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This is episode 442 with professor of biological anthropology, endurance athlete and researcher, Dr Andrew Best.
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My guest today is a former marathoner, and my hunch is that many of you are now aware of his work.
Dr Andrew Best is a professor at the Massachusetts College of Liberal Arts, and his research focuses on evolutionary questions about human physiology.
His latest study is making the rounds in the running community.
It's titled Ultra Endurance Athletes and the Metabolic Ceiling.
They looked at the maximum sustainable amount of energy expenditure possible.
So in other words, how much you can train sustainably over long time periods.
Our conversation focuses on sustainable training how to avoid the failure points that cause us to stop training, the value of durability, the effect of running economy and a lot more.
A big lesson from his research and how to avoid many of the reasons why we have to curb our training is to fuel your training well all the time and to put a big emphasis on staying healthy.
These are likely the two biggest ways to make your training more sustainable over the long term.
And if you want to figure out how to run with fewer injuries, get our free series at strengthrunningcom.
Slash prevention.
This is available to all of our listeners and you'll learn more about the major causes of injuries, strength routines and training tips to reduce your injury risk, and a lot more.
And now without further delay, please enjoy my conversation with Professor Andrew Best.
Alright, here we go.
Andrew, welcome to the podcast.
Hey, thanks for having me, Jason.
The endurance community has been talking about you, Andrew.
You have published a fascinating research study last fall on caloric expenditure.
But maybe we can use layman's terms and say the limits of endurance, which is such a wonderful phrase, I think.
Now, the title of the study was Ultra Endurance Athletes and the Metabolic Ceiling.
What an amazing title.
Already has piqued everyone's interest.
Can you give us maybe the CliffsNotes version of what you were trying to understand with this study and then what you ended up learning?
Yeah, sure.
I'll give you some of the background first to tell you why we did this and how this came about.
So in 2019, a paper came out by Caitlin Thurber and some other authors, and the senior scientist was Herman Ponser, one of my co-authors on this project.
And they basically proposed that there's – a maximum number of calories that a human can burn over an extended time period, and that it doesn't really matter what physical task you're doing.
So it could be running farming pregnancy, any physiological task that requires a lot of energy that there's a common limit To how many calories you can burn, regardless of that task.
And the number came out to be around two and a half times your basal metabolic rate.
So for any listeners who don't know, your basal metabolic rate means the number of calories per day that it takes just to run your body.
So to run your organs, your immune system, you know, digestion, all that stuff.
So it's the number of calories you would burn if you laid in bed for 24 hours and didn't move a muscle right.
And for a 150-pound person, there's some variation, but it's probably around 1,700 calories a day.
So if we do that math, then for that average person that means that long-term sustainably – that person can only burn about two and a half times that basal metabolic rate for their total calorie expenditure.
So that's calories they spend on everything from just living to running and training, playing with your kids all that kind of stuff.
So if we were to put some numbers to that, if we just make it simpler, because my brain's a little foggy right now If your BMR is about 1500 calories a day, then that would put you at what.
So double is 3,000.
So what?
3,750 calories a day.
So for the average person we're talking about burning just under 4000 calories a day would really be the limit that you can do sustainably over months and months without losing body mass right.
So when we say sustainable, we mean that you're in energy balance.
You're able to absorb that many calories and store them as carbohydrate and fat and muscle tissue, et cetera.
So that was the number that came out of it.
Someone years ago, Wester Terp, another researcher, had proposed a similar number.
But Ponser and Thurber and these other authors had some real good data behind it.
They looked at people running across the US in the race across the USA.
They looked at Tour de France cyclists.
They compiled a bunch of previously published data.
And this is a number they came up with.
And so I had been – so we basically started this project in, I think 2020 2021, just getting some energetic data on ultramarathoner Joe McConaughey, also known as the string bean.
And Herman and I decided.
Well, we think ultrarunners are a really good population for testing this idea.
That the total number of calories you can burn over time sustainably is limited to roughly two and a half times your basal metabolic rate.
So we recruited at first just ultra marathon runners.
We looked for really competitive people, folks who are tier three, four or five according to a standardized classification framework.
We ended up also getting a couple of cyclists and Ironman triathletes, but people who also do ultra distance type stuff.
And we measured their energy expenditure during training and life.
Um, and also during ultra marathon events.
So we could have some data points just from the races.
Um, cause we're not just interested in, you know, what are the ultimate limits over time?
What are the sustainable limits?
We also wanted some more data points um painting that picture of how many calories can you expend over shorter time periods too, you know, which we can talk more about later.
So that's what we did.
Um, I can talk more about the methods if you like.
Um, But what we ended up finding is that in this cohort of 14 really competitive endurance athletes and we weren't very specific with who these people were because we didn't ask if we could identify them
So we weren't able to include some of the really exciting information about who these people are.
But I can tell you that at least one of them is a world record holder.
One of them is a world champion.
One of them is an age group world record holder at some ultra distances.
So we had some serious people and they were not breaking this two and a half times BMR threshold.
A couple people did.
They were like 2.6, 2.7 over the course of a year.
But on average, the cohort was just under or just around 2.5 BMR.
And the reason we're not really placing a lot of stock in those individual data points is just that there's a bit of error built into our methods.
So we're not going to say, oh, man, this guy broke 2.5.
This cohort of people who we'd expect would be breaking the ceiling if it was higher weren't.
So this is not the be-all, end-all.
To say 25 times BMR is the limit, but it's telling us there's something physiological going on that is – determining how much physical work a person can do.
