How Far Is Too Far for Ultra-Endurance Athletes? This Study of Metabolism Found Out

Ultra-endurance athletes can push their bodies to the limits, whether on a racecourse or bike path or in a swimming pool. But where does that limit end?

To find out, a group of ultramarathoners, cyclists and triathletes joined anthropologists to test the limits of human energy expenditure, burning thousands of calories a day. Most ran an average of 4,000 miles over the course of one year.

The results were published October 20 in the journal Cell Biology—and suggest that the human body can only sustainably burn about two-and-a-half times the calories it requires at rest.

“What are the ultimate limits to human physical performance when the factors that limit most of us are removed?” study lead author Andrew Best, an anthropologist at the Massachusetts College of Liberal Arts, says to Laura Sanders at Science News. He teamed up with other anthropologists and a group of 14 ultra-endurance athletes to determine the maximum limit of calories a body can burn before giving up, a threshold known as the metabolic ceiling.

Though most humans will never reach that limit, Best adds in a statement, he thought a group of “really competitive ultra-athletes” might be able to help define it, or even break it.

The athletes worked hard during the study year, running thousands of miles and enduring thousands of hours of training. Over short periods of a few intense days, they used up to seven times their basal metabolic rate (BMR), the amount of energy the body requires while at rest. Some ultramarathoners burned 11,000 calories in a single day.

But over long periods of time, athletes studied could not use up more energy than 2.5 times their BMR. When averaged over 30 weeks and over the entire year, the athletes’ energy use did not exceed that metabolic limit.

“If energy expenditure exceeds 2.5 times BMR for a prolonged period, the energy intake of athletes wouldn’t be able to keep up, and they would have to start using body stores, which could include muscle breakdown,” says Nigel Turner, a metabolic researcher at the University of New South Wales Sydney in Australia, to Nature’s Mohana Basu. Turner was not involved in the study.

The researchers also found that as athletes neared this ceiling, they subconsciously limited energy usage elsewhere, by reducing activities such as fidgeting or walking.

Researchers used water containing elevated levels of hydrogen and oxygen isotopes to track the athletes’ energy expenditure. They deduced the amount of carbon dioxide the athletes exhaled—and therefore how much energy they were using—by measuring the levels of the hydrogen and oxygen isotopes in athletes’ urine and subtracting it from the amount they had consumed.

This method has recently become affordable, Best tells Nature, and the authors write in the study that this research is the first to measure metabolic limits using this method. The study is also the longest of its kind, tracking athletes for an entire year.

“We’re starting to get a more complete picture of what the requirements are for these long, arduous work bouts,” says Andrew Creer, an exercise physiologist at Utah Valley University who was not involved in the study, to Science News.

With just 14 participants, the study’s sample was relatively small and mostly male, and authors note that some factors, such as athletes’ activity outside of the tracked exercise, are difficult to control for. It’s also possible, they write, that some outliers may be able to operate beyond this metabolic limit.

Bryce Carlson, an anthropologist and ultra-endurance athlete not involved with the study, tells Scientific American’s Jason Dinh that some athletes could possibly surpass the body’s natural metabolic ceiling by sidestepping limits to nutrient absorption, especially with a recent surge in sports nutrition options.

“Is it an absolute human limit that we will never be able to surpass?” Carlson asks the outlet. “Or is it a historically contingent limit that we just haven’t passed yet?”

An average person would probably run into other restrictions, like injury or fatigue, long before they get close to the metabolic limit. But by studying those who can reach the upper limits of energy use, scientists can understand “the capabilities and limitations of human physiology” more broadly, the authors write in the study.

“We’re studying the Ferraris to learn about the Hondas,” Best tells Science News.