The Limits of Human Endurance Might Be Our Guts

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Yeah! [beeping]

Ah! Go!

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How far can we push the human body?

That's a question I've tried to answer time and time again

for Wired's video series Almost Impossible.

That's right at the threshold. [clapping]

I've tried running as fast as an elite runner

trying to break two hours in the marathon,

and I've tried matching the power output of elite cyclists.

And while those tests felt like

they were taking forever at the time, for me anyway,

they really represented these really short bursts of energy.

When it comes to endurance events

that last weeks or even months on end,

scientists and athletes really aren't sure

what the upper limits are on human performance.

But a new study that looks at energy expenditure

in everything from multi-day ultramarathons to pregnancy,

could shed light on just how far humans can push

themselves for prolonged periods of time.

To learn more about what that study found

we caught up with Dr. Herman Pontzir.

He's one of the lead authors on the study.

I'm a Professor of Evolutionary

Anthropology at Duke University

and I study human evolution and metabolism.

You were looking at at a pretty

gob stopping endurance event, right?

Yeah.

This whole study got started when Bryce Carlson,

who is a co-author on the study,

he was organizing the science team for this crazy event

called the Race Across the USA.

And people ran from the Pacific Coast to Washington D.C.

They did a marathon a day, six days a week, for five months.

It's 3,000 plus plus miles, 140 days.

My lab, we specialize in studying energy expenditure.

He asked if we wanted to tag along

and do the energetics of that race,

and of course we said absolutely, you know, can't miss it.

So that was the impetus for this whole study.

We measured energy expenditures

at the beginning of the race and at the end,

and as you can imagine,

they're burning tons and tons of calories every day.

You actually compared the Race Across the USA athletes

to a bunch of other data that's been collected on athletes,

on people who do manual labor,

on people who are just undergoing

normal biological processes, right?

And what did that comparison show you?

Right, so we took the data from

the Race Across the USA athletes,

and when you place it against triathletes,

ultramarathon runners, Tour de France cyclists,

Arctic trekkers, mothers who are pregnant,

scoured literature to find all the longest-lasting,

highest intensity events we could find.

And when you put it all together you end up sort

of mapping out this beautiful boundary

of what the human body is capable of,

the sorta limits of endurance.

And as you might expect,

you can burn a lot of calories for a short amount of time,

you can sustain fewer calories burned

for a couple of weeks or a couple of months.

And then the level at which you

can maintain your expenditure goes down, down, down

as the duration goes longer and longer.

It's not the same system actually,

but it's analogous to sprinting versus

marathon running in track events,

where you can run super fast for 100 meters.

If you need to go a mile then you have

to pace yourself and go slower.

That same kind of distance versus

intensity relationship, only with us,

we're seeing a time versus intensity relationship,

and we're talking over much longer time periods

than even your standard marathon.

We're looking at things that are days, weeks, months long.

This is six marathons a week for multiple weeks on end.

Yeah. [laughs]

Yeah, or in pregnancy, you know,

nine months of pretty intense expenditure.

What is the rate that you or I, or an elite athlete,

or seemingly anyone, right,

could sustain kind of indefinitely in an ultra,

ultra, ultra endurance capacity, so to speak?

It comes down to two and a half

times your resting metabolic rate.

What does that mean?

So that means in real world terms,

most people are burning around 1,600 to 2,000

calories a day, just at rest, at baseline,

and so multiply that by two and a half

and you get the level at which your

body is able to put calories back.

So somewhere between 4,000 to 5,000 calories a day

would be the maximum sustainable

amount of energy expenditure at which you're able to meet,

whatever you burn that day you're able

to put back at the end of the day.

You did find that some people do exceed that

two and a half multiplier in short bursts

or for shorter sustained efforts, right?

So right, so you can go above that ceiling,

you can go above that two and a half BMR ceiling for awhile.

We wouldn't consider that to be truly sustainable forever

because if you're negative energy balance,

as we say, if you're losing weight,

then obviously you can't push yourself like that forever,

bad things happen.

Yeah, that's a good way to put it.

What do you think is keeping that

limit at 2.5 times metabolic rate?

What seems to be sitting at two and a half times

basal metabolic rate limit

is how quickly your body can digest calories

and get them into your body in a useful way.

And we see that no matter what the activity is,

if you're trekking, if you're in the Tour de France,

if you're pregnant,

if you're doing a triathlon or ultramarathon,

no matter what the activity is,

when we calculate how many calories

people are able to absorb into their bodies,

it seems to be that two and a half

basal metabolic rate limit.

