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A conversation on the limits of human endurance

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Just how far can the human body go, and is there a limit on what we’re capable of enduring? These questions have fueled scientists’ imaginations for centuries. Now we may have a clearer answer.

In early June, a team of researchers published a paper in the academic journal Science Advances that suggested the limit of human endurance is burning energy at 2.5 times your basal (resting) metabolic rate.

In other words, the point at which your body is burning more calories than it can consume. Once you hit that rate, and everyone does eventually, your body begins to lose weight and it’s a long slow grind down to nothing.

The test subjects were a group of runners who set out to complete the Race Across America, a series of marathons stretching across the country over a period of 20 weeks. Like the other endurance disciplines the researchers tested, the runners began burning far more than 2.5 their BMR, but eventually settled into a consistent L-curve pattern.

It’s a fascinating discovery for the world of human endurance where theories have ranged from the physiological to the neurological. One that has become prominent in recent years is the Central Governor theory, which suggests your brain is the ultimate arbiter of your body’s limits.

We talked to Herman Pontzer, an evolutionary biologist and anthropologist at Duke University, who played a leading role in the study. (Portions of this interview have been lightly edited for clarity.)

SB NATION: Let’s start with a layman’s summary of what your team found.

HERMAN PONTZER: We were invited by Bryce Carlson, who is the race director for Race Across America, where people ran a marathon a day every day for five months from the Pacific coast to Washington, D.C.

What we found is they’re burning tons of calories a day. Something like 6,000 calories a day at the beginning of the race. A bit less toward the end as their bodies adjust to the workload. That’s what we started out to do: figure out the energy cost of something that extreme.

And then when we took that extreme event and put it into the context of other really long, multi-day even multi-month events that have been measured. Everything from the Tour-de-France to shorter events like Iron Man triathlons to longer events like pregnancy.

What we found is that we had mapped out the boundaries of what humans are capable of. We mapped out the limits of human endurance. We didn’t start out with that intention. But we actually had an entry way into mapping out this whole boundary and understanding the envelope of human capability.

SB: And that is, essentially, you are spending up to 2.5 times your basal metabolic rate.

HP: That’s a complicated piece that either we haven’t done a great job of explaining it every time or people pick it up differently every time. You can definitely burn more than 2.5 times your BMR. When you run an ultramarathon you are burning well over your 2.5 times BMR during that event.

We wanted to ask two related questions, but they’re different questions. One is, what are the boundaries of human capability? That’s the complicated thing. You can do higher intensity work for a shorter amount of time, you can do less intensity work for a longer amount of time. It’s analogous to the fact that you can sprint for a 100 meters but you have to run a lot slower if you want to run a marathon.

There’s that same kind of speed versus duration relationships at these higher levels of endurance multi-day things playing out. It’s a different physiology, but it’s an analogous system. So we can map out that sort of sloped relationship between duration and intensity.

Then we can ask another question, which is: What can you do forever? As we slide along that intensity gradient, is there a point where we can actually go forever and really be sustainable for years? That’s a slightly different question.

What we found is when you look along that gradient from high intensity to lower intensity long-term endurance events, people at above 2.5 times their BMR are losing weight. We made the argument that if you’re losing weight you can’t do that forever because obviously, you’d die.

It’s a two-part answer and those things got conflated in funny ways in the press. ‘Oh, that means that all you can ever do when you’re doing the Tour de France is 2.5 times your BMR.’ and that’s not what the paper says at all. I can understand how that happens, but you’d do well to make sure that’s clear.

SB: There are so many different theories on the limits of endurance, like lactate threshold (muscle tolerance for lactic acid buildup) and VO2Max (oxygen uptake efficiency) . There’s other factors like heat and hydration, and the Central Governor theory that suggests the limits are in your mind. Has there been much of a reaction from those quarters?

HP: The short answer is yes. People who take the Central Governor view of things, and there’s two school of thought. There’s the Samuel Marcora view and the Tim Noakes view. I’m sure you’ve read Alex Hutchinson’s book (Endure), which is fantastic. I can’t do any better than that, summing up those schools of thought.

The Central Governor theory -- I don’t think anyone would disagree with this -- even if your brain is the organ that’s shutting you down and setting your limits, it’s doing that based on all your other systems. It’s doing that based on how hard your heart and lungs and mitochondria are cranking away. It isn’t this peripheral switch that shuts you down, but those peripheral signals are important.

What we haven’t done a good job yet — and this is an exciting area of science — is trying to walk those two areas toward each other and find out where we meet in the middle. Because temperature will shut you down and running out of carbs and bonking will shut you down.

But I don’t disagree at all. I think it’s right on, that the brain is the central organ that’s setting the limits. But what the brain’s listening to, exactly, and how it’s perceiving your expenditure is an open question. We’ve got to figure that one out.

SB: To be clear, when we talk about the limits of endurance, people can push really, really far.

HP: Absolutely. We did the best we could and got every credible measurement of every high-end ultra-endurance event that we could find and that’s how we designed that limit. For example, we have the Tour-de-France in there, we’ve got Iron Man triathlons, we’ve got the Western States ultra marathon, we’ve got the guy who just broke the Appalachian Trail record.

We’ve got all the stuff we could find that had a credible energy expenditure attached to it, and we couldn’t find anything that breaks the limit. During the review process, one of the reviewers asked about a study that we had somehow missed. It had a person do 31 Iron Man triathlons in 31 days.

SB: Oh, my God.

HP: I thought, oh boy, I better check that one out. So we estimated out his energy expenditure out as well as you can do and it fits the line perfectly. I really don’t have any credible measurements that surpass it.

That’s not to say that somebody won’t break through that barrier. That’d be awesome. I’d love to see that. The data brought us here and the data will move us forward.

SB: Can you train your BMR? Is there any practical application for athletes?

HP: Two things there. Let’s assume for a moment that it’s a really hard limit like we really think it is. As you get better and better at these things, what you’re training is how close you can approach that ceiling before your body shuts itself down. When you train at these high-end workloads, that’s what you’re training.

The second thing I’d say, it might be something like the 100-meter sprint record or the two-hour marathon. We seem to be up against a real physiological limit of what the human body is capable of in those track and field events that have been run for a century. I hope to see someone run a 2-hour marathon, that would be amazing. But nobody’s going to run a 1:30 marathon.

I suspect, and what we suggest in the paper, is that what we’ve mapped out is that same kind of limit, but for multi-day events.

SB: Why does this work appeal to you?

HP: I’m an evolutionary biologist and evolutionary anthropologist. I want to understand how the human body evolved and how it works and how the past shapes the present. I’m interested in physiology through that lens. I’ve always been drawn to these questions about energy expenditure because that’s the currency of life.

Life is a game of turning energy into kids, from a crude biological perspective. We don’t really understand much about a species until we understand how it’s spending these calories. Now today with the obesity crisis, which is really a crisis of too much food energy coming in, not enough being burned off, these energy metabolic questions have real practical applications.

They have cool applications in sport and exploration, so there’s this big wide open arena of understanding better our metabolism. It’s a fun place to work.

SB: That leads to the other big takeaway, which is that pregnant women are the ultimate endurance athletes because they can sustain this rate for nine months.

HP: That’s right! When I was wondering what are some of the hardest, longest things that humans do, as a biologist, pregnancy came right to mind because we know that’s a huge energy cost for moms. It made sense to plot that out and I was really tickled how pregnancy sits right on that same boundary.

We’ve mapped out this envelope of human capability and on the high intensity short end you’ve got ultra marathons and Iron Mans. In the middle length you’ve got Tour-de-France. And out there on the other anchor holding down the other end of the envelope is nine months of pregnancy.