It really is too bad that these articles get written without any real thought put into them, clickbait I guess. They say the limit to human endurance is around 4000 calories burned a day, that is funny. Because I remember when me and my buddies were just training everyday for triathlon, and going through 6500 calories a day, no problem. Think I read somewhere that Phelps was consuming 6 to 7k calories a day too. So I'm pretty sure it is possible to efficiently burn through a lot more than 4k. And isn't training for a long endurance sport basically the same as a month or years long stage race??

If you are going to test the limits of human endurance on land, probably should start with the guys that run sub 2;10 marathons, and do the highest mileage. Perhaps maybe use the top ultra guys out there too. In swimming, find those rare swimmers that do 20 to 30k a day in the water. Using some event that really does not even begin with these kind of folks, well is just not illuminating for those of us that understand this stuff..

The 2.5x BMR asymptote to the curve occurs 300+ days in!

With all due respect to Monty, I have to say it would surprise me if you were putting in 300+ x 6500kcal days in a row. At, say an avg output of 200W and assuming a BMR of 1600kcal, that would be about 7 hours of cycling every day for a full year or ~a 2500 hour training year :-)

4000kcal/day expenditure under the same constraints is ~3.5hrs/day or a 1270 hr training year -- much closer to lining up with what happens in the 'real world' of elite endurance IME.

---

Alan Couzens, M.Sc. (Sports Science)
Exercise Physiologist/Coach
Twitter: https://twitter.com/Alan_Couzens
Web: https://alancouzens.com

It really is too bad that your posts get written without any real thought put into them,
phelps was exaggerating and so are you. When I get actual caloric intake counts from athletes training 20+ hours a week, if often far less than what they think, more in line w/ the 4.5k a day not the 6k they all seem to think they consume regularly.

Hey, let a brother have his "back in the day" story. :)

1600Kcal/day for BMR?

Where you getting that? For a 50 kg 1.6M tall kid?

Every calc I've done puts mine at 2200+ and I'm only 85 kg or so. I do 6 plus days a week over 4000 KCAL and at least 4 over 5k.... That's on 15 hrs a week of training

You're missing the forest for the trees. 1600 is the BMR for an average untrained person, per this article. If you'd like to apply your anecdote and use 2200 in his calculation, it's still an unrealistic training load for 300+ days in a row. The exact value doesn't detract from his conclusion.

The title does say "limit" not average so that should count for something.

But the research is saying 2.5x your BMR is the limit for long term sustainability. Doesn't matter what your BMR is or that it will change, their claim is that you will only be able to sustain 2.5x that value. I would think most athletes have a higher BMR than the regular person, so their max sustainable calorie intake would be higher.

---

Strava

When I get actual caloric intake counts from athletes training 20+ hours a week//

Ha, there is your problem. The guys I routinely trained with(the Germans) trianed 35 to 40 hours a week. Granted it wasn't for the entire year, but for the 6 months they would spend in San Diego, I bet it averaged mid 30+ hours a week. Now for me, I would only do some to the weeks with them, I was not an ironman specialist, and I just would rather go hard than long..


My 6500 came from a food study someone did on us for a month during high training in the winter. Of course it was not a yearly average, but I bet my buddies was pretty close to that..


Anyway, there you go, my back in the day story. (-;

Sorry to tell you this, Dave, but 85kg puts you in the "Big Unit" category when it comes to endurance sport :-) Most of the elite males (triathletes) that I've tested for BMR are in the 1600-1700 kcal range (@ ~70+/-kg body mass)

---

Alan Couzens, M.Sc. (Sports Science)
Exercise Physiologist/Coach
Twitter: https://twitter.com/Alan_Couzens
Web: https://alancouzens.com

If we are talking about the limit, it would be interesting to know how Kristian Blummenfelt scores. I seem to recall that he put up a story on instagram in which he showed 15xx hours over the last 365 days. I don't know what his BMR is but I think he is around 75-78kg.

Quoting for good measure...

---

808 > NYC > PDX > YVR
2024 Races: Taupo

Dang, this old man was in agreement with monty's observations and right away you bring us back to reality. Good on you.

