Printed October 20, 2025 01:37PM
Again in 2019, scientists proposed a new theory of endurance. For efforts lasting greater than a few day, they prompt, the final word restrict is dictated by how a lot meals you’re in a position to digest. Your coronary heart and thoughts and muscle groups can adapt to do wonderful issues, however all of them want gasoline. Essentially the most energy you possibly can digest appears to be about 2.5 occasions your resting metabolism—in order that’s what limits how a lot bodily exercise you are able to do day after day over weeks, months, or years.
This concept of a “metabolic ceiling” sparked a lot of dialogue, however it additionally left some open questions. Does it actually apply to top-level endurance athletes—like, say, Kilian Jornet, who simply completed climbing 72 14,000-foot summits and biking 2,500 miles in simply 31 days whereas quaffing olive oil for energy? A newly revealed research in Current Biology units out to reply a few of these questions, measuring calorie information from 14 world-class ultrarunners and triathletes and analyzing the coaching logs of notable athletes like Jornet. Right here’s what they discovered.
What They Did
The research was led by Andrew Better of the Massachusetts Faculty of Liberal Arts and Herman Pontzer of Duke College, the latter of whom was one of many key authors of the unique 2019 paper. The important thing information within the paper comes from 14 ultra-endurance athletes who drank particular isotope-labeled water that enabled the scientists to calculate precisely what number of energy they have been burning at completely different occasions. They collected this information throughout occasions like a six-day ultramarathon, a 24-hour file try, and Joe McConaughy’s 13-day FKT on the Arizona Path. Additionally they collected calorie information throughout a number of coaching weeks, for causes we’ll get into beneath.
The calorie information from races blew via the theorical restrict of two.5 occasions resting metabolism. That’s as a result of you possibly can afford to enter calorie debt for brief intervals of time, that means that you just’re burning saved fats (and typically muscle) and reducing weight. “Joe misplaced tons of weight operating the Arizona Path,” Finest informed me. However that may’t proceed indefinitely. In case you’re burning 9,000 energy a day (as Jornet estimates he was throughout his most up-to-date problem) however solely consuming 7,000 energy a day, you would possibly be capable to preserve doing that for a month or two, however you’ll finally hit a wall.
That’s why Finest additionally measured energy throughout coaching weeks. By taking at the least two measurements for every runner, one throughout a contest or arduous coaching week and the opposite throughout a comparatively simple coaching week, he created a personalised system for every runner to estimate what number of energy they burn as a operate of how a lot they’re operating. Then he utilized this system to a yr’s value of coaching information to see what number of energy they may burn over a 12-month interval moderately than simply throughout every week or two of competitors. That’s the place the two.5 resting metabolism restrict exhibits up once more.
Right here’s a graph displaying “metabolic scope” (which is what number of energy per day you’re burning expressed as a a number of of resting metabolism) for various durations:
The darkish blue circles on the left aspect of the graph present the direct measurements of calorie burn throughout coaching and racing. There are values as excessive as seven occasions resting metabolism, which corresponds to a one-day file try on a 90-mile path.
The sunshine blue circles are calculated from the athletes’ coaching logs primarily based on coaching intervals of varied lengths. For instance, on the six-week mark (42 days), you possibly can see a variety of sunshine blue circles between about 2.5 and 4. The circle at 4 corresponds to a runner who ran an astounding 1,989 miles over a six-week interval, which is 332 miles per week. However that was throughout a 46-day FKT try on the Appalachian Path, so clearly not a degree the topic might maintain for a complete yr.
As you lengthen to longer durations like 30 or 52 weeks, you possibly can see that the sunshine blue circles all cluster round 2.5. Some are just a little larger, others just a little decrease, however none of those elite extremely athletes are sustaining values which might be considerably larger than the proposed restrict.
What about true super-elites like Kilian Jornet and triathlon star Kristian Blummenfelt? Primarily based on their publicly out there coaching information, together with the coaching hours-to-calories system that the brand new research generated, Finest estimates that Blummenfelt averages about 2.8 to 2.9 occasions his resting metabolism over the course of a complete yr, whereas Jornet hits 2.75. So the perfect of the perfect could edge barely above the standard restrict of two.5, however not by a lot.
What It Means
There are two attention-grabbing options within the graph I included above. The primary and most vital is the flat line on the fitting aspect of the graph, which corresponds to the proposed asymptote of two.5 primarily based on the boundaries of digestion. The brand new outcomes bolster my confidence that this actually is a constant phenomenon. If Jornet isn’t breaking it (by a lot), I don’t know who’s. So I used to be shocked, once I checked in with Herman Pontzer, to search out that he’s much less assured than he was in 2019 that that is an ironclad rule.
Certainly one of his causes is that extra information has emerged from elite cyclists at Grand Excursions the place they appear to be burning monumental numbers of energy with out reducing weight—which suggests that they’re absorbing a comparable variety of energy. A study of seven cyclists within the Giro d’Italia, for instance, discovered that they burned greater than 4 occasions their resting metabolism over the course of 24 days with out reducing weight. It might be that sports activities scientists’ quest to supply ever-more digestible carbohydrates is enabling cyclists to push again the boundaries of digestibility.
The opposite attention-grabbing function within the graph is the form of the curve on the left. You see an identical curve if you plot your velocity in shorter distance (i.e. just a few hours or much less) races in opposition to the time elapsed, as I did for my very own observe occasions here. In that state of affairs, the asymptote corresponds to a amount known as vital velocity, which represents your long-term sustainable tempo. The form of the curve is dictated by one other parameter typically known as anaerobic capability, which you’ll consider (very loosely) as the quantity of vitality you’re in a position to “borrow” when operating sooner than vital velocity earlier than you hit a wall. Milers and different middle-distance runners are likely to have a really excessive anaerobic capability.
One thing has to dictate the form of Pontzer’s multi-day vitality curve, and at this level he’s undecided what that one thing is. Intuitively, you possibly can consider it as analogous to anaerobic capability: you possibly can “borrow” plenty of energy for a brief time period, placing you method above the two.5 line; or you possibly can borrow a lesser quantity over an prolonged time period. If you wish to preserve going for, say, six months, you possibly can’t actually borrow something: energy out needs to be balanced by energy in.
However what determines the form of that curve? In case you’re carrying plenty of physique fats, does that allow you to borrow extra for longer? Or, extra possible, when you’ve educated your metabolism to burn fats extra quickly, does that elevate the curve? Does the exact form of the curve rely upon the combo of fats and carbohydrate that you just burn at completely different train intensities? Or are there different non-metabolic elements that come into play, like muscle restoration or psychological fatigue? The physiology of multi-day endurance challenges continues to be a comparatively younger scientific subject—which suggests there must be a lot of extra insights, and much extra enjoyable, nonetheless to return.
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