This Is the Point at Which Human Endurance Tops Out

Scientists analyzed some of the most brutal tests of endurance that humans put themselves through to figure out that the average person can’t go on past burning 4,000 calories in a day, or the equivalent of 2.5 times the body’s resting metabolic rate.

The Duke University researchers who conducted the study monitored people who competed in the Race Across the USA, a 3,080 mile run from California to Washington D.C. in 140 days that requires competitors to complete six marathons per week for months on end.

The resting metabolic rate — the number of calories the body burns at rest — of the racers was recorded both before and after running and then compared against each other, the BBC reported. The researchers found that there was a pattern between the amount of time spent racing and the energy used: the longer the event, the harder it becomes to burn through calories.

Put simply, this means that people can go far beyond what their bodies are capable of in the short-term but that level becomes unsustainable fairly fast.

“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,” Dr Herman Pontzer from Duke University told the BBC.

Interestingly, however, the study found that while most people would consider running a marathon outside their own realms of possibility, it’s actually nowhere near where we totally crap out. This is likely because of our digestive systems, not the strength of our hearts or lungs like many may have assumed. The body cannot digest, absorb and process enough calories and nutrients to sustain competition at that high a level for a prolonged period of time.

Fun fact: The researchers noted that women basically live at the ultimate top of what the human body can endure during pregnancy.

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