How far can the human body push itself? For most of us, it’s hard to imagine being able to handle anything more than running or swimming a few miles – if we had absolutely had to – but ultra athletes can perform shocking feats. To figure out the limit, scientists analyzed these intense athletes and measured their “resting metabolic rate,” which is the number of calories the body burns when it’s relaxing. The study began with athletes participating in Race Across The USA. Within 140 days, these runners covered over 3,000 miles. That’s six marathons a week.
Researchers measured the resting metabolic rate before and after the races. After collecting this data and data from other athletic events, scientists believe the cap is 2.5 times the body’s resting metabolic rate, or around 4,000 calories a day. If a person goes higher than that, they can’t keep it up. That doesn’t mean a person can’t get higher than 2.5. A runner doing just one marathon can use a huge 15.6 times their resting metabolic rate, while cyclists during the 23-day Tour de France used 4.9 times. However, these are short-term events. If these athletes wanted to keep it up for a long time, they would have to pace themselves down to that 2.5x rate.
Why just 2.5 times? Researchers actually think it’s because of the human digestive system, and not a person’s lungs, muscles, or heart. The body simply can’t process enough nutrients and calories to support a higher energy use, long-term. During short events, like a marathon, the body can start to rely on fat or muscle mass, and recover those afterwards. If it was forced to do that over a long period of time, it would start to shut down.
The most interesting finding from this study doesn’t have anything to do with athletes. Researchers learned that pregnant women actually live very close to the limit of human endurance. For anyone who’s been pregnant, this probably isn’t a surprise.