Which is the greater physical feat: Completing the Tour de France or carrying a baby to term?
The answer is likely both, according to a new paper published today in the journal Science Advances. By studying six runners who competed in a 14-week race in which participants run about a marathon per day six out of seven days a week, a research team led by a scientist at Hunter College found that there is an upper limit to the amount of energy that human bodies can expend consistently over time. This limit is consistent with other work that has been done on endurance athletes who compete in shorter competitions, and nearly the same as those of people who are pregnant and lactating. Together, these two factors suggest that there’s a ceiling to the amount of energy humans can expend for a period of time—and pregnancy pushes these limits, too.
For those of us who are not pregnant or elite athletes, our metabolism is fairly consistent. The number of calories we burn day to day may vary slightly depending on the workouts we do (or don’t), but evens out to a consistent, baseline rate.
That all changes in the world of intense athletic competitions, though. For example, over the course of an Ironman triathlon—which consists of a 2.4-mile swim, a 112-mile bicycle ride, and a marathon (26.2 miles)—an athlete will burn close to 10 times the number of calories she typically burns in a day. The next day as she’s recovering with her feet up, her metabolism will return to its normally level.
Yet if she’s competing in a way that requires her to push herself over many weeks or months, like running consecutive marathons, trekking across Antarctica, or competing in a multi-day cycling challenge, her metabolism will still be above her baseline rate, but will gradually slow down as it adapts to the exertion.
Caitlin Thurber, an anthropologist at Hunter College and lead author of the paper, measured the caloric output of six athletes as they completed in the 2015 Race Across the USA. She found that toward the end of the race, athletes were burning 600 fewer calories per day than expected—5,400 instead of 6,000. (Only three of the six original athletes the team studied completed the course; one became injured, and two others decided to take an even longer course across the country). Their metabolism slowed to about 2.5 times their basic metabolic rates.
“We were able to show that in the face of running a marathon a day, your body finds a way to save calories,” Herman Pontzer, an evolutionary anthropologist at Duke and co-author of the paper, said in an interview.
When the research team compared the number of calories burned from the Race Across the USA athletes to other endurance athletes whose caloric output had been measured in previous studies, they noticed a clear trend: the longer the endurance challenge, the fewer calories burned per day. The maximum amount any athlete burned petered out to be about 2.5 times their baseline rates—roughly what the runners burned in their last week of racing.
They also looked up data on the number of calories women burn while pregnant and lactating. A study from 2005 showed (paywall) they tend to burn roughly twice as many calories as normal.
This realization was an exciting moment for Pontzer and his team. Among all apes, humans have one of the most energy-taxing pregnancies due to a combination of the length of gestation and the size of our babies. We’re also the best endurance athletes, as measured by our distance running capabilities. Our species’ metabolic cap could be the reason for both, he explained.
Humans may have evolved to be capable of incredible endurance feats, which had the side effect of allowing us to have large babies. The reverse could also be true: evolving to have large babies gave us the capabilities to carry out extreme sports over long periods of time. “There’s no reason it can’t be both,” Pontzer says.
It would make sense, too, that people who are pregnant and lactating only burn about twice as much energy as they normally would, instead of the 2.5 times like the athletes did. Unlike athletes in endurance competitions, who are trying not to lose weight, people who are pregnant have to gain weight.
The hypothesis that we have metabolic limits that affect our athletic ability and pregnancy may be impossible to prove. Evolution is complex, those who are pregnant are often excluded from studies, and extreme endurance athletes are sparse among the general population. This study’s six participants and the handful of other athletes’ data included in the analysis is hardly enough to definitively label a trend.
That said, in addition to requiring a higher metabolic rate, bodies undergo hundreds of other unique changes as they carry a child to term. It’s a complex, biological function that has allowed our species to thrive for thousands of years. That in itself should make it one of the most awesome feats of humanity.