Wissenschaftler erforschen die Zusammenhänge zwischen der Menge an Kalorien, die Sie verdauen können, und den ultimativen Grenzen der Ausdauer | Ultra-Ausdauersportler und die Stoffwechselgrenze

Some highlights from the magazine piece:

>Back in 2019, scientists proposed a new theory of endurance. For efforts lasting more than about a day, they suggested, the ultimate limit is dictated by how much food you’re able to digest. Your heart and mind and muscles can adapt to do amazing things, but they all need fuel. The most calories you can digest seems to be about 2.5 times your resting metabolism—so that’s what limits how much physical activity you can do day after day over weeks, months, or years.
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>This idea of a “metabolic ceiling” sparked lots of discussion, but it also left some open questions. Does it really apply to top-level endurance athletes—like, say, Kilian Jornet, who just finished climbing 72 14,000-foot summits and cycling 2,500 miles in just 31 days while quaffing olive oil for calories? A newly published study in Current Biology sets out to answer some of these questions, measuring calorie data from 14 world-class ultrarunners and triathletes and analyzing the training logs of notable athletes like Jornet. Here’s what they found.
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>The key data in the paper comes from 14 ultra-endurance athletes who drank special isotope-labeled water that enabled the scientists to calculate exactly how many calories they were burning at different times. They collected this data during events like a six-day ultramarathon, a 24-hour record attempt, and Joe McConaughy’s 13-day FKT on the Arizona Trail. They also collected calorie data during one or more training weeks, for reasons we’ll get into below.
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>The calorie data from races blew through the theorical limit of 2.5 times resting metabolism. That’s because you can afford to go into calorie debt for short periods of time, meaning that you’re burning stored fat (and sometimes muscle) and losing weight. “Joe lost tons of weight running the Arizona Trail,” Best told me. But that can’t continue indefinitely. If you’re burning 9,000 calories a day (as Jornet estimates he was during his most recent challenge) but only consuming 7,000 calories a day, you might be able to keep doing that for a month or two, but you’ll eventually hit a wall.
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>That’s why Best also measured calories during training weeks. By taking at least two measurements for each runner, one during a competition or hard training week and the other during a relatively easy training week, he created a personalized formula for each runner to estimate how many calories they burn as a function of how much they’re running. Then he applied this formula to a year’s worth of training data to see how many calories they could burn over a 12-month period rather than just during a week or two of competition. That’s where the 2.5 resting metabolism limit shows up again.
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>There are two interesting features in the graph I included above. The first and most important is the flat line on the right side of the graph, which corresponds to the proposed asymptote of 2.5 based on the limits of digestion. The new results bolster my confidence that this really is a consistent phenomenon. If Jornet isn’t breaking it (by much), I don’t know who is. So I was surprised, when I checked in with Herman Pontzer, to find that he’s less confident than he was in 2019 that this is an ironclad rule.
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>One of his reasons is that more data has emerged from elite cyclists at Grand Tours where they seem to be burning enormous numbers of calories without losing weight—which implies that they’re absorbing a comparable number of calories. A study of seven cyclists in the Giro d’Italia, for example, found that they burned more than four times their resting metabolism over the course of 24 days without losing weight. It may be that sports scientists’ quest to produce ever-more digestible carbohydrates is enabling cyclists to push back the limits of digestibility.
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>The other interesting feature in the graph is the shape of the curve on the left. You see a similar curve when you plot your speed in shorter distance (i.e. a few hours or less) races against the time elapsed, as I did for my own track times here. In that situation, the asymptote corresponds to a quantity called critical speed, which represents your long-term sustainable pace. The shape of the curve is dictated by another parameter sometimes referred to as anaerobic capacity, which you can think of (very loosely) as the amount of energy you’re able to “borrow” when running faster than critical speed before you hit a wall. Milers and other middle-distance runners tend to have a very high anaerobic capacity.

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Journal link: [Ultra-endurance athletes and the metabolic ceiling](https://www.cell.com/current-biology/abstract/S0960-9822(25)01129-7)

Summary:

>Endurance limits, measured as the maximum sustainable rate of caloric expenditure, constrain human performance from athletics to manual labor. Early work on the metabolic limits of humans and other species focused on establishing a single estimate for maximal sustained expenditure, often expressed as a multiple of basal metabolic rate (BMR; kcal/day) and known as “metabolic scope” (MS). More recently, Thurber and colleagues proposed a duration-dependent metabolic ceiling for humans, in which the limit of sustained expenditure falls in a semi-log manner from ∼10× BMR for events lasting 1 day and reaches an asymptote of ∼2.5× BMR at approximately 28 weeks. We tested this metabolic ceiling model in 14 highly trained, elite, and world class ultra-endurance athletes (12 males and 2 females; 37.2 ± 7.0 years). We measured MS (total energy expenditure [TEE]/BMR) using doubly labeled water during ultra-endurance competitions spanning ∼24 h to ∼13 days, and lower- and higher-workload training weeks, to generate predictive equations for MS as a function of exercise volume. We then applied these equations to participants’ training records to calculate their maximum MS for 1-, 3-, 6-, 12-, 30-, and 52-week periods. Maximum MS for competitions and training periods up to 12 weeks approached, but never exceeded, the proposed metabolic ceiling. Four athletes exceeded 2.5× BMR at 30 and 52 weeks (maximum: 2.74), but mean MS for the sample at 30 weeks (2.43 ± 0.17; TEE: 4,085 ± 642 kcal/day) and 52 weeks (2.39 ± 0.17; TEE: 4,020 ± 641 kcal/day) did not exceed the proposed limit. These results suggest that exceeding the hypothesized duration-dependent limit of energy expenditure is rare.