Ultimate limit of human endurance found

Scientists have defined the ultimate limit of human endurance by analyzing a 3000km race, the Tour de France and other elite events.

They showed that the ceiling is 2.5 times the resting metabolic rate of the body, or 4,000 calories per day for an average person.

Anything higher than that was not sustainable in the long run.

The research, conducted by Duke University, also showed that pregnant women were endurance specialists and lived almost to the limit of what the human body can bear.

The study began with the race across the United States, in which athletes traveled 3,080 miles from California to Washington in 140 days.

Competitors ran six marathons a week for months and scientists studied the effects on their bodies.

The resting metabolic rate – the calories that the body burns when it is relaxing – was recorded before and during the race.

And the calories burned during an extreme endurance test were recorded.

The study, published in Science Advances, showed that energy consumption was high, but that it had finally stabilized at 2.5 times the resting metabolic rate.

The study revealed a trend between the duration of a sporting event and energy expenditure; and that running a marathon can be beyond many, it is far from the limit of human endurance.

• Marathon runners (only that one) used 15.6 times their resting metabolic rate
• Cyclists in the 23 days of the Tour de France used 4.9 times their resting metabolic rate
• A 95-day Antarctic hiker used 3.5 times the resting metabolic rate

"You can do very intense things for a few days, but if you want to last longer, you have to remember them," said Dr. Herman Pontzer, of Duke University, at BBC News.

"Each data point, for each event, is mapped onto this magnificent barrier of human endurance.

"No one we know has ever managed to cross it."

The study showed that during pregnancy, women's energy consumption reached 2.2 times their resting metabolic rate.

The researchers say that the number 2.5 could be due to the human digestive system, rather than anything that affects the heart, lungs or muscles.

They discovered that the body can not digest, absorb and process enough calories and nutrients to maintain higher energy consumption.

The body can use its own resources by burning fat or muscle mass – which can be recovered later – during shorter events.

But in extreme events – at the limits of human exhaustion – the body must balance its energy consumption, say the researchers.

Dr. Pontzer said the results could eventually help the athletes.

"On the Tour de France, knowing where your ceiling is is where you can take a step back.

"Secondly, we talk about endurance for days, weeks, and months, so it's quite applicable to workout patterns and the question of whether they fit the metabolic limits to long-term body. "

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