---

Indoor Triathlete - I thought I was right, until I realized I was wrong.

I was about to post the journal article on this but remember seeing your post. I was interpreting this article to suggest the body's metabolism changes during the course of extreme endurance events to regulate energy expenditure and reducing its need for increased caloric intake. I remembered reading a study similar to this with husky's back in the day during the course of the Iditarod. I'm not a scholar of this type of research, but I feel like this has been disseminated with them (the dogs) for a lot longer. Its interesting the correlations, but I don't think its particularly surprising of animal's bodies to adapt temporarily to the catalyst affecting it.

https://frontierscientists.com/...urning-capabilities/


I was curious, but the human study seemed to only measure these rates before the race,during the first week, and then the last week. Did I miss this in the journal or perhaps get lost in jargon? It said there was a logarithmic relationship, but at the same time admits that the timing of metabolic compensation was unclear. So if the relationship is dependent upon duration of the event, if looked at with finer resolution would you see metabolic flux punctuated at times by changes in the fluctuations? I would assume the body obviously doesn't know what the duration is, so I am imagining more of a stepped-like decrease in the average metabolic compensation between the fluctuations. Its hard for me to articulate, but I am picturing like Jenks natural breaks in how the metabolic rate values get parsed into different classes that is dependent on duration (marathon, ultra, extreme-ultra, etc or whatever these durations would be called).

---

Use this link to save $5 off your USAT membership renewal:
https://membership.usatriathlon.org/...A2-BAD7-6137B629D9B7

The 2.5x BMR asymptote to the curve occurs 300+ days in! //

Well there is the disconnect in this discussion, that and the fact they are using regular people I guess? Do we really have any studies of elite athletes doing 300+ day races? Do we really have any good athletes doing that long of distances? I was thinking about the IronCowboy doing his 50 days of ironman's. What if he dialed back the run to say a 10k, would he be able to go a couple 100 days doing 8 to 12 hours a day? It would be interesting to test some of the high mileage runners for a year, I know it is not racing every day, but running 20+ miles a day certainly should push the body to endurance limits. Or maybe amaro cyclist that takes very few days off, and rides 4 to 8 hours a day..I now my German buddies would do some crazy miles and hours per week, and some would flourish, while others would wilt. A sorting out of the truly endurance gifted I suppose, like Iron Cowboy. Besides being able to train a 1/3 of the day, you also have to be able to hold your body together though all of that.


Read your blog on the evolution of adding mileage to elite runners, very interesting. We had kind of the opposite dynamic happen in triathlon. There was a long period where miles were increased, up until the point I described, 40 hour weeks were routine. But then I believe it was Dave Scott who finally saw that it was just too much, too slow. Then people began to dial it back down, and go harder. Of course we are unique in that we train for 3 sports, two of which allow unlimited hours without gravity damaging the body.


I really dont know what guys like Frodo or Gomez do these days, but I suspect it is closer to 30 hours, +/- a good amount...
And as I recall, there were runners experimenting with 150+MPW, as well as swimmers doing 30k a day during my era. SO I think the upper limits were experimented with, and tossed out for going faster and shorter, all relative of course...

You all realize that for one of the hardest 6 hour days in cycling that the pros are doing about 7000 kj. And you're trying to claim that you've got people training at nearly that every day for 6 months. That's 300's NP for that long. Even then, not sure how the ratio of KJ to calories works for the pros due to how efficient they burn versus we do. Probably not 7000 calories in those 6 hours.

I call BS.

The 4k/day seems reasonable. That 6700 cal a day burn rate doesn't seem right at all.

