Like all great villains, lactic acid has been misunderstood. We鈥檝e been blaming it for the pain we suffer during intense exercise for more than two centuries. There鈥檚 nothing worse, we say, than the 鈥渓actic burn鈥� that locks our failing muscles into immobility. More recent tellings of the story have tried to rehabilitate lactic acid鈥檚 reputation, insisting that it鈥檚 actually trying to fuel our muscles rather than shut them down. But that version 诲辞别蝉苍鈥檛 capture the full complexity, either.

Into this confusion steps , from veteran physiologists Simeon Cairns and Michael Lindinger. It鈥檚 a dense 35-page doorstop titled 鈥淟actic Acidosis: Implications for Human Exercise Performance,鈥� and the clearest conclusion we can draw from it is that the precise causes of muscle fatigue during intense exercise are still a topic of active research and vigorous debate among scientists. But the sudden popularity of baking soda as an acid-buffering performance aid has renewed conversations about how, exactly, lactic acid works in the body鈥攁nd how we might counteract it. Here are some highlights from the latest research.

The Lactic Backstory

The first scientist to draw the connection between exercise and lactic acid was J枚ns Jacob Berzelius, the Swedish chemist who devised the modern system of chemical notation (H2O and so on). Sometime around 1807, he noticed that the chopped-up muscles of dead deer contained lactic acid, a substance that had only recently been discovered in soured milk. Crucially, the muscles of stags that had been hunted to death contained higher levels of lactic acid, while deer from a slaughterhouse who had their limbs immobilized in a splint before their death had lower levels, suggesting that the acid was generated by physical exertion.

A century later, physiologists at the University of Cambridge used electric stimulation to make frogs鈥� legs twitch until they reached exhaustion, and high lactic acid levels. The levels were even higher if they performed the experiment in a chamber without oxygen, and lower if they provided extra oxygen. That finding helped establish the prevailing twentieth-century view: your muscles need oxygen to generate energy aerobically; if they can鈥檛 get enough oxygen, they switch to generating energy anaerobically, which produces lactic acid as a toxic byproduct that eventually shuts your muscles down.

There are two small problems鈥攁nd one big one鈥攚ith this picture. The first detail is that, while lactic acid can be measured in the muscles of dead deer and frogs, it 诲辞别蝉苍鈥檛 actually exist in living humans. In the chemical milieu of the body, what would be lactic acid is split into two components: lactate and hydrogen ions. That鈥檚 not just being persnickety about terminology: lactate and hydrogen ions behave differently than lactic acid would. In fact, they can have separate and sometimes even opposing effects.

The second detail is that lactate (and hydrogen ions) aren鈥檛 really produced because your muscles are 鈥渞unning out of oxygen.鈥� The chemical reactions that use oxygen to turn food into muscle fuel are efficient but slow, great for powering relatively easy and sustained exercise. But they can鈥檛 provide energy fast enough to supply an all-out sprint. For that, you鈥檒l eventually need to rely on lactate-producing anaerobic reactions, even if you鈥檙e huffing pure oxygen from a can.

The big problem with the old view of lactic acid is the idea that it’s a metabolic villain. It turns out that, far from being an inert byproduct, lactate can be recycled into fuel for your muscles. In fact, one of the key superpowers that well-trained athletes develop is the ability to reuse lactate more quickly. This rehabilitation of lactate鈥檚 reputation has been going on for now (though it still has ), but athletes are still left with an unanswered question: if lactate isn鈥檛 what causes muscle fatigue, what is?

What the New Review Reveals

The first thing that Cairns and Lindinger establish is that, yes, levels of lactate and hydrogen ions increase during intense exercise. This is most obvious during intense exercise lasting between about one and twenty minutes. Longer bouts of exercise are less intense, so they can be mostly fueled by non-lactate-producing aerobic energy, and bouts of exertion shorter than one minute simply don鈥檛 have time to produce much lactate.

The evidence is now clear that lactate itself 诲辞别蝉苍鈥檛 interfere in any significant way with muscle function. But lactate and hydrogen ions are produced simultaneously in exactly the same quantities during anaerobic exercise, which complicates the 鈥渓actic acid is a good guy after all鈥� narrative. Lactate may be great, but it comes with an equivalent helping of hydrogen ions鈥攁nd that may be a problem.

When you increase the concentration of hydrogen ions in a solution, you鈥檙e increasing its acidity. That鈥檚 how the pH scale is defined: it鈥檚 a measure of hydrogen ion concentration. During intense exercise, the pH in your fast-twitch muscle fibers (which seem to be particularly susceptible to hydrogen ion buildup) can drop from around 7.0 to 6.0. That change represents a ten-fold increase in the concentration of hydrogen ions鈥攁 situation that can wreak havoc on muscle contraction.

