Scientists have discovered the ultimate limit of human endurance

Scientists at Duke University have worked out the ultimate limit of human endurance by analysing a range of extreme endurance events.

The study found, which was published in Science Advances, that the limit of energy expenditure for human beings was two and a half times the body's resting metabolic rate (the rate at which the body burns through calories when it is relaxing). In other words, 4,000 calories a day for an average human being. Anything more than that would not be sustainable in the long term.

As part of the study, researchers analysed the Race Across the USA, a 3,000-mile run from California to Washington DC, where participants ran six marathons a week for months. Researchers recorded resting metabolic rate before and during the race, as well as calories burned.

They found that energy use starts off high but eventually leveled off at 2.5 times the resting metabolic rate. They also found a pattern between the length of a sporting event and energy expenditure. For example, the longer the event, the harder it is to burn through the calories. For example:
  • Marathon (just the one) runners used 15.6 times their resting metabolic rate
  • Cyclists during the 23 days of the Tour de France used 4.9 times their resting metabolic rate
  • A 95-day Antarctic trekker used 3.5 times the resting metabolic rate
According to the study, the body can use up its own resources burning through fat or muscle mass - which can be recovered afterwards - in shorter events. However, in extreme events - at the limits of human exhaustion - the body has to balance its energy use.

Interestingly, the study also found that pregnant women live at nearly the limit of what the human body can cope with. During pregnancy, a woman's energy use peaks at 2.2 times their resting metabolic rate.

Speaking to BBC News, the study's co-author Dr Herman Pontzer, from Duke University said that no one, as far as their research has shown, has ever sustained levels beyond the limit discovered in the study. “This defines the realm of what’s possible for humans. So I guess it’s a challenge to elite endurance athletes.Science works when you’re proven wrong. Maybe someone will break through that ceiling some day and show us what we’re missing,” he said.

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