You all realize that for one of the hardest 6 hour days in cycling that the pros are doing about 7000 kj. //

But what if it were an hour of swimming, an hour+ of running, and 5 hours of cycling? That was a routine day for those guys. And yes, at some point they come as efficient as they can be, and would burn less calories than they did at the beginning. I really don't know how many calories everyone burns, just gave my small sample in a short period at my peak. Maybe they counted them wrong, but lots of guys got 6k numbers back then too. What do they say an ironman burns, 10k or so? Ironwocboy did 50 of those, 50 days in a row, how does that jive??

that is an interesting point..
my son is 80kg and loses weight during swim season, eating 5-6k calories daily. He says, "I get so tired of eating"..
suspect his BMR is unusually high, partly from being young, partly from lifting weights and building a lot of muscle, partly genetic - he metabolizes anesthetics very fast for example, woke up twice during wisdom tooth extraction and they kept having to dope him up again..



thank you, interesting study. The odd thing about dogs is they can burn fat directly for energy so their metabolism is quite different from humans. I learned this from my gun dog - the energy bars for hunting dogs are mostly fat, since that is what they use rather than carbs.
https://scottlindenoutdoors.com/dog-energy-bar/
"Kronch Pemmikan was developed by Henne Pet Food of Denmark, in conjunction with the Danish army’s Sirius Patrol, the dogsled teams working the harshest, most demanding environment on earth: the icy northern shores of Greenland. If it can fuel their dogs, it can energize your hunting dog."

Alex Hutchinson makes sense of it at,

https://www.outsideonline.com/2397917/human-endurance-limit-study

Here's the study linked in the article:

https://advances.sciencemag.org/content/5/6/eaaw0341

The suggestion in the study is that the limit is energy intake/absorbtion. In other words its digestivly limited, not muscular performance limited. So, it doesn't matter which muscles you use on any given day or what your mode of exercise is. Ultimately, you can't ABSORB enough (even if you eat more). The study makes this claim by looking at overfeeding studies versus energy output versus weight gain/loss, and shows that the human body is limited long-term to the ~2.5xBMR absorbtion value, out at the 300 consecutive day range.

You can do more than that for sustained periods of time, but you will loose weight as your body consumes itself to make up the absorbtion deficit. Eventually, you run out of fat stores and muscles to consume.

Intensity, that's how.

IF = NP / FT

Assume 330w NP for a Paris Roubaix rider. Assume 420w hour power. That's nearly .8.

Relating Paris Roubaix to Kona, the intensity factors workout overall to be about .8. But, those are races. Not training days done everyday for months.

You're saying that the people in training are going to go at Paris Roubaix and Kona I.F. everyday, and for essentially as long or longer.

The math simply doesn't work. It only works, nearly, on race day. Even then, probably not as KJ is not equal to calories in elite endurance athletes. Their economies are better than Joe Schmuck like me who can approximate 1:1 for KJ and calories.

Your math is essentially saying that we should already have a few dozen successful Iron Cowboy or Ashley Horner types out there.........if they are already burning that everyday in training anyway!

That was a good read. He explains things very well.

You're saying that the people in training are going to go at Paris Roubaix and Kona I.F. everyday, and for essentially as long or longer. //

No, not what I'm saying. But first of all, I was using 10k as my Kona burn, perhaps that is wrong. So not doing a Kona everyday, but 60% of a Kona to hit the 6k. But it makes more sense as I think about it, you are talking about absorption, not burn necessarily. But here is my question on that front. If every single factor in the human body works differently from others, like there are people born with 60% HCT, others less than 30, VO2 max over 90, others under 30, size of hemoglobin or mitochondria, lung volume, and on and on. Would not absorption rates also be variable, like there is the worlds best and worst? Perhaps the outliers I'm talking about are at the high end of that bell curve? Or are we to assume that this 4k is the limit, as in upper limit?


It would be fun to see what the Ironcowboy actually did, moving for 12 to 18 hours a day for 50 days, and how much he ate, and how much weight he lost too. I forgot to figure in out those weight losses in these events, but he was pretty lean going is as I recall..Good stuff here, getting some old brain cells working again, and increasing my daily burn rate...(-;

The Hutchinson article touches on this a bit. Because their conclusions are based on some sparse data,they can't really pinpoint a specific number. Rather they just show that it clearly flattens out. As such this represents a clear limit on long term endurance performance. At some point (maybe 2.5 bmr, maybe 3.0 bmr) you can't absorb calories fast enough to sustain aerobic activity and if you do this long enough you'l run out of fuel (ie fat, muscle etc). So perhaps what this study does is provide a starting point into further research pinpointing what this number actually is, how it works, can it be trained. etc.