When your coach tells you “run until you can’t run anymore,” it’s now more than just a vague motivational phrase. Scientists have actually pinpointed the limit of human endurance, and no matter how hard you persevere, there’s pretty much no surpassing it. The research, conducted by Duke University, showed that the endurance cap for the average person is 2.5 times the body’s resting metabolic rate, or 4,000 calories a day. The study analyzed Race Across the USA, where athletes ran 3,080 miles from California to Washington, DC. Along the way, scientists investigated the effect of the run on their bodies.
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Scientists Have Found the Limit of Human Endurance
The study uncovered an important pattern between the length of a sporting event and the amount of energy expended. The longer the event lasted, the harder it became to burn through calories, meaning you can safely surpass your base metabolic rate during short periods of exercise, but that level of exertion is ultimately unsustainable.
Dr. Herman Pontzer of Duke University told the BBC, “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. Every data point, for every event, is all mapped onto this beautifully crisp barrier of human endurance. Nobody we know has ever pushed through it.”
One of the reasons for this endurance limit is because the body simply can’t absorb, digest, and process enough calories and nutrients to sustain more energy use. It can burn up its own resources for shorter events, but longer events require a balancing of energy use, and this is where the limits of human endurance are typically reached. 
H/T: BBC