The Limits Of Human Endurance

How do you study the upper limits of human endurance? Professor Herman Pontzer of Duke University tracked long-distance athletes running across the country. He talks with NPR's Lulu Garcia-Navarro.

LULU GARCIA-NAVARRO, HOST:

What is the limit of human endurance? For years, scientists have been attempting to understand how far we can push the human body. Herman Pontzer of Duke University is a professor of evolutionary anthropology, and he is one of those researchers. He joins us now from Durham, N.C.

Welcome to the program.

HERMAN PONTZER: Thanks for having me.

GARCIA-NAVARRO: So your study came out last week in the journal Science Advances. You studied athletes who basically ran a marathon a day for months. They got one day off a week to rest. First off, who are these people? (Laughter)

PONTZER: Yeah, they don't seem like normal people, do they? - to (laughter)...

GARCIA-NAVARRO: No.

PONTZER: ...Jump at an opportunity like this. So it was organized by a coauthor on the study, Bryce Carlson, who's done a lot of really amazing endurance events like this. And he helped organize this - yeah, this race across the USA that went from the Pacific Ocean down to Washington, D.C., over five months and asked if I wanted to tag along and asked if my lab wanted to measure energy expenditures of the racers. And of course we said sure. It sounded, you know, too good to pass up - this unique opportunity to get people pushing the limits.

GARCIA-NAVARRO: So what did you find out?

PONTZER: Well, you know, it's analogous to running track and field events. So you can - you know, if you sprint for a hundred meters, you can go very fast. If you have to run a mile, it's a bit slower. If you have to run a marathon, it's even slower still. And as you push yourself for longer and longer, the limits of what you're able to do go down and down. It's this sort of curved relationship between intensity and duration.

GARCIA-NAVARRO: What exactly determines our limits?

PONTZER: That limit to what we can do indefinitely, forever, is determined by how quickly we can get calories back in because we have to be able to replenish the calories we burn each day if we want to keep ourselves in energy balance. That energy-balance limit, which is the limit to what we can sustain forever, is set by how fast our guts can bring calories into our bodies.

GARCIA-NAVARRO: How did you collect the data?

PONTZER: So we used this really cool isotope-tracking technique. People drink some isotopically enriched water. It's totally safe. But what it allows us to do is calculate how much carbon dioxide your body produces. And that gives us a really precise measure of how many calories you're burning every day.

GARCIA-NAVARRO: The sample size, though, was quite small - just six people. How confident are you in the results?

PONTZER: Well, if it was just this one study with these racers across North America, then that's right, it would be too small of a study to say much. But what we did is we took this study of six racers, and we compared it to all the other work that's ever been done measuring energy expenditures in triathletes, hundred-mile ultramarathon runners, Tour de France cyclists, people who have trekked across Antarctica to reach the South Pole. And, you know, if you put all these studies together, you plot out this very crisp, clear boundary that is the boundary of human ability.

GARCIA-NAVARRO: And you had an interesting takeaway, and it had to do with pregnant women.

PONTZER: That's right. So when you're mapping out the longest duration events you could find that are the most intense, what you find is that pregnancy is the full end of the spectrum of what humans are capable of. So nine months long...

GARCIA-NAVARRO: Tell me about it.

PONTZER: Yeah - incredible...

GARCIA-NAVARRO: (Laughter).

PONTZER: ...Amounts of energy expenditure. I don't think any woman who's gone through pregnancy is surprised by this. It's pushing your body to the very limits of what you can do. It's the - you know, it is the ultra ultramarathon of human endurance - is pregnancy. And it plots right on that same boundary of human capability with Tour de France cyclists and Arctic trekkers and everybody else.

GARCIA-NAVARRO: Well, that is a study that I think many women could get behind. That was Dr. Herman Pontzer, associate professor of evolutionary anthropology at Duke University.

Thank you so much.

PONTZER: Thank you.

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