A team of evolutionary anthropologists led by Herman Pontzer from Duke University (USA) was able to determine the limit of a person's physical endurance, after which the body begins to draw energy from its fat reserves.
According to Science, the study was based on measuring the BMR (basal metabolic rate). This is the main indicator of the body's metabolism: the amount of energy expended by a person at rest.
Pontzer's team resorted to the help of athletes. Back in 2015, there was a race across the USA. Five men and one woman took part in marathon races for 20 weeks, covering a total of 4957 kilometers.
The researchers measured all six runners' initial BMR score. Subsequently, their physical condition was monitored using the analysis of isotopes of deuterium and oxygen-10, which replaced ordinary hydrogen and oxygen in the drinking water of the marathon runners.
The study showed that energy consumption after about 20 days of continuous exercise began to decline and reached 2.5 BMR. Previous studies have shown that with intense physical activity for several hours, the BMR increases up to five times, after which it begins to level out.
It has been established that with prolonged exercise, when the metabolic rate drops to 2.5 BMR, a key moment comes: the body begins to burn calories faster than it can get energy from food. It is then that the biologically established limit of human endurance comes, the authors of the work believe.
Once the athlete reaches this "ceiling", his body begins to draw energy from its fat stores. Otherwise, a person will not be able to withstand the new load.
Researchers note that many athletes are aware of this feature of the body. Therefore, before competitions with prolonged physical activity, where very high endurance is required, they begin to accumulate fat reserves.
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