And that number seems to be fairly predictable in people who are making a living at burning as many calories as they can.
So it's funny, we're mostly talking about this in the context of running and sports, and that's what we're doing here today.
But that wasn't really... impetus behind this.
I mean, Herman and I are both biological anthropologists.
We're not exercise physiologists.
I kind of moonlight as one with the methods I'm using, and it's one of my interests.
But really, I'm an anthropologist.
So what we're really interested in is why is there this common limit?
It's something about our biology, and we don't think it's unique to running.
I think that if you I don't know, had someone doing any kind of physical activity, that's the limit.
And we can get into what we think determines that limit later, but that's basically what we found.
So it's basically, it's just further evidence that there is this threshold that most people can't break.
Well, Andrew, I think it's super interesting.
And Shirley has some applications for ultra marathoners and those runners who are interested in the really long stuff, the prolonged type of events that might really put a dent into their caloric needs.
One of the aspects of this that I'm particularly interested in is the overtime piece of it.
So this idea that the limit for your energy expenditure is about two and a half times your basal metabolic rate,
When you say over time, how much time are we talking here?
Like is this you know, over the course of a year, which basically means that there's going to be some periods where you can burn more calories, but then there's going to be other periods where you do need some rest and recovery to kind of get yourself back to neutral.
Quote, unquote there.
But how do you think about the time piece of this?
Yeah.
So I would say further work is needed to really drill into where this two and a half threshold kicks in.
Gosh, I wish I could share my screen.
But if you look at the paper, there's one of the figures showing the relationship between energy expenditure and time.
And this is pretty intuitive to all runners, right?
So if I could just step back for a minute and if I totally lose your question please, you know, pull me back.
It's intuitive to all of us that if I say, run as hard as you can for five minutes, you're going to be able to burn more energy per unit time than if I say run as hard as you can for three hours, right.
You can amp up your metabolic rate much higher for very short periods of time.
And if I say you know, run as hard as you can for 24 hours, for most of us that won't even be running.
We'll have to do a lot of walking, right?
And if I say, do as much physical activity as you can over a year, obviously the slope of that line falls even lower.
So that's kind of intuitive.
So what we're really doing is just extending the slope of that relationship between duration and energy expenditure further out than we normally think of it.
So that initial part of the curve over the shorter durations is really intuitive to runners, right?
If you're going out for a five-mile all-out race, you're going to work harder and burn more calories per unit time than you would if you were doing a 10-mile all-out race.
Okay, so this is the same idea, just extend it out.
I think I've already lost... what your question was.
Can you remind me where we're going with this?
Well, I was asking about the over time piece of the equation here and asking about the two and a half times your kind of resting metabolism.
What kind of a time period is that is?
Is that a year?
Is that a month?
Is that a week?
Is that lifetime, like?
I'm interested in the scale gotcha gotcha, okay.
So i'll just extend this curve kind of farther out.
If you're super, super fit, if you're a tour de france cyclist, say right.
Or one of the guys in our study um, ran the appalachian trail actually the last couple summers.
Um Well, it's running and hiking.
It's a mix, right?
But he was going for the record and he actually got pretty close.
So for those 46 days, he was at a metabolic scope or a multiple of his BMR of about four.
So someone who's super fit and has the musculature and the soft tissue that's been trained enough and strong enough to handle this kind of output can maintain this multiple of BMR of about four for about a month and a half.
So that means that the two and a half doesn't really kick in until later.
We're thinking it's somewhere around maybe 30 weeks.
And So how long could someone in theory maintain two and a half?
I mean, potentially years.
I think if you, it would take a lot of luck.
You'd have to not get injured, not get sick, still be highly motivated.
Because again, for the average person, we're talking about burning roughly 4000 calories a day on average, every day.
So where does the two and a half kick in?
Where does that slope actually level off and kind of become an asymptote, right?
Where it's not actually falling anymore?
Probably around 30 weeks.
We're talking a matter of months.
Can you burn more than that for short periods?
Of course, absolutely.
The guy that ran the Appalachian Trail.
He was at a multiple of BMR of about four but he had to borrow so much from the energetic tank.
He was wasting away his own body, right?
Even people who through-hike the Appalachian Trail, let alone trying to run it, end up losing a lot of body weight, fat and muscle, and even probably some organ weight.
Your organs also shrink too when you're in that kind of starvation mode.
So he had to pay that debt back.
So two and a half is really we're saying that's the number we think where there is no debt, where you're able to absorb enough nutrition and process it and assimilate it to adequately recover and maintain body weight.
You're not breaking down muscle faster than you can repair it, right?
You're not –
I don't know, all the other things that happen during exercise, right?
Like you're not using up all of your oxidative enzymes faster than you can replenish them.
All that stuff.
So two and a half kicks in somewhere in the matter of months, around 30 weeks, we think.
At that point, in theory, it's sustainable.
Although in practice I would be really surprised if anyone's ever actually maintained two and a half for more than a couple of years, just because of the sheer output of training you need to be doing.
So that's another point.
I've been saying this isn't just about athletes, but it's really only athletes who would ever be at this level of energy expenditure.
Because the only organ in your body that can elevate your whole metabolic rate enough to have a total energy expenditure that high is your muscles.
So it's only through muscular activity something that an endurance athlete would be doing where we're going to see numbers like this.
Andrew, it seems that this has much more to do for runners with training than it is for racing.
Because even if you're doing...
