Is There A Limit to Human Metabolism? What New Research Suggests

A recent study published in Current Biology has provided fresh insights into the limits of human metabolism. Researchers tracked ultramarathon runners over a year, revealing that while athletes can burn up to 11,000 calories per day during extreme endurance events, this level of energy expenditure is not sustainable over time.

The study, titled Ultra-Endurance Athletes and the Metabolic Ceiling, confirms the existence of a physiological limit to human metabolism. It reveals that even the most elite athletes cannot sustain extremely high energy output over long periods without facing physical strain and performance decline. The findings highlight a metabolic ceiling - a point beyond which the body cannot maintain prolonged exertion without consequences, reinforcing the natural limits of human endurance.

The study found that over long periods, around 30 weeks or more, the human body can only sustain energy use up to about 2.4 times its basal metabolic rate (BMR). BMR is the minimum energy needed daily for basic functions like breathing and staying alive.  

While people can temporarily burn up to 10 times their BMR during intense activity, the body can't keep that up for long. Researchers had earlier suggested that the long-term metabolic limit was around 2.5 times BMR, but this is the first time it has been clearly tested, says researcher Andrew Best.

"I get the sense a lot of people were surprised" by the findings, study co-author Andrew Best, a biological anthropologist at Massachusetts College of Liberal Arts, who is himself a marathon runner told Scientific American. "I was surprised, too.... We had a guy who ran 4,500 miles a year on trails-pretty rugged stuff-and he was well under the limit."

Researchers track real-time calorie burn in elite athletes using innovative water-based method

According to the science journal Nature, Andrew Best and his team studied 14 elite endurance athletes (12 male and two female), including ultra-runners, cyclists, and triathletes. The participants drank special water containing harmless heavy forms of hydrogen (deuterium) and oxygen (oxygen-18). These elements leave the body through sweat, urine, and breath. By analyzing how much of these substances came out in urine - especially oxygen-18 lost through carbon dioxide - the researchers could accurately estimate how many calories each athlete burned. 

This method allowed scientists to measure energy use in real-life competitions instead of relying on lab tests, said Best.

The research brings scientists a step closer to understanding how much energy the human body can process, shedding light on the boundaries of endurance and performance.