Ultimate limit of human endurance found

June 6, 2019 3:33 PM

The ultimate limit of human endurance has been worked out by scientists analysing a 3,000 mile run, the Tour de France and other elite events. They showed the cap was 2.5 times the body's resting metabolic rate, or 4,000 calories a day for an average person. Anything higher than that was not sustainable in the long term. The research, by Duke University, also showed pregnant women were endurance specialists, living at nearly the limit of what the human body can cope with.
posted by bq (20 comments total)

The research, by Duke University, also showed pregnant women were endurance specialists, living at nearly the limit of what the human body can cope with.

This does not surprise me in any way.
posted by aclevername at 3:37 PM on June 6, 2019 [51 favorites]

Fascinating:

The researchers argue the 2.5 figure may be down to the human digestive system, rather than anything to do with the heart, lungs or muscles.

They found the body cannot digest, absorb and process enough calories and nutrients to sustain a higher level of energy use.

The body can use up its own resources burning through fat or muscle mass - which can be recovered afterwards - in shorter events.

But in extreme events - at the limits of human exhaustion - the body has to balance its energy use, the researchers argue.

So it’s true: an army really does travel on its stomach.

I wonder if I.V. nutrition could permit people to break through the barrier...
posted by darkstar at 3:57 PM on June 6, 2019 [11 favorites]

I'm really curious at what point "the long term" kicks in. The Tour de France is about 3 weeks, and as they say, racers are at 4.9× their BMR, which sounds about right.

The Race Across America is a very different kind of bike race: the clock never stops, and it has been completed in about 9 days. Many racers hit the limit of how quickly they can metabolize food before they hit the halfway point. So I wonder if this is a sliding scale: if you're trying to metabolize 12,000 kcal/day, you can keep that up for X days, but if you're trying to metabolize 16,000 kcal, you can only keep it up for Y days.
posted by adamrice at 3:59 PM on June 6, 2019 [5 favorites]

Speaking as a currently pregnant person, feeling this today big time
posted by potrzebie at 4:03 PM on June 6, 2019 [23 favorites]

The study itself is open access and linked from the BBC article.

I'm really curious at what point "the long term" kicks in.

Their model shows a precipitous drop from 1 to 25 days and then a long slow slide after that. Looks like around 200 days is the point at which their model reaches 2.5x.
posted by jedicus at 4:14 PM on June 6, 2019 [6 favorites]

And humans gestate for about 280 days. Implying that our pregnancies are limited by metabolic capacity? Would there have been a change after we started cooking food, or is it possible to max out our digestions on plausible raw food?
posted by clew at 5:19 PM on June 6, 2019 [15 favorites]

I wonder if I.V. nutrition could permit people to break through the barrier...

Ketones, maybe?
posted by meowzilla at 5:27 PM on June 6, 2019 [1 favorite]

> Their model shows a precipitous drop from 1 to 25 days and then a long slow slide after that.

The individual event types on the Metabolic Scope and Duration chart have some interesting characteristics. Arctic trekking is kind of flat from 0-10 days, has a significant drop 10-20 days, and then at 30 days pops back up to above the 0-day mark, and then kind of tapers off downward. Cycling is kind of flat for days 0-20, curves downward until day 60, after which it curves upwards until day 80 and is mildly down and up through the 140 day mark (all days are -ish, since the chart is kind of coarse).
posted by ardgedee at 5:32 PM on June 6, 2019 [4 favorites]

I wonder if I.V. nutrition could permit people to break through the barrier...

Not in its current form. PN (parenteral nutrition, or IV nutrition) carries a pretty substantial risk of metabolic complications that tend to be worse the more energy it provides. Livers do not love PN. At some point hopefully the science of PN will evolve such that it can better mimic physiological nutrition, but it's definitely not there yet.
posted by obfuscation at 6:05 PM on June 6, 2019 [8 favorites]

Yeah, parenteral nutrition is a poor substitute for the real thing, has to be infused very slowly (e.g., over 24 hours), and unless it’s a very basic/supplemental mixture, requires a central line. It’s a crude hack to bypass the GI tract for patients without better alternatives.
posted by dephlogisticated at 6:23 PM on June 6, 2019 [3 favorites]

or is it possible to max out our digestions on plausible raw food?

Koebnick (1999) would imply not . Fonseca & Herculano (2012) points that way too.
posted by the man of twists and turns at 7:17 PM on June 6, 2019 [2 favorites]

the Maternal Energy Hypothesis (pdf), for all mammalian brains (Martin 1996)!
posted by clew at 7:49 PM on June 6, 2019 [3 favorites]

The authors did include pregnancy in their analysis of extreme workloads. It is in table S1 from the original article's supplemental materials. They list pregnancy as 280 days at 2.0 +/- 0.5 x resting metabolic rate. The other workload summaries in the table are also interesting.
posted by RichardP at 10:34 PM on June 6, 2019

This is so fascinating. I do endurance cycling and the longer events (1000km+ self supported / nonstop) definitely comes down to planning around two things: how little sleep can you function on, and how fast can you put stuff in your mouth while moving.

Chewing becomes a chore, your jaw hurts. You have to eat high calorie stuff but you get tired of sweet things real quick. A large part of training for these longer events is figuring out how you can personally eat enough calories under these conditions without getting sick.
posted by bradbane at 11:51 PM on June 6, 2019 [16 favorites]

Very interesting article, even if I'm posting this from my office chair unlike bradbane above me. I do have some arctic warfare training "back in the day", and endurance and getting enough calories inside you are big concerns there as well. I think the Norwegian Army standard field rations are around 3800 kCal / day, while the arctic rations are considerably fortified from that. But getting enough of both calories and water in a stressful setting is not a given, so the training includes a lot of indicators to monitor to keep an eye of the condition of your body (i.e. colour of urine).
posted by Harald74 at 12:15 AM on June 7, 2019 [1 favorite]

And humans gestate for about 280 days. Implying that our pregnancies are limited by metabolic capacity?

Alice Roberts did a documentary (review, I can't find a find a copy online) on why humans are born so prematurely. Her conclusion was that nine months was due to an energy limit. Human brains, even unborn ones, use up a lot of energy.
posted by antiwiggle at 2:20 AM on June 7, 2019 [1 favorite]

And humans gestate for about 280 days. Implying that our pregnancies are limited by metabolic capacity? Would there have been a change after we started cooking food, or is it possible to max out our digestions on plausible raw food?

Previously on MetaFilter

This was a really cool article about both that exact question, and about evolutionary biology as an explanatory tool.
posted by Mayor West at 6:44 AM on June 7, 2019 [2 favorites]

I remember reading that swimmer Michael Phelps ate 10000-12000 calories daily during training.

That seems to be far off this estimated value (2-3x), and obviously training takes place over a longer timeframe than an endurance event.

I wonder if the estimates of his calorie intake were simply wrong, or Phelps is an unusual specimen.
posted by theorique at 6:56 AM on June 7, 2019

Theorique, Phelps is a big dude, so his BMR - the thing being multiplied by 2.5 - was higher than us mere mortals.
posted by notsnot at 8:49 AM on June 7, 2019

Very loose analogy: training a big computer model uses as much energy as several cars (fuel and manufacture). Thinking is expensive.
posted by clew at 1:51 PM on June 7, 2019