You know, even if you're a Tour de France cyclist, it's like a 22-day event, which is far less than 30 weeks.
So presumably you can jack up your metabolism, experience a tremendous amount of physical activity and get through it just fine, as almost all Tour de France cyclists do.
But it's when your training is really demanding for a very long time period.
It seems like is is where there can potentially be breakdown.
Yeah, absolutely.
So the place that this is applicable for athletes isn't in racing, like you said, it's training.
And you know we could argue and I think people do about you know how important volume is, or you know how much volume you need.
I think most endurance athletes and coaches would agree that as much volume as you can handle is probably best if you want to maximize performance right.
So that's really where this becomes.
A limit is You can go above two and a half times BMR for maybe a few months at a time if you're in a really huge, high volume base training block.
But you're going to have to repay that debt later by training less.
This is maybe the ultimate training limitation, but I think it's worth noting that most of us are never going to be limited by this two and a half times BMR.
Because the things that limit most of us are really much more pedestrian.
Like we're going to get injured before we ever hit this energetic limit.
Yeah.
I mean.
So these are people who are biomechanically just – really durable, i think.
And you know i don't know exactly what goes into that.
You know it's genetics, that's what you're born with, but it's also i don't know the kinds of strength training you do.
It's, it's your running technique, it's all that kind of stuff.
So it's really just like the best of the best and the most durable, where this will be the ultimate limit.
So if you have the biomechanically perfect runner who's really motivated to run 150 miles a week and they can without getting injured, then for them the limitation really seems to be energetic.
That's really what we're saying.
And what that means is that the things limiting endurance, performance in training over months and years, those are very different physiological limiting factors than the things that limit us over minutes, hours and days, which I think we all know right.
So the things that limit performance over 30 minutes well, that's maximal oxygen uptake and utilization, right.
So it's the rate of oxygen absorption and carrying by the red blood cells and how many mitochondria you have and how efficient they are at using oxygen.
If we're talking about running performance over a couple hours, then we're looking more at running economy, substrate usage, that kind of stuff.
If we're talking about an ultramarathon.
The limiting factors for most people then are, you know, gut function, whether you're able to take in enough calories, you know, without having GI issues.
And muscle damage is the other one.
But over months and years.
The fact that there's this common number tells us that the physiology that is limiting is different.
And I think the other interesting thing that also tells us that the limiting factor is different is is that there's no single physiological measure that we normally take in a runner that can predict who's going to win an ultra marathon.
It's not always won by the person with the highest VO2 max, the best lactate threshold, the best running economy, those kinds of things that we'd measure in the lab to say you're a great runner.
It's not that they don't matter, but they're not very predictive of ultramarathon performance.
And they're certainly not very predictive of how much training someone can do over months and years.
Yeah, so I'm putting on my coach's hat right now and trying to think of some interesting ways that this might be directly applicable to runners.
I think the number one lesson is...
This probably isn't something that most runners even really have to worry about, because I'm thinking about it.
You know if you're roughly 150 pounds with a resting metabolism or basal metabolic rate of about 1700 calories a day.
We use your prior math.
We're talking about an extra 2000 calories a day from exercise.
Running burns roughly 100 calories per mile at an easy effort.
And so we're talking about maybe 20 miles of running a day, and almost nobody is actually running that amount.
Now, of course, we can throw in a dynamic warmup ahead of time, some strength training, you know, the other moving around during the day.
Maybe it's closer to 15 miles a day, but still most runners are not even getting in that kind of volume.
So on the one hand, it's really only something the best of the best have to worry about.
Um, but I think it also just lends a lot of weight to the ideas of focusing, fueling the work that you're doing, so that you're not getting into some kind of metabolic hole where you're under fueling your activity.
And also, it's okay to have really high periods of intense work during training where you might vastly exceed this two and a half times your, you know uh, basal metabolic rate, as long as you're then allowing your systems to recover, you're having some easy you training and even some recovery time.
We're not training at all after a major goal race, so that you know you can allow that body to recover and and kind of, you know, get back to neutral.
Are those fair conclusions that a coach might have about this research?
I think so yeah, and i think this is another case of athletes and coaches recognizing something um, Before the science pinpoints exactly why it's true.
We've known for a long time.
Also, your body tells you, I'm really tired.
I need to back off.
I think listening to fatigue to a certain extent will lead to the same thing of you back off and you replenish your energy stores and you repair, et cetera.
Yeah, I don't think the two and a half number is applicable as a constraint for all that many athletes.
But the general principle of you need to be fueling maybe more and better than you think you do, I think is very important.
And it dovetails with you know, in the last what like five or 10 years, there's been a lot more understanding about red S right.
So relative energy deficiency and sports syndrome.
Um, And there's a paper that came out, I think, last year that if you're interested I could look up for your viewers basically suggesting that a lot of the studies looking at overtraining syndrome were actually missing the point.
And the point was that those athletes weren't overtrained.
And I'm quoting that for anyone who can't see it.
They're actually just under-fueled, right?
So unless you're really near that two and a half times threshold, you're probably not training too much.
It's that you're not fueling to adequately recover.
And if I were a coach and I'm not, but just in my own athletic life I've stopped counting calories.
I mean, when I was running marathons, I wanted to be a certain weight.
And that was probably too light.
But I would sort of track calories and I didn't want to gain an extra pound.
And I think the understanding now is that having an extra pound of body mass is way less deleterious to performance than being even a couple hundred calories under fueled.
