Not even the world’s fasting ultra-marathoners could surpass that limit.
"This defines the realm of what's possible for humans," said study co-author Herman Pontzer. He is an associate professor of evolutionary anthropology at Duke University.
According to the research, the body starts to break down its own tissues to make up for the caloric deficit beyond the threshold of 2.5 times a person's resting metabolic rate.
Team leaders Pontzer and John Speakman of Scotland's University of Aberdeen and the Chinese Academy of Sciences, think the limit may be explained by the digestive tract's ability to break down food.
In other words, eating more won't necessarily help someone make Iditarod history. "There's just a limit to how many calories our guts can effectively absorb per day," Pontzer said.
For the research, the team measured daily calories burned by a group of athletes who ran six marathons a week for five months as part of the 2015 Race Across the USA, a 3,000-mile race from California to Washington, D.C. Other feats of human endurance, including punishing 100-mile trail races and pregnancy were also factored in.
The data, over time, made an L-shaped curve.
The athletes were burning 600 fewer calories a day than expected based on their mileage after 20 weeks of running back-to-back marathons. The research found that the body "downshift" its metabolism to help stay within sustainable levels.
"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," said Caitlin Thurber.
"You can sprint for 100 meters, but you can jog for miles, right? That's also true here," Pontzer said.