This idea of a \u201cmetabolic ceiling\u201d sparked lots of discussion, but it also left some open questions. Does it really apply to top-level endurance athletes\u2014like, say, Kilian Jornet<\/a>, who just finished climbing 72 1,400-foot summits and cycling 2,500 miles in just 31 days while quaffing olive oil<\/a> for calories? A newly published study in Current Biology<\/a> 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\u2019s what they found.<\/p>\n What They Did<\/p>\n The study was led by Andrew Best of the Massachusetts College of Liberal Arts and Herman Pontzer of Duke University, the latter of whom was one of the key authors of the original 2019 paper. 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<\/a>\u2019s 13-day FKT on the Arizona Trail. They also collected calorie data during one or more training weeks, for reasons we\u2019ll get into below.<\/p>\n The calorie data from races blew through the theorical limit of 2.5 times resting metabolism. That\u2019s because you can afford to go into calorie debt for short periods of time, meaning that you\u2019re burning stored fat (and sometimes muscle) and losing weight. \u201cJoe lost tons of weight running the Arizona Trail,\u201d Best told me. But that can\u2019t continue indefinitely. If you\u2019re 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\u2019ll eventually hit a wall.<\/p>\n That\u2019s 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\u2019re running. Then he applied this formula to a year\u2019s 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\u2019s where the 2.5 resting metabolism limit shows up again.<\/p>\n Here\u2019s a graph showing \u201cmetabolic scope\u201d (which is how many calories per day you\u2019re burning expressed as a multiple of resting metabolism) for different durations:<\/p>\n The dark blue circles on the left side of the graph show the direct measurements of calorie burn during training and racing. There are values as high as seven times resting metabolism, which corresponds to a one-day record attempt on a 90-mile trail.<\/p>\n The light blue circles are calculated from the athletes\u2019 training logs based on training periods of various lengths. For example, at the six-week mark (42 days), you can see a range of light 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 period, which is 332 miles per week. But that was during a 46-day FKT attempt on the Appalachian Trail, so clearly not a level the subject could sustain for an entire year.<\/p>\n As you extend to longer durations like 30 or 52 weeks, you can see that the light blue circles all cluster around 2.5. Some are a little higher, others a little lower, but none of these elite ultra athletes are sustaining values that are significantly higher than the proposed limit.<\/p>\n What about true super-elites like Kilian Jornet and triathlon star Kristian Blummenfelt? Based on their publicly available training data, along with the training hours-to-calories formula that the new study generated, Best estimates that Blummenfelt averages about 2.8 to 2.9 times his resting metabolism over the course of an entire year, while Jornet hits 2.75. So the best of the best may edge slightly above the usual limit of 2.5, but not by much.<\/p>\n What It Means<\/p>\n 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\u2019t breaking it (by much), I don\u2019t know who is. So I was surprised, when I checked in with Herman Pontzer, to find that he\u2019s less confident than he was in 2019 that this is an ironclad rule.<\/p>\n 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\u2014which implies that they\u2019re absorbing a comparable number of calories. A study<\/a> of seven cyclists in the Giro d\u2019Italia, 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\u2019 quest to produce ever-more digestible carbohydrates<\/a> is enabling cyclists to push back the limits of digestibility.<\/p>\n 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<\/a>. 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\u2019re able to \u201cborrow\u201d 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.<\/p>\n Something has to dictate the shape of Pontzer\u2019s multi-day energy curve, and at this point he\u2019s not sure what that something is. Intuitively, you can think of it as analogous to anaerobic capacity: you can \u201cborrow\u201d a lot of calories for a short period of time, putting you way above the 2.5 line; or you can borrow a lesser amount over an extended period of time. If you want to keep going for, say, six months, you can\u2019t really borrow anything: calories out has to be balanced by calories in.<\/p>\n But what determines the shape of that curve? If you\u2019re carrying a lot of body fat, does that enable you to borrow more for longer? Or, more likely, if you\u2019ve trained your metabolism to burn fat more rapidly, does that raise the curve? Does the precise shape of the curve depend on the mix of fat and carbohydrate that you burn at different exercise intensities? Or are there other non-metabolic factors that come into play, like muscle recovery or mental fatigue? The physiology of multi-day endurance challenges is still a relatively young scientific field\u2014which means there should be lots of more insights, and lots more fun, still to come.<\/p>\n For more Sweat Science, join me on Threads<\/a> and Facebook<\/a>, sign up for the email newsletter<\/a>, and check out my new book The Explorer\u2019s Gene: Why We Seek Big Challenges, New Flavors, and the Blank Spots on the Map<\/a>.<\/p>\n","protected":false},"excerpt":{"rendered":"Published October 20, 2025 01:37PM Back in 2019, scientists proposed a new theory of endurance. For efforts lasting…\n","protected":false},"author":2,"featured_media":239746,"comment_status":"","ping_status":"","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[61],"tags":[76663,97,269,80377,132298,80379,122019,70096],"class_list":{"0":"post-239745","1":"post","2":"type-post","3":"status-publish","4":"format-standard","5":"has-post-thumbnail","7":"category-nutrition","8":"tag-editor-awise","9":"tag-health","10":"tag-nutrition","11":"tag-parent_category-health","12":"tag-tag-endurance-training","13":"tag-tag-evergreen","14":"tag-tag-nutrition","15":"tag-type-article"},"_links":{"self":[{"href":"https:\/\/www.newsbeep.com\/us\/wp-json\/wp\/v2\/posts\/239745","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.newsbeep.com\/us\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.newsbeep.com\/us\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/us\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/us\/wp-json\/wp\/v2\/comments?post=239745"}],"version-history":[{"count":0,"href":"https:\/\/www.newsbeep.com\/us\/wp-json\/wp\/v2\/posts\/239745\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/us\/wp-json\/wp\/v2\/media\/239746"}],"wp:attachment":[{"href":"https:\/\/www.newsbeep.com\/us\/wp-json\/wp\/v2\/media?parent=239745"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.newsbeep.com\/us\/wp-json\/wp\/v2\/categories?post=239745"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.newsbeep.com\/us\/wp-json\/wp\/v2\/tags?post=239745"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}![\"The](\"https:\/\/www.newsbeep.com\/us\/wp-content\/uploads\/2025\/10\/graph.jpg\"\/)
The longer the duration, the lower the daily calorie burn you\u2019re able to sustain. (Photo: Current Biology)<\/p>\n