There is a limit to human endurance and it is determined by the gut, not the mind or muscles, according to scientists.
US and Scottish researchers studying humans involved in the most demanding physical feats, from running six marathons a week to pregnancy, have shown that the number of calories the body can absorb a day determine the upper limit of human performance.
The threshold amounts to 2.5 times above the level they would use energy if they were at rest; beyond this limit, the body begins to break down fat, muscle, and connective tissue to make up for the shortfall in calories.
“This defines the realm of what’s possible for humans,” said study co-author Dr. Herman Pontzer, an evolutionary anthropologist at Duke University, North Carolina. “There’s just a limit to how many calories our guts can effectively absorb per day.”
The team compiled new data on energy use from the 3,080-mile, five-month Race Across the USA. The researchers found that athletes’ energy expenditure started off high, before plunging to flatten out at this 2.5-times limit.
Urine samples were used to gauge calorie consumption at the start of the race in California, and after 20 weeks of back-to-back marathons nearing Washington DC.
They found competitors were burning 600 fewer calories a day that would be predicted by their initial pace.
If they had carried on at their original rate of energy expenditure the participants would not have been able to complete more than half the race.
While this was the longest event where metabolic data has been evaluated, researchers found cyclists in the summer Tour de France, 100-mile marathon runners, and explorers crossing Antarctica showed a similar “downshift” in energy expenditure to the 2.5-times base-rate ceiling.
There was no apparent temperature effect, and researchers said this was “unexpected” as previous theories had suggested that the body’s maximum energy consumption is set by its ability to dissipate heat.
The research also found pregnant mothers’ bodies burnt calories at close to 2.5-times energy-expenditure limit seen in endurance athletes, suggesting the endurance threshold applies independently of the muscles or organs involved.
“Humans have evolved greater endurance capabilities than other apes, which has typically been ascribed to selection for increased physical activity, particularly long-distance running,” the authors wrote in the journal Science Advances.
But raising this endurance threshold might have also had advantages in allowing longer pregnancies, or larger – more energy intensive – brains than our primate ancestors.