned 600 fewer calories a day than expected. This suggests that their bodies lower their metabolic rate so that, at least for a time, they’re not using energy faster than it can be replenished — all in order to keep the athletes going. The number of calories the body requires at rest is known as the resting metabolic rate (RMR).

The researchers found that we can expend much more energy than our resting metabolic rate allows for. For example, the people this study focused on expended 15.6x times the energy while running for a marathon, 4.9x for a Tour de France, and 2.2x for a pregnancy. However, this regime can only be sustained for a short while.

In grueling physical activities that last for days, weeks, or months at a time, humans can only burn calories at 2.5 times their resting metabolic rate — only slightly more than women’s metabolic rates during pregnancy. So, the same limits that prevent Ironman triathletes from easily breaking speed records may prevent babies from growing too big in the womb.

“It’s a great example of constrained energy expenditure, where the body is limited in its ability to maintain extremely high levels of energy expenditure for an extended period of time,” Caitlin Thurber, co-author of the new study.

“You can sprint for 100 meters, but you can jog for miles, right? That’s also true here,” Pontzer said.

The reality is that after the body’s energy stores — excess muscle, fat, and glycogen — are depleted, all you’re left with is calorie intake from your diet. If you cross the 2.5x resting metabolic rate limit, the body will start to consume itself, and that’s simply not sustainable for endurance.

The findings were reported in the journal Science Advances.