What do extreme marathoners, mountain hikers and professional bicycle racers all have in common? They push their bodies to unsustainable (不可持续的) extremes of physical stress.
We tend to think there's no limit to human physical achievements. But a new study, published in Science Advances, says there is a definite limit to human endurance (耐力). Beyond that, our bodies begin to break down.
To find the limit, a US research team tracked marathon runners over a five-month period, measuring competitors' initial basal metabolic rates (BMR, 基础新陈代谢率) – the amount of energy they expend when they rest. Then they looked at how many calories (卡路里) each runner burned per day.
The team found that the maximum amount of energy a human can expend is 2.5 times of BMR. It equals 4,000 calories of energy per day. These calories can help a trained athlete to run about 42 kilometers in a day.
But people can't keep using this amount of calories all the time. "You can do really intense stuff for a couple of days, but if you want to last longer then you have to dial it back," US evolutionary anthropologist Herman Pontzer told the BBC.
Pontzer explained that the problem is our guts (消化道). "There's … a limit to how many calories our guts can effectively absorb per day," he said in the Daily Mail. At that point, the body is burning calories more quickly than it can absorb food and convert (转化) it into energy.
These new findings could help athletes to best work underneath this ceiling. For example, they could manage their daily exercise time based on their BMR, to get enough calories and keep going.
However, Pontzer's team can't rule out the possibility of someone breaking this limit. "So I guess it's a challenge to elite endurance athletes," said Pontzer. "Science works when you're proven wrong. Maybe someone will break through that ceiling some day and show us what we're missing."
