From the famous Ironman to the Tour de France, through to the Marathon of the Sands (250 km through the Moroccan desert) or the Transpyrenean, all of them take the resistance of the most tanned athletes to the limit. But what is that limit? A new study on the energy expenditure of some of the longest and most exhausting sporting events in the world suggests that no matter what the activity, they all reach the same metabolic limit, a maximum possible level of effort that humans can withstand in the long term.
The researchers, led by Herman Pontzer, discovered that when it comes to physical activities that last for days, weeks and months, humans can only burn calories at 2.5 times their resting metabolic rate. Not even the fastest ultramarathoners in the world (athletes specialized in races over 100 km) were able to exceed that limit.
“This defines the scope of what is possible for humans,” Pontzer explains in a statement.
According to the results, published in Science Advances, beyond the threshold of 2.5 times the resting metabolic rate of a person, the body begins to break down its own tissues to compensate for the caloric deficit. An explanation for this limit may be the ability of the digestive tract to break down food, the authors point out. In other words, eating more will not necessarily help someone make history in endurance events and that “there is a limit to the amount of calories we can absorb per day,” adds Pontzer.
To reach this result, the Pontzer team measured the daily calories burned and analyzed the urine of a group of athletes who ran six marathons a week for five months as part of the Race Across USA, a race of more than 4,500 kilometers from California to Washington. The team also analyzed other events with similar requirements.
Pontzer’s team concludes that the determining factor for resistance events lies in the digestive process, that is, the body’s ability to process food and absorb calories and nutrients to feed body processes. “I guess it’s a barrier, a limit for elite endurance athletes,” adds Pontzer. “Science works when it shows you are wrong. Maybe someone will break that roof one day and show us what we are wrong about.”
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