That's probably telling us about

the rate at which you can digest calories

from your food in through your intestines,

through your liver and get into your body in a useful way.

So you mentioned that this is operating

on a different system from what separates,

say a 100-meter sprinter from a marathon runner,

and it sounds to me that the system actually

has to do with your digestion.

Is that right?

If you look at shorter events like marathons,

sprints, you know miles,

the limits there seem to be based on

basically on your muscles and muscle ability and fatigue.

When we look at these really long events,

the limit is your guts, basically.

You actually approach this problem

from a couple of different angles,

and one of them, what struck me as really creative,

which was you actually also looked at overfeeding studies.

[Herman] Yeah.

Can you talk to me about why you did that

and what it showed you?

In science when you're kind of doing

outside of the box stuff, which I think this study is,

you end up kind of following your nose

and trying to figure out, go where the evidence takes you.

And so when we had this sense that

from the weight loss in the endurance events

that energy absorption was a big part of this,

that had us wondering, well,

how can we look at studies where people

have maxed out their abilities to absorb calories?

Well, the whole point of the study

was to maximize and push your digestive system

to the brink and absorb as many calories

as you could and as fast as possible.

That would be an overfeeding study.

So there have been studies done,

both in different cultural groups,

where they go on these sort of

month and two month long binges

as part of their culture where

it's manual in their early 20s usually just trying to

pack on weight for a month or two.

As well as like laboratory studies

where people are in a metabolic ward

and a very much more sort of clinical laboratory setting,

and they're just plied with all

the calories they can get.

The goal is to gain as much weight as possible.

It's like the biggest winner instead of the biggest loser.

No matter what you do with that,

no matter how you test it we get the same answer,

which is, again, you're able to take in

about two and a half times your

basal metabolic rate of calories.

Again, that 4,000 to 5,000 calorie range

for most people per day as the max,

even when you're just jamming yourself full of calories.

So given that you also looked

at studies that looked at pregnant women,

does this tell us anything new about

pregnancy as a biological process?

Kinda puts a new light on it for sure.

I don't think it's gonna change

the way that mothers should be cared for

or take care of their nutrition or anything like that.

We're not doctors and we're not proposing

some new way of taking care of pregnant women.

But what I think this does say is

pregnancy takes mothers to the same brink,

the same boundaries of human ability

as a Tour de France race, as a triathlon,

as an ultramarathon.

And so you know moms' metabolic machinery

is getting pushed to the limit,

and it's just I think one more reason

that we have to be really sure

that we get mothers all the nutritional help they need,

all the medical help they need to keep them healthy,

because it isn't easy,

and I don't think that that's news to

any woman who's gone through pregnancy.

But just one more piece of evidence

to just how tough it is on the body.

Comparing it to a marathon almost,

it really doesn't do it justice, right?

It's like multiple marathons.

Yeah, that's right.

The people who ran across the USA for five months,

a marathon a day, were you know not really

pushing it any harder than a pregnant mother does.

That's amazing.

Metabolically speaking, that's incredible.

[laughing]

Do you think it's conceivable that

somebody could exceed this limit?

Well, since we published this study,

we've gotten, Twitter's a wonderful thing

because you just have to say it's not possible to do X,

and then you'll get a thousand people

who want to tell you about how they did X

or somebody that they know did X.

It's great, actually, and it's got a lot of people excited,

and we're excited about it.

Do I think it's possible?

Of course.

The data kind of brought us here,

and the data will take us to some place new

if that's what we can find.

All I can say is we looked as hard as we could

to find all the high intensity,

long-lasting endurance activities we could find

that had any kind of credible measures

of expenditure on them.

So, is it possible?

Sure.

And hopefully I'll get a chance to see it happen,

and then the science will go on from there.

For those of us who aren't running

six marathons a week for multiple weeks on end,

what is the takeaway for the study?

Well, the takeaway is this.

One, is that there are real limits to what your body can do,

and we can now map that out.

You don't have to run an ultramarathon

or be in the Tour de France to potentially

be kind of coming up against these limits, right.

So I think that for a lot of people out there

who are recreational athletes and love to push themselves,

I think this tells you maybe some guidance

about what you can expect your body

to be able to handle over the long-term.

Thank you so much for talking with us.

This was a lot of fun.

Yeah, it was a great conversation.

Thanks.

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