The body can take advantage of other energy stores initially, giving that initial burst of calories burned, but once you burn up all the fat and excess muscle in the body, all that's left is caloric intake, which is ultimately the limit of what the human body can do, energy-wise.
The research "defines the realm of what is possible for humans", according to Pontzer.
A new report from BBC News details the work of scientists to study some of the most extreme athletes in the world to determine what, if any, limit there was on human endurance and reveals that they did in fact conclude that there is an ultimate limit to what the human body could do.
"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", Thurber said.
When tracked over time, the data revealed that although athletes' energy expenditure started out high, it eventually plunged and leveled out to burn calories at 2.5 times their resting metabolic rate.
"You can do really intense stuff for a couple of days", Duke University's Herman Pontzer told BBC News, "but if you want to last longer then you have to dial it back". That's why just eating more won't improve endurance.
For the study, the researchers measured daily calories burned by athletes who ran six marathons a week for five months as part of the 2015 Race Across the US, which stretched 3,000 miles from California to Washington, D.C. They also looked at other feats of endurance, such as 100-mile trail races and pregnancy.
"There's just a limit to how many calories our guts can effectively absorb per day", Pontzer explained in an article outlining the research. After 20 weeks of running back-to-back marathons, the athletes were burning 600 fewer calories a day than expected based on their mileage. "That's also true here", Pontzer said. That finding challenges the idea, proposed by previous researchers, that human endurance is linked to the ability to regulate body temperature.
No one has ever sustained levels above this limit, Pontzer said.
They also found that the maximum sustainable energy expenditure in endurance athletes was only slightly more than women's metabolic rates during pregnancy. This suggests that the body can power down its metabolism to keep the body going.
So the same physiological limits that keep Ironman triathletes from shattering speed records, for example, may also constrict how big babies can grow in the womb.
The study, published in Science Advances, found that the calories burned started high before falling over time and leveling off around 2.5 times the competitor's RMR. "Maybe someone will break through that ceiling some day and show us what we're missing".