Researchers have found that no matter how much strength and grit the most intense ultra-athletes have, they ultimately cannot exceed the human body's 'metabolic ceiling' over a long period.
The metabolic ceiling refers to the maximum number of calories a body can burn at one time. Previous research has suggested that people can burn up to 10 times their 'basal metabolic rate' (BMR), the minimum amount of energy their body needs to function while resting.
However, this can only be maintained in short bursts.
"Every living thing has a metabolic ceiling, but exactly what that number is, and what constrains it, is the question," says anthropologist, endurance athlete and lead author of the study from Massachusetts College of Liberal Arts in the US, Andrew Best.
"To find out, we asked, 'if we get a group of really competitive ultra-athletes, can they break this proposed metabolic ceiling?'"
Best and the team recruited 14 ultra-marathon runners, cyclists and triathletes and tracked their activity over a 52-week period as the athletes trained and participated in competitions.
The athletes drank water containing deuterium and oxygen-18. These are naturally occurring isotopes of hydrogen and oxygen which contain additional neutrons.
When the researchers measured these isotopes in urine samples, they were able to calculate the calories burned and the amount of carbon dioxide exhaled.
They found that while the athletes burned 6 to 7 times their BMR during multi-race days, this level was not sustained over longer periods. Over longer periods, 30 and 52 weeks, the athletes' burn rates dropped down to an average of 2.4 times their respective BMRs.
"If you go over the ceiling for short periods, that's fine. You can make up for it later," says Best.
"But long term, it's unsustainable because your body will start to break down its tissue, and you'll shrink."
These findings demonstrate the limits of human biology and that even the most dedicated athletes have a metabolic ceiling that is exceedingly difficult to overcome.
The researchers also found "tentative evidence" that as ultra-running athletes began to increase the energy expended running, they unconsciously reduced the amount energy devoted towards other non-running activities.
"Your brain has a really powerful influence on how much you fidget, how much you want to move, and how encouraged you are to take a nap," says Best.
"All these fatigues we feel save calories."
However, the researchers stress that most people won't come anywhere near reaching this limit.
"For most of us, we're never going to reach this metabolic ceiling," says Best.
"It takes running about 11 miles [17.7km] on average a day for a year to achieve 2.5 times BMR. Most people, including me, would get injured before any sort of energetic limit comes into play."
While the authors acknowledge that their limited sample size may have missed individuals who are capable of breaking the metabolic ceiling, they hope further research will continue to investigate how this metabolic ceiling can shape other essential functions.