Science finally finds human endurance limit, even the world's fastest ultra-marathoners can't surpass it

Humans can only burn calories at 2.5 times their resting metabolic rate. Beyond that, the body starts to break down its own tissues to make up for the caloric deficit


                            Science finally finds human endurance limit, even the world's fastest ultra-marathoners can't surpass it

Is there a limit to human endurance? Science has finally answered the question and has also put a cap on it. Research from Duke University now reveals that when it comes to physical activities, which lasts for days, weeks, or months, humans can only burn calories at 2.5 times their resting metabolic rate. Not even the world's fastest ultra-marathoners managed to surpass that limit, show the findings.

"This defines the realm of what's possible for humans," said study co-author Herman Pontzer, an associate professor of evolutionary anthropology at Duke University, in a university release. From the Ironman triathlon to the Tour de France, some competitions test the limits of even the toughest endurance athletes. The results of the study on energy expenditure during some of the world's longest, most grueling sporting events show that no matter what the activity, everyone hits the same metabolic limit -- a maximum possible level of exertion that humans can sustain in the long term.

Humans can only burn calories at 2.5 times their resting metabolic rate, beyond that, the body starts to break down its own tissues to make up for the caloric deficit, says the study. (Source: Getty Images)

While it is often said the breaking point is in one's head, the study indicates that it could be in one's gut. The paper, published in 'Science Advances' reveals that beyond the threshold of 2.5 times a person's resting metabolic rate, the body starts to break down its own tissues to make up for the caloric deficit.

According to team leaders, Pontzer and John Speakman of Scotland's University of Aberdeen and the Chinese Academy of Sciences, the explanation for this limit could be attributed to the digestive tract's ability to break down food. What this means is that eating more may not necessarily help as there is simply a limit to how many calories the gut can absorb effectively daily.

For the study, the team measured daily calories burned by a group of athletes who ran six marathons a week for five months as part of the 2015 Race Across the USA, a 3,000-mile race from California to Washington, D.C. The team also considered other feats/events of human endurance, which include 100-mile trail races as well as pregnancy.

"The limits on maximum sustained energy expenditure are unclear but are of interest because they constrain reproduction, thermoregulation, and physical activity. Here, we show that sustained expenditure in humans, measured as maximum sustained metabolic scope, is a function of event duration. We compiled measurements of total energy expenditure and basal metabolic rate (BMR) from human endurance events and added new data from adults running approximately 250 km/week for 20 weeks in a transcontinental race. For events lasting 0.5 to 250-plus days, the maximum sustained metabolic scope decreases curvilinearly with an event duration, plateauing below 3× BMR. This relationship differs from that of shorter events (for example, marathons). Incorporating data from overfeeding studies, we find evidence for an alimentary energy supply limit in humans of approximately 2.5× BMR. Greater expenditure requires drawing down the body's energy stores. Transcontinental race data suggest that humans can partially reduce total energy expenditure during long events to extend endurance," says the paper.

Explaining these findings, the researchers said in the release that when they plotted the data over time, they found an L-shaped curve. The athletes' energy expenditure started out relatively high, but inevitably plunged and flattened out at 2.5 times their basal metabolic rate for the remainder of the event.

Co-author Caitlin Thurber analyzed the urine samples collected during the first and final legs of Race Across the USA. Results reveal that after 20 weeks of running back-to-back marathons, the athletes were burning 600 fewer calories a day than expected based on their mileage. The findings suggest that the body can "downshift" its metabolism to help stay within sustainable levels.

"It is a great example of constrained energy expenditure, where the body is limited in its ability to maintain extremely high levels of energy expenditure for an extended period of time," Thurber said in the release.

All the endurance events followed the same L-shaped curve, whether the athletes were hauling 500-pound sleds across Antarctica for days in sub-freezing temperatures, or cycling the Tour de France in summer. That finding challenges the idea, proposed by previous researchers, that human endurance is linked to the ability to regulate body temperature.

"One limiting factor for endurance events, researchers found, lies in the digestive process, the body's ability to process food and absorb calories and nutrients to fuel bodily processes. What is interesting is that the maximum sustainable energy expenditure found among endurance athletes was only slightly higher than the metabolic rates women sustain during pregnancy. This suggests that the same physiological limits that keep, say, Ironman triathletes from shattering speed records may also constrain other aspects of life too, such as how big babies can grow in the womb," the release added.

According to the research team, no one has ever sustained levels that are beyond the threshold limit. "So I guess it is a challenge to elite endurance athletes. Science works when you are proven wrong. Maybe someone will break through that ceiling some day and show us what we are missing," said Pontzer.