The Limits of Human Endurance

Norbert, what'd you think of the people who ran three thousand miles across the country to raise money for childhood obesity? Remember that? It was in two thousand fifteen. Yeah, I remember and I thought kudos to them but it just sounded nuts. I mean can the human body take that? Good question, scientists actually studied these ultra-runners to see if there is a measurable upper limit to human endurance.

The race, called Race across America, took place over twenty weeks from LA to Washington D-C. Researchers measured five men and one woman by giving them water containing stable isotopes of hydrogen and oxygen. It allowed them to measure these isotopes in the runners' sweat, urine, and breath in order to find the amount of carbon dioxide they produced. Carbon dioxide levels are directly related to how many calories they've used. By the end of the race, the data clearly showed that there IS an upper limit to human endurance.

This can be described as the amount of energy used when exercising compared to what's used at rest which is called the basal metabolic rate or BMR. So, for short periods of exercise, human endurance is limited to five times a person's BMR. But for long endurance sports, the study found in all cases, the runners' energy expenditure leveled off at two and a half times the BMR. At this point, the body cannot convert food to energy fast enough and begins to use stored fat.

So, one lesson from this study is that athletes preparing for long endurance events eat more to store fat. Fascinatingly, pregnancy's metabolic toll is similar to an ultramarathoner's! My wife will have one more reason to make me thank her!