There’s a limit to the total energy output we can sustain during high-energy activities. This applies to everything from exploration to growing babies and is determined not by our muscles and lungs but by how much energy we can get from eating.
According to a study looking at data on ultra-athletes, explorers and pregnant women, over a period of several months even the fittest people can only expend two and a half times as much energy as they use when resting. Above this limit they lose weight.
“I think this is a pretty hard limit,” says Herman Pontzer of Duke University in North Carolina. “All the training people do is to push up against this limit.”
His team measured the total energy used by runners in the Race Across The USA in 2015, a 5000-kilometre race lasting 140 days. They then compared their results to those from other energy-intensive activities including trekking, cycling and pregnancy.
Because bigger people use more energy, the team divided the total energy used by each individual by their resting energy consumption to eliminate differences due to body size.
In relatively short events such as a marathon, people’s total use can be nearly 20 times higher than their resting energy use. But energy output falls sharply over longer periods. In 25-hour ultramarathons it is around 9 times. For 10-day treks it’s around 7. For the 23-day Tour de France cycle race it’s 5.
The team conclude that this curve flattens out at 2.5 times resting energy use over longer periods of several hundred days. People can’t sustain energy outputs higher than this without losing weight.
“That crossover point, where energy expenditure is finally low enough that you are able to replenish it every day is actually just a little bit above what women are experiencing in pregnancy,” says Pontzer. “In pregnancy, you have to be able gain weight, so you have to be below that ceiling, but that ceiling is not much above what women do in pregnancy.”
Surprisingly, the team think this limit exists because our bodies can’t get any more energy from food. Extracting energy from food requires chewing it, breaking it down, absorbing it through the gut, metabolising it in the liver and so on, and the body’s capacity to do this is limited.
Over short periods we can exceed this “alimentary limit” by using stored food reserves, but over longer periods there is no way round it. People taking part in ultra-endurance events struggle to maintain their weight, Pontzer says. “People who are trying very hard not to lose weight and are packing themselves full of high-energy food, they still lose weight.”
The team might well be right, says Klaas Westerterp of Maastricht University in the Netherlands, who studies human energetics. He has found that endurance athletes given easy-to-digest liquid foods do better.
Ultra athletes are sometimes put on IVF drips to bypass the gut, Westerterp says. “What you see in the hotels is incredible.”
Some animals such as migratory birds might be able to sustain much higher energy outputs than humans. But humans have far greater endurance than our closest ape relatives, and not just in activities like running. Women have bigger babies than apes and can have them more often.
“Pregnancy is the longest, most difficult thing that humans can do,” Pontzer says “It’s on the same boundaries of human ability as the Tour de France.”
It’s possible that natural selection for investing more in pregnancy led to us evolving the greater endurance that enabled us to become runners, rather than vice versa, he says.
Journal reference: Science Advances, DOI: 10.1126/sciadv.aaw0341