No matter how hard an athlete trains or whatever the activity is, there's a maximum possible level of exertion that a human being can sustain.

Humans can only burn calories at a rate 2.5 times of their resting metabolic rate, according to new research. Even the world's most elite ultra-marathoners will be unable to exceed this threshold without the body breaking down its tissues.

Testing The Limits

The study published in the journal Science Advances tracked the daily calories burned by athletes who ran six marathons a week for five months. These athletes are part of the 2015 Race Across the USA, which takes them 3,000 miles from California to Washington D.C.

Additionally, the researchers also considered other examples of human endurance, such as 100-mile trail races and pregnancy.

Upon plotting the data, the team found that it followed an L-shaped pattern with the athletes' energy expenditure starting out relatively high. However, over time, it inevitably drops and remains steady at 2.5 times their resting metabolic rate for the rest of the event.

In an analysis of urine samples taken at the first and final legs of the race, the athletes were found to burn 600 fewer calories than what's expected from their mileage.

According to the study authors, these findings suggest that the body can actually automatically "downshift" its metabolism so that it stays 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," explained study co-author Caitlin Thurber in a news release from Duke University.

New Insights About Human Endurance

The study authors suggested that one of the possible reasons for this limit is the digestive tract's own ability to break down food, which means eating more won't necessarily help people push past the limits.

After all, as study co-author Herman Pontzer of Duke University pointed out, there's a limit to the amount of calories the human gut can absorb every day.

Interestingly, all the subjects from the various endurance events featured the same L-shaped curve, from dragging 500-pound sleds through sub-freezing Antarctica to biking in the summer for Tour de France. It challenges findings from previous studies that link endurance to regulation of temperature.

Furthermore, the maximum sustainable energy expenditure among endurance athletes are only a little bit higher than the metabolic rates sustained during pregnancy. It's possible that the physiological qualities limiting endurance athletes are the same qualities that limit other aspects, such as the size of babies in the womb.