Mind & Body

Scientists Have Pinpointed the Energy Limit of the Human Body

If you've ever shopped for a new car, you've probably compared stats between models: How much fuel does it use? How fast can it go? How quickly can it accelerate? For decades, scientists have been asking the same things about the human body. Last month, scientists announced that they'd determined the human body's maximal energy expenditure — basically, the maximum mileage we can get out of the tank. The takeaway? Pregnancy isn't that different from an ultramarathon.

Get Your Motor Runnin'

Bodies aren't sports cars, so our land speed and fuel consumption stats are a little harder to come by. Scientists can get some of these answers by studying world-class athletes like Usain Bolt, who holds the human footspeed record of nearly 28 miles per hour (45 kilometers per hour). But so far, the human equivalent of miles-per-gallon has been tricky, since we use different amounts of energy depending on the task at hand.

For example, most people know that you burn more calories running than you do sitting at a desk. The rate at which your body uses energy at rest is called your basal metabolic rate (BMR), and it varies from person to person depending on their sex, weight, and other characteristics. The rate at which you use energy increases as your activity level increases. The ratio between your active metabolic rate at any given time and your BRM is known as your metabolic scope. The closer to 1 that number is, the closer you are to your resting rate; the higher that number, the more energy you're using. Humans generally max out at a metabolic scope of 5, but some species can get as high as 7.

For this study, which was published in the journal Science, the researchers wanted to find the human body's maximum sustained metabolic scope — in other words, the point where the body's rate of energy use over time outweighed its ability to absorb food and turn it into energy. To do that, they needed to find a group of humans who made superhuman demands of their metabolic systems. They found it in a 2015 endurance event called Race Across the USA: a 20-week, 3,080-mile (4,957-kilometer) race from Los Angeles, California to Washington D.C. Six runners agreed to be their human sports cars — er, guinea pigs.

Going the Distance

The team started by measuring the BMRs of all six runners, then had them drink doubly labeled water. That's H2O that's been replaced by harmless but rare isotopes of hydrogen and oxygen — specifically, deuterium and oxygen-18. As the isotopes came out in the runners' sweat, urine, and breath, the researchers could measure how much carbon dioxide they produced and, therefore, how many calories they were burning. The team made these measurements before the race began, during the first week, then during the final week.

What they found was that the runners' metabolic scope went from 1.8 before the race to 3.8 after a week of running. But by week 20, it had leveled off to 2.8. From the data, scientists could tell that the plateau was due to the runners' bodies simply using less energy. Some of that was due to them just losing weight and running fewer miles per day as the race wore on, but the remainder — about 600 calories a day — couldn't be explained by such obvious factors. Their bodies, it seemed, were adjusting to ensure they could cover the long road ahead. If they'd kept at their original energy usage, the researchers write, they would have petered out around 10 weeks. But they didn't.

The team analyzed the runners' data alongside data that had been collected from similarly long endurance events, including the Tour de France, triathlons, shorter ultramarathons, and Arctic expeditions. In all cases, the participants' metabolic scope started high, then plateaued after about 20 days to settle around 2.5. After that plateau, the human body has to turn to other sources of energy besides food — namely, its own fat stores.

But the big surprise happened when they compared these challenging athletic events to a more everyday challenge: pregnancy. While pregnant humans don't experience that initial spike in metabolic scope, it turns out that their bodies burn energy at about the same rate as an ultra-endurance athlete late in their event.

As Herman Pontzer, an evolutionary anthropologist at Duke University in Durham, North Carolina and corresponding author on the study, told Science, "To think about pregnancy in the same terms that we think about Tour de France cyclists and triathletes makes you realize how incredibly demanding pregnancy is on the body." For your next baby shower, you might want to bring some Gatorade.

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Dive deeper into the science of endurance in our interview with Alex Hutchinson on The Curiosity Podcast or in his book "Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance." We handpick reading recommendations we think you may like. If you choose to make a purchase, Curiosity will get a share of the sale.

Written by Ashley Hamer July 2, 2019

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