So under fueling is way more of a risk to your performance and your durability than over fueling and having a little bit of extra body mass.
Because all the things that come with under fueling, they will directly impact performance, right?
I mean, if you're just a little bit carbohydrate depleted, which is one of the first things that will happen if you're under fueled right, you're not going to have those 2500 calories worth of glycogen in your liver and muscles.
That's going to directly and immediately impact performance.
And then take that over weeks and months and that's going to impact bone density.
Um, you know testosterone and estrogen levels in both men and women, which will affect recovery and estrogen bone density, right?
Um, And it leads to stress fractures and you know you're not properly recovering.
So I think the message here really is fuel, maybe even more than you think you need to, until you start really gaining too much weight, and then you can dial it back.
But if you're training really hard, you really need to be eating a lot of carbohydrates.
And that's the other piece we haven't really gotten into.
And our study did not look at the breakdown between carbohydrates and fats.
But I am a bit of an evangelist, for you know carbohydrates for various reasons.
Yeah, for very good reason.
It is definitely the body's preferred fuel source for higher intensity exercise.
And I did get a question from one of our listeners about the best macro split of all.
This required energy.
Now, I know you're not a nutritionist.
You guys didn't even look at this.
I'm not a dietician myself.
Are you able to comment at all about that macro split?
Is it completely individual or is there an ideal split, depending on how much energy this hypothetical athlete might be burning?
I think there's a couple things that I could say about it, but none of them are really my own ideas.
But I think they're things that bear repeating, right?
And I think they're things that shouldn't be all that contentious.
So, number one remember that fat requires more oxygen to oxidize than carbohydrates, and that's really the reason that carbohydrates are their preferred fuel at any kind of higher intensity.
And that's not even a very high intensity, right?
Like a lot of the training runners do, especially if you're a shorter distance athlete, you're going to be relying primarily on carbohydrates.
So it's not just like, Oh, only when you're anaerobic, do you burn carbs?
No carbs are the preferred fuel source for really a lot of different intensities, right?
Um, It's only when you're going really easy that fats really become the dominant fuel source.
So there's that.
Burning carbs is more efficient in terms of oxygen.
So what does that mean?
If you're a fat-adapted athlete and I'm not trying to rain on that parade, but one fact is that if you're training in a low-carb state and you're trying to be in ketosis and increase your muscles' ability to oxidize fat,
That might be useful.
I don't think there's great data on this actually, but that might be theoretically useful for ultra marathoners, because you only have two and a half thousand calories worth of carbs in your body.
So that's not going to get you very far.
You need to take in carbs and also burn fat.
But even if you're a marathoner, you're still burning primarily carbs.
And if you train yourself to burn more fat...
Yeah, you're going to be sparing your glycogen for later in the race in theory.
But number one, you're increasing your ability to oxidize fats partly at the expense of carbs, right?
So your carbohydrate burning machinery, you know the enzymes and stuff, are tuned down a bit because you're not using them as much.
But also, you're going to require more oxygen just to oxidize those fats.
And that's a huge limiter in running performance is how much oxygen you can get to your muscles and how much ATP you can make with it.
So if you're training yourself to burn more fats, that's probably a performance limiter, especially over shorter distances.
So...
I'm not going to recommend a specific macro breakdown between carbs and fat.
Certainly during exercise, if you're taking in fuel, it should be mostly carbs.
There's not a lot of benefit, I think, to fats, unless you're going for many hours and days at a time.
But even in terms of ultra running, you know fat is a pretty good fuel for super long distance stuff, especially because what we've seen, both in some research we've done and some other numbers I've seen ultra runners trying really hard to replace calories during a race might take in about 50 percent of the calories that they're burning.
Right uh, which is fairly heroic, but that also means that every ultra marathoner is going to be out of glycogen and they're going to have to be relying on fat.
So you have to have some fat stored in your muscles and your listeners probably know that you store droplets of fat right in your slow twitch muscle fibers.
So they're, they're right there as a fuel source.
And you also access the fat that's under your skin, right?
Um, So you have to have some fat on you for that, and you have to have some fat in your diet for that.
So that was kind of the long answer.
You know, is there a good macro split?
I think, you know, carbohydrate heavy.
And I think that the evidence that we have now and I'm not a nutritionist, but would probably suggest that, rather than training your body to spare carbohydrates, it's probably better to train it to burn them faster and just plan to always be topped off with carbs and taking some as you go.
Yeah, that tracks with pretty much every conversation I've had with a registered dietitian who works with endurance athletes.
It also seems just to be the practice of elite ultra runners these days.
Ultras right now are being won in record-setting times, at paces that you would think are not capable of being run for say, 100 miles.
But there's this quote-unquote carb revolution going on in the running community where these ultra runners are taking in 150 grams of carbohydrate or more per hour And you know they're basically staying in high zone two, maybe a little bit in low zone three for a majority of the ultra marathon.
And, you know, they can just kind of fly for 12, 13, 14, 15 hours.
And it's really, it's really interesting to see because of the addition of so much more carbohydrate during the race itself.
Yeah, it's been fun as a former trail runner and now as a sort of fan of ultra and trail running, as I research it and I also spectate it just as a boring fan now.
It's been fun to see the professionalization of trail and ultra running really just in the last couple of years and how it's gotten so much more competitive that yeah, like you're saying, you know the performance level is is even higher.
Uh, and I think the same thing has been happening in the tour de France.
You know um?
You know, the average speed of the Peloton has gotten faster and faster, and it's not all attributable just to bike technology and aero stuff.
