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At the upper limits of human endurance – think Ironman, ultra-marathons and the Tour de France – much is made of the power of the mind: conquer your thoughts, some say, and anything is possible. But new research has put pay to the idea that our endurance knows no bounds, with a study from Duke University suggesting that no matter what the activity, everyone hits the same metabolic limit: a maximum possible level of exertion that humans can sustain in the long term. Whether an athlete is competing for days, weeks or even months, researchers found humans can only burn calories at 2.5 times their resting metabolic rate. "There's just a limit to how many calories our guts can effectively absorb per day," said study co-author Herman Pontzer.

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