Pregnancy pushes a woman's body to the limits nearly as much as extreme endurance sports, research finds A new study of energy expenditure suggests that everyone has a limit People can only burn calories at 2 and a half times their resting metabolic rate Not even the fastest ultra-runners managed to surpass that limit, they found The body starts to break down its own tissues to make up for the calorie deficitBy Victoria Bell For Mailonline Published: 14:20 BST, 6 June 2019 | Updated: 11:43 BST, 7 June 2019 93 shares 58 Viewcomments All humans have a limit to their endurance as they can only burn calories at two-and-a-half times their resting metabolic rate, a study on energy expenditure has found. The research suggests that not even the world's fastest ultra-marathoners managed to surpass that limit. They suggest that their findings define the realm of what's possible for humans and that no matter what the activity - everyone hits the same metabolic limit. This is the maximum possible level of exertion that humans can sustain in the long term. Beyond that threshold of 2.5 times a person's metabolic rate, they found that the body starts to break down its own tissues to make up for the calorie deficit. Scroll down for video All humans have a limit to their endurance as they can only burn calories at two-and-a-half times their resting metabolic rate, a study on energy expenditure has found. The research suggests that not even the world's fastest ultra-marathoners managed to surpass that limit HOW DID THEY MAKE THESE FINDINGS? They compiled measurements of average energy expenditure and metabolic rate for human endurance events, including marathons, the Tour de France, swimming, arctic trekking, and pregnancy. By looking at the data over time and found an L-shaped curve. The athletes energy started out relatively high and then petered out and flattened out at 2.5 times their basal metabolic rate for the rest of the event. After analysing urine samples collected during the first and final legs of Race Across the USA they found that after twenty weeks of running back-to-back marathons, the athletes were burning 600 fewer calories a day than expected based on their mileage. This suggest that the body can 'downshift' its metabolism to help stay within sustainable levels. Eating more will not boost your performance because the limit may be down to the digestive tract's ability to break down food, the study claims. Co-author Associate Professor of evolutionary anthropology Herman Pontzer at Duke University in North Carolina said: 'There's just a limit to how many calories our guts can effectively absorb per day.' The findings suggest activities involving different muscle groups and organ systems are all united by the same energy intake controls. They compiled measurements of average energy expenditure and metabolic rate for human endurance events, including marathons, the Tour de France, swimming, arctic trekking, and pregnancy. By looking at the data over time and found an L-shaped curve. The athletes energy started out relatively high and then petered out and flattened out at 2.5 times their basal metabolic rate for the rest of the event. After analysing urine samples collected during the first and final legs of Race Across the USA they found that after twenty weeks of running back-to-back marathons, the athletes were burning 600 fewer calories a day than expected based on their mileage. This suggest that the body can 'downshift' its metabolism to help stay within sustainable levels. Beyond that threshold of 2.5 times a person's metabolic rate, they found that the body starts to break down its own tissues to make up for the calorie deficit. Pictured, the team measuring the resting energy expenditure for a Race Across the USA runner Co-author Caitlin Thurber said: 'It's 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.' 'You can sprint for 100 meters, but you can jog for miles, right? That's also true here.' All the endurance events followed the same L-shaped curve, whether the athletes were hauling 500-pound sleds across Antarctica or cycling the Tour de France. This finding challenges the idea, proposed by previous researchers, that human endurance is linked to the ability to regulate body temperature as these Antarctica is freezing and the Tour de France takes place in summer. The maximum sustainable energy expenditure found among endurance athletes was only slightly higher than the metabolic rates women sustain during pregnancy. This suggests the same physiological limits that keep Ironman triathletes from shattering speed records may also constrain other aspects of life too, such as how big babies can grow in the womb. While there is considerable research interest in the physiological limits on energy expenditure for humans, studies that have explored this vary in their results, and few studies have explicitly explored how energy expenditure varies with event duration. As far as the researchers know, no one's ever sustained levels beyond this limit. Professor Pontzer said: 'So I guess it's a challenge to elite endurance athletes. 'Science works when you're proven wrong. Maybe someone will break through that ceiling some day and show us what we're missing.' The study was published in the journal Science Advances. Read more: Share or comment on this article: All rights reserved for this news site dailymail and under his responsibility