It's probably also got something to do with nutrition and just packing in tons and tons of carbohydrates.
Um And so what we think really defines this two and a half times limit is that's probably also the rate of calorie absorption, right?
So, going back to this 2019 paper by Caitlin Thurber and Herman Ponser and colleagues, they also incorporated some data from overfeeding studies, where folks were eating – excess calories every day, four or 5000 plus calories a day, and not being physically active.
And they basically just tracked how much weight they gained.
And it was way less than you'd predict based on how many calories they were eating.
And so what that means is that you're not always absorbing all the calories you eat.
So some of it just goes right through your intestines and into the toilet, right?
And The limit to absorption, based on those data, seems to be around two and a half times your basal metabolic rate, so 4000 calories a day for the average-sized person.
Well, not the average-sized person, 150-pound person, right?
That also means, though, that there are some ways around this metabolic ceiling.
One way I don't know if it's a way to get around it, but one way to make sure you can get really close to it is to make sure that you're taking in as much carbohydrate as you possibly can absorb.
And so your listeners probably know you are breaking down all the carbohydrates you eat into simpler carbohydrates such as glucose and fructose, and you've got different transporters in the lining of your intestines for both of those types of sugars.
And so there's a maximal rate of glucose absorption and a maximal rate of fructose absorption.
So if you really are near that limit, you want to make sure that you're maximizing both of those.
And so that's why we're seeing this two-to-one ratio of what glucose to fructose in a lot of sports products now,
You know, there actually is some science behind that.
You know that's to maximize how much of each of those types of sugar you can absorb at the same time.
And another way around it, and I don't know how much this is happening or if it's happening at all, but that would be intravenous glucose administration, right.
So if, for example, folks in the Tour de France, because we know Don't sue me.
Don't anybody sue me.
We know that pro cycling has a long history of performance enhancing, drugs and other sort of questionably you know, ethically and legally questionable things.
If you could bypass the gut and just intravenously administer glucose, you in theory could be, you know, replenishing your carbohydrate stores faster if you're not limited by that uptake limit in your intestines.
Can we put an IV bag at the back of one of the bikes on the Tour de France and test this?
I would love that.
I mean, that would be a really interesting study, actually.
And it's just, I don't know if anyone's done this.
Just do it in the lab.
You know, just, I don't know if you could do that in the lab.
You'd have to have someone be going for a really long time.
But that would be interesting to see.
That would help to prove whether the intestinal absorption rate really is the limit.
If you could use more glucose than you're able to absorb.
Yeah, a great future study.
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Well, Andrew, we've been talking a lot about very long-term metabolisms.
Let's tighten the window.
Is there a ceiling for your metabolic rate or how much energy you can expend in, let's say, 24 hours?
You know, once it gets to 30 weeks and longer okay, it's about two and a half times your resting metabolic rate.
But what about when, you know, you maybe have a 24 hour event?
Is there a limit?
Or can you theoretically, just you know, if you're fueling kind of well, can you just hammer for 24 hours and produce, you know, just an incredible amount of energy burn.
Well, there's definitely a limit, but it's going to be defined by very different things.
So we think it's only over the course of months and longer that the limit really is set by your rate of energy intake and absorption and recovery.
In a 24-hour cycle scenario.
Um, it's pretty similar to you know.
If you were to ask me, like you know, what's the limit to how many calories you can burn in an hour, You know?
To that I would say well, that really depends on your maximal oxygen uptake, your maximal carbohydrate and fat oxidation.
So it's, you know, how many mitochondria do you have in your muscles?
How well trained are they?
How many red blood cells do you have to carry oxygen?
All the stuff that we think of as as defining the limit to running performance levels.
Well, running performance is really the same thing as saying energetic performance, right?
So generally I mean, even though there are differences in economy between athletes usually the person who can burn more calories per unit time is doing more muscular work and is going faster generally.
So over 24 hours, say, yeah, I mean, we had...
There's actually a friend of mine that was in the study.
He ran the Massachusetts section of the Appalachian Trail.
He did it in just under 24 hours.
And it's a really gnarly 90 miles, you know, super technical, lots of up and down.
And he burned, I think, just under 12,000 calories, you know, for that effort.
And he's like 140 pounds.
So, I mean, the limit, and we're not saying he's the limit.
I'm sure someone like, you know, Killian Jornet could do it even faster, right?
But the things that define that limit are the things we're much more familiar with considering in our training and our racing.
And that's muscle durability, glycogen stores, maybe even things like lactate thresholds.
So how efficient are you at using lactate as a fuel?
So over these shorter durations, the limits to energy expenditure are huge, the limits to performance.
They're the same things.
Sounds like it's fitness related.
It's really based on, you know, colloquially speaking, it's your fitness level, which is a combination of all those different metrics that you talked about.
It also sounds like one of the best ways to almost reduce your metabolic rate and increase your ability to burn more calories is to get into better and better shape.
Your ability just to train more to handle more volume.
It will then have a more efficient.
You'll create a more efficient metabolism which will then help your long distance running.
Would that be fair too?
I would say it's more that – so there might be an element of that, right?
So let's come back to that in a minute.
I think it's more simply that you're making a bigger engine.
So endurance performance, especially over shorter durations, really is just about the size of your engine.
It's how much fuel can you burn, right?
Not so much the size of the gas tank, but how big is the engine?
So how much oxygen can you use?
How much ATP can you get out of that so that you can do muscular work?