The idea that hydrogen ions are what cause muscle fatigue isn鈥檛 entirely straightforward either, though. When you start hard exercise, the concentration of hydrogen ions actually decreases for about 15 seconds while you use up another source of fast-acting muscle energy called phosphocreatine. And yet your muscles are already getting fatigued during this initial burst, losing some of their maximal force, while hydrogen ion levels are still lower than normal.

There鈥檚 also a disconnect when you stop exercising, or take a break between hard intervals. Hydrogen ion (and lactate) levels keep climbing for a few minutes, which is why the highest lactate levels are generally recorded several minutes after hard exercise. But you don鈥檛 get weaker after you stop exercising; you get stronger as you recover, despite the rising concentration of hydrogen ions. So hydrogen ions may play a role in muscle fatigue, but they can鈥檛 be the whole story.

Another possibility is that hydrogen ions may interact with other molecules to disrupt muscle contraction. The most prominent candidates are potassium and phosphate, both of which increase during exercise and are associated in some studies with muscle fatigue. What these and other candidates have in common is that there are a ton of conflicting results: they have different effects on muscle fibers depending on the level of acidity, the muscle temperature, and the test protocol. This suggests鈥攏ot surprisingly鈥攖hat there isn鈥檛 a single molecule that causes your muscles to lose their power. Instead, it鈥檚 the whole cocktail of things going on inside your muscles during hard exercise that matters.

What About the Burn?

Most of the research that Cairns and Lindinger describe deals with muscle properties: how quickly are your fibers losing their twitch force, and why? It鈥檚 true that, as a middle-distance runner, I鈥檝e sometimes staggered down the finishing straight of a race with the sense that my legs were literally ceasing to function. It鈥檚 an awful feeling to experience, but satisfying to look back on: you know you left nothing out there.

Far more common, though, is a softer limit. You feel a red-hot burn and spreading numbness in your legs, and you choose to back off a bit. This feeling that we used to describe as 鈥済oing lactic鈥� is significant in its own right. In interviews with athletes who鈥檝e begun using baking soda, a common theme is that they鈥檙e able to push harder for longer before feeling that burn in their legs, which in turn enables them to race faster.

One theory about the feeling of going lactic is that you鈥檙e literally starving your brain of oxygen. If you push hard enough, it鈥檚 not just your muscles that go more acidic; your whole bloodstream follows. Thanks to a phenomenon called the Bohr effect, rising acidity reduces the ability of your red blood cells to ferry oxygen from your lungs to the rest of your body, including your brain. In one study, all-out rowing caused oxygen saturation to drop from 97.5 to 89.0 percent, which is a big drop鈥攂ig enough, perhaps, to slow you down and contribute to the out-of-body feeling at the end of hard races.

We also have nerve sensors that keep the brain informed about the metabolic status of the muscles. These group III/IV afferents, as they鈥檙e known, keep tabs on the real-time levels of molecules like lactate and hydrogen ions. If you block these nerves with spinal injections of fentanyl, exercise feels great鈥攖oo great, in fact, because you鈥檒l lose all sense of pacing, go out too hard, then hit the wall.

The most telling finding about the lactic burn, in my view, was where they injected various molecules into the thumbs of volunteers in an attempt to reproduce that familiar feeling. Injecting lactate didn鈥檛 do it. Neither did injecting hydrogen ions, or ATP, a fuel molecule whose levels are also elevated during hard exercise. Injecting them in pairs didn鈥檛 do it either. But injecting all three at the levels you鈥檇 experience during moderate exercise produced a sensation of fatigue in their thumbs, even though they weren鈥檛 moving them. And injecting higher levels turned fatigue into pain.

That鈥檚 a distinction I try to keep in mind in the late stages of hard workouts, and at the crux of races. That burning feeling is real, and it鈥檚 associated with lactate and acidity and muscular fuel levels. But it鈥檚 just a feeling. The lactate and ATP are actually helping me. The hydrogen ions, in combination with various other metabolites accumulating in my muscles, not so much. They鈥檒l eventually stop me. But until they do, I can keep pushing.

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For a while, it really looked as though beet juice would beat the odds. Most hot new performance-boosting supplements, even if they claim to be backed by science, don鈥檛 hold up to scrutiny. But after making thanks to high-profile adherents like marathon star Paula Radcliffe, the first wave of high-quality studies supported the idea that beet juice really does improve endurance.

After a decade, though, the bloom had partly faded. There were concerns about its gastrointestinal effects (much as there were