Scientists found that human's energy expenditure had a maximum possible level of exertion that can sustain in the long term, no matter what the activity is.

The study published on Wednesday in the journal Science Advances showed that humans can only burn at 2.5 times their rate of metabolism at rest or the basal metabolic rate, in the long run.

Not even the world's fastest marathoners could surpass that limit, according to the team leaders Herman Pontzer at Duke University, and John Speakman at University of Aberdeen and the Chinese Academy of Sciences.

The researchers measured daily calories burned by a group of athletes who ran six marathons a week for five months in a 4,800-kilometer race in the United States in 2015.

They found that the athletes' energy expenditure started out relatively high, but rapidly plunged and flattened out at 2.5 times their basal metabolic rate for the remainder of the event.

When they went beyond the threshold, the body starts to break down its own tissues to make up for the caloric deficit, according to the study.

The researchers said such a limit was due to the digestive tract's ability to break down food and absorb calories and nutrients to fuel bodily processes. It challenged the previous theory that human endurance is linked to the ability to regulate body temperature.

After 20 weeks of running back-to-back marathons, the athletes were burning 600 fewer calories per day than expected based on their mileage. It showed that the body can downshift its metabolism to help stay within sustainable levels.

Also, the researchers found the maximum sustainable energy consumption among those endurance athletes was only slightly higher than the metabolic rates women sustain during pregnancy. They suggested that the same body limits that keep athletes from breaking speed records were also constraining how big babies can grow in the womb. (Xinhua)