And that's the primary thing we're doing with training, with endurance.
Training is making the engine bigger, making the heart, the lungs, the circulatory system and especially the muscles better, not so much at being more economical with their fuel, but just using more of it faster.
That's really the biggest thing.
Differences in economy absolutely do matter.
But I don't know that that's so much a metabolic economy.
So improvements in economy that you get through training as a runner uh, i think are really more biomechanical, right?
So, like your, your movements get more efficient, you're.
You're wasting less muscular energy, you know, with um too much whatever, like your arms flailing off to the side uh, you know, the more that you train, your body kind of learns like oh, here's how much hip flexion i need, Here's how much knee extension I need.
Here's how much plantar flexion force I need.
I don't want to create more than is useful.
All those kinds of things.
So the efficiency or economy improvements that you get with training absolutely matter.
And if you have two people with the same VO2 max or the same lactate threshold and one can do more, can run faster at a given energetic output yeah, that person's going to win.
Absolutely.
But I think even more important, especially over shorter distances, is the size of the engine.
So we're primarily training the size of the engine.
And then, especially if you're doing a longer distance event, then you're training the economy.
But more than metabolic economy, it's just movement economy.
Yeah, the movement economy is, I think, what runners are more familiar with when we talk about economy.
That's your biomechanics.
It's like the oxygen cost of running.
If you can be more efficient, it will be less oxygen expensive for you.
Yes, yes.
Andrew, I have a couple of listener questions for you that are a little bit off the general topic here that we're discussing, but I think they're really interesting anyway.
I think we discussed this, but one of our listeners wants to know what are the theoretical or real failure points.
Because, like you said, it's usually not your metabolism that's causing you to slow down or stop training.
You know, is it bone, muscle, joints fail before your heart and lungs?
Or even is it mental or psychological?
I think it really depends on the duration of the task, right?
So if we're talking about a 5K race, you know, so, okay.
So the answer, I think, to most of these is fatigue.
And it is your brain strongly encouraging you to slow down.
And we all know, as endurance athletes, that you can ignore that because your brain is really overprotective of you.
It's very, very hard to actually get yourself to a point where you've expended so much energy so quickly that you're near death, which is really what we mean by an ultimate limit.
My body thought I was the last time I raised an 800.
Well, I think the 800 might be kind of a special case.
What a miserable event that is.
Yes, sir.
I bet a lot of your listeners have probably read Alex Hutchinson's fantastic book, Endure.
Great book.
I think it's from that book.
He uses the example of someone winning the 10K in the Olympics.
If they were really at their physiological limit – then when they finish the race they shouldn't be able to get up and do a victory lap.
But they do right.
So if those people super highly motivated, everything is on the line, if they're not able to run themselves to near death, then that tells us that the brain is really, really protective.
There's a lot of research here.
Um, so number one, yeah, it's really fatigue over any distance.
It's your brain sensing all of these peripheral, uh, inputs, right?
Like muscle pH and you know body temperature um you know CO2 levels.
You know lack of oxygen to certain tissues.
These are all warning signs and your brain coalesces them into this feeling of fatigue and you try to fight through it.
And the better you can fight through it, the faster you go.
And that's why, you know, mental toughness is so important for an endurance athlete.
But if we're talking about the ultimate limits, so let's say that you know you're able to really push past a lot of that fatigue, Which would really take a super long.
It's really only the really really long distance events where you're probably able to push through fatigue for long enough that you really do actually do some real damage.
So like ultra marathons, muscle damage is usually the limiting factor.
So let's say that you fueled really well.
You were able to tolerate all the fuel you were taking in.
You know the heat didn't affect you that bad.
You're not super dehydrated.
Um then it's usually muscle damage and you know, people end up just like really not being able to run anymore.
Right.
I mean um.
So muscle damage is usually an ultimate limiting factor, but I think, over most of the distances that your, that your listeners, compete in um.
So we've already established that it's fatigue and that fatigue is not entirely real.
But what are the peripheral things that your brain is sensing and telling you are fatigue?
Over a shorter distance race, it's probably going to be like blood pH, right?
So the hydrogen ions are accumulating in your muscle and your blood and your brain is freaking out about that.
It's oxygen deprivation.
It's that stitch in your stomach which really comes, because you are purposely shunting blood away from your intestines.
Because your brain basically doesn't know the difference between hard exercise and a life or death situation.
It's all fight or flight.
It's all the sympathetic nervous system.
So you're flooded with adrenaline.
And your body is putting everything it can into what your muscles are doing, at the expense of your intestines and some other things.
So intestinal discomfort and that real knot in your stomach, especially in like an 800 or like a 5K, even.
It's probably going to be an ultimate limiting factor.
But if we're talking about training, if we just had someone run until they can't run anymore there's a gun to their head, this is some terrible kind of crazy experiment then it usually ends up being muscle damage, I think.
And if the muscles didn't break down first, then it's just going to be lack of energy.
I mean, you burn through all of your carbohydrates in a matter of hours.
And then it's just on to fat stores.
And depending how much of that you have, you're just going to run out.
Yeah, so much depends on the distance of the race.
And you saying that the shorter events, it's all blood pH just gives me PTSD flashbacks to my college track career where yeah, everything was pretty much based on how much anaerobic discomfort that you can endure over time.
And then it's almost like a checklist of failure points, right?
So if the distance is long enough where blood pH isn't going to get so acidic, then it's probably a durability issue.
But if you're durable enough where you don't just get hurt, you're one of these biomechanically efficient runners where you can kind of run forever and not get hurt.
Okay, you're not going to just hurt something, but you might just want to quit.
And then it's the psychology aspect of it.
It's like okay well, if it's not a blood pH issue, you're durable and you're not getting hurt and you still want to keep going then it's probably that muscular endurance issue where you might get a visit from Uncle Rabdo and suffer from that kind of muscle breakdown.
And it seems like if it's not one of those three or four things...
It's probably not going to be anything else, because those are the big kind of issues that come up, it seems, for every runner and why they have to stop exercising.
And I would also add, you know, your brain is always monitoring how much fuel you have.
And when the gas tank starts getting empty, your brain starts to freak out.
So when your glycogen is depleted, we all know that feeling of bonking when your blood sugar crashes.
It happens to me like... almost every morning, actually, which maybe I need to eat breakfast sooner.
But I don't want to do anything physical in that state.
My brain is telling me I'm going to make you fall over and lay on the ground until you put some more food in your face.
So one of the big drivers of fatigue, especially over longer distances, really is glycogen depletion, which we know is something we can push through.
We can eat more.
When your glycogen is depleted, you have to slow down, but you can rely on fat.
But your brain is still saying, Uh-uh, danger.
We don't want you to run out of glycogen.
Well, Andrew, what about different types of runners and how many calories they burn?
So one of our listeners asks do elite runners burn fewer calories to run the same pace than a recreational runner?
Or is the cost largely fixed per unit of body mass and distance?
Right.
So let's assume these two people are the same weight, because the biggest predictor of how much energy it takes to move your body is your body mass, right.
So if we have two athletes that are the same weight one is recreational, one is elite there are usually differences between those two people in running economy, which in the exercise physiology world is usually measured as you know how many liters of oxygen it takes to run at a given pace.
So you have these two athletes run at the same speed, say six minute miles per Measure how much oxygen it takes them per minute to go at that pace.
There are some pretty significant differences on the order from what I've looked up.
Around 15 differences in economy between recreational and truly elite cream-of-the-crop runners, which isn't nothing.
That means that those elite runners can move faster at a given oxygen cost.
And you might think, well, oxygen costs, so what?
Well, that's a pretty close proxy for energetic cost.
So, first of all, oxygen cost.
I mean someone who can do more with the oxygen they're taking in.
They're going to go faster because there are hard limits to how much oxygen you can take in.
I mean really that's like the prime determinant of performance that, like something like 10 to 30 minutes, is how much oxygen you can take in and use.
So someone who can do more work with the same amount of oxygen is going to be faster.
But also, you know, the amount of oxygen that you're using is roughly proportional to how many calories you're burning, to how much actual work you're doing.
If you really want to know you know what the energetic cost is per mile then you have to know also the substrate breakdown, right?
Because as we said, Fats take more oxygen to oxidize than carbohydrates do.
So you'd have to have the mask on the person and measure something called the respiratory exchange ratio, which tells you how much carb versus fat they're burning.
So for the stuff I do, I think of running economy more in terms of calories per kilometer, per kilogram.
So it's based on body weight.
It's relative to body weight, but it's not so much.
How much oxygen does it cost to move at this speed?
What we care about more is how many calories does it take to move a kilometer, right?
And the rule of thumb is usually like one calorie per kilometer per kilogram of body mass.
And just for fun, I looked back at some of the data from our runners in our most recent study.
We had five people who ran the Cocodona 250.
So 250-mile race.
It takes a couple days, and there's some walking involved.
There's also heat, and they're carrying things.
But our numbers were we saw anywhere from about 0.9 up to 1.3 calories per kilogram per kilometer.
Um, so there is variation there.
But to answer your initial question, I know we got way off on this tangent here
Yeah.
Um, the the elite athlete is burning less energy.
Um, at a given speed, or even, you know, over over a mile, you know per mile than than the recreational one is.
That, of course, doesn't mean they're probably burning less calories overall, because it means they can do more work.
They're, they're covering ground faster.
And that means that they are burning more calories.
But no, they are more efficient.
Yeah, it's almost like you might run for the same amount of time, but they'll run two or three miles longer than you in the same amount of time.
And energy cost really is per unit distance, not per unit time.
Exactly.
Okay, another question for you, Andrew.
This is a really interesting one.
During exercise.
Does energy expenditure per unit of time drift upward, downward or remain stable across multiple hours, and why?
Yeah, so I've only seen some limited data on this, but it's kind of intuitive and it's probably what your listeners would think.
Yeah, your economy tends to drift downwards a bit the longer you go.
And I think that's largely because you just become biomechanically less efficient, right?
As you become tired, you know, you're not, your form is not as perfect as it was before.
I have some very concerning photos from mile 24 of a marathon that show exactly this, Andrew.
Did you go from forefoot striking to heel striking?
Yeah, kind of.
Yeah.
All of my bad habits were magnified a little bit.
Which is not to say that heel striking is worse than forefoot running in terms of energy cost.
The data on that are totally mixed.
But changing your biomechanics during a race from what your body normally wants to do to to what it ends up doing when you're tired usually translates to you are less economical.
So yeah, and I don't know what the magnitude of these differences are.
I'd have to look up some of the studies.
But yeah, you absolutely get less efficient the farther you go.
That probably also applies just to like training fatigue.
You're probably less efficient at the end of a 100-mile week than you are at the beginning, if it's the first week you're doing that.
And then the end of 100 mile week is just the beginning of the next 100 mile week.
So really, where does any of this begin?
Well, I mean, you know, the more that you do that, you do get more economical.
So you know you can have differences, or training can induce changes improvements, I should say up to about 8 in a running economy.
Yeah.
And one of my prior podcast guests who just wrote a book about the marathon and he has a PhD in.
But it's a interesting aspect of exercise physiology.
And, according to him, you do get about 10 uh less economical at the end of a marathon compared with the beginning of a marathon, which you know, I think anyone who's run a marathon and kind of checks in with their body at the end of the race can tell that they are not running as efficiently as they were when they were fresh, had full glycogen stores and you know uh, a positive outlook at the beginning of that race.
Yeah, and your quads are not smashed to pieces yeah that's, i think we've all experienced that.
Well Andrew, this was a super interesting conversation.
And I think we learned a little bit more about the value of fueling the value of higher volume training to increase that economy so that it positively impacts your economy over time.
And all these interesting ways of thinking about fueling kind of the maximum amount of energy you can put into your training over that long term.
So I think it's super interesting for all of us endurance runners.
Is there anything else that about this study or about the general topic that we might not have covered in as much detail as you wanted, or something related that you think we might benefit from?
I mean, I think you've you've sort of hit upon all the things that would be relevant to runners at this point.
Um, I like these conversations because when we've started this project and even when we finished it, I was thinking the whole time this is theoretical.
It's really cool.
I like it.
I don't know what the practical application is going to be.
And the more of these conversations I have, you know, the more of them come out.
Where would you like to see future studies go?
What would you like to see them look at as sort of a next step in this field of study?
Differences between sports, for one thing.
I suspect that the mechanical damage that you do as a runner probably limits even in a really well-trained runner, probably limits that metabolic ceiling versus someone like a cyclist.
You know, and there there are some anecdotal reports, even some that are published.
So I guess that's not anecdotal, you know, showing that you know some people might be breaking this ceiling over, might be might be breaking the ceiling over shorter distances.
And they're cyclists, you know, I don't think runners are doing that.
So I would like to see differences in in long-term expenditure, sustainable expenditure in impact versus non-impact endurance sports.
So that's one.
There are probably outliers to this.
I heard that Killian Jornet had a response to our paper and he said something about it on his social media.
Maybe somewhere.
I couldn't find it.
Killian, if you're listening, I'd love to know what you think, because if anyone's an outlier...
I think it's you, man.
Yeah, get Killian in the lab.
We've got to hook him up to some testing here.
Yeah, I think I'd be really interested in not just cycling, but also cross-country skiing.
Cross-country skiing is typically the sport that the highest VO2 maxes are measured.
And also long-distance swimming.
Those long-distance swimmers can go... for like half a day swimming.
I mean, they swim for longer periods of time than many cyclists spend time in the saddle.
And I would just be interested in the energy expenditure, but also, if they're in the water, the effect of the water on their metabolism.
You know, one of the things that I remember reading about Michael Phelps and his, you know, kind of famous 7000 calorie a day diet because he's in the pool for four hours a day, or whatever it might have been is that it wasn't just the energy expenditure of his workouts.
It was also just the effect of being in the water and the fact that it's just a really good conductor of heat.
And in this case it's basically making you colder and your body has to work harder to maintain its core body temperature.
And that results in more calories being needed.
So any study on that kind of thing, I think, would just be super interesting and teach us a lot more about the sports specific demands.
You know, on that note, I just kind of want to reinforce.
All right, so I only really got into science.
So I was a science teacher for a long time.
I've only been a biology professor for about five years.
So I've only really been doing science for about 10 or 12 years.
And the first thing I noticed was that we don't know nearly as much as we think we do.
Not that what we know is wrong, but that so many questions that I have have not been tested.
We assume all kinds of things but in terms of actual controlled studies very little has been done regarding really metabolism and exercise metabolism.
So, for example, we don't really know yet whether lots of endurance exercise reduces your BMR or whether it increases it.
On the one hand, if you're always recovering, your BMR might be elevated because your body's working harder to repair itself.
But on the other hand, some of Herman Ponser's work shows that the more calories you spend on exercise, it seems you pull back on calories somewhere.
And your total energy expenditure is a little lower than you'd predict.
We still don't know why.
So there's a lot left to do here.
So our study really just like I don't know, threw a pebble here know, into the wishing well of uh making up a weird metaphor, here we.
It's just like one of hundreds of studies that need to be done to start figuring out how metabolism actually adjusts during exercise, what these limits are and why.
Well, at a minimum, i hope it stokes the curiosity of other scientists and and they get in on on some of these studies that we've talked about, because I have many questions and and I love having folks like you on the podcast to itch my own uh curiosity and and learn more about this kind of stuff.
So Andrew, thanks so much.
And you know, if folks want to learn more about you and your work is there anywhere they can go to do that.
Yeah.
So, um, yeah, if you want any of my papers, I can usually share them directly.
So Look me up on Google Scholar.
Some of the paper links are there.
I have an Instagram.
It's kind of just for my own running and biking and a little bit of science.
I'm Shreddy Professor.
Proud of that name.
But, you know, I don't know.
I'm off Twitter, you know, ever since Elon took it over.
So those are kind of the ways.
But anyone who has any questions, feel free to email me and I will do my best to get back to you with something.
Well, amazing offer.
Thank you, Andrew.
And thanks for your time today.
I really appreciate it.
Thank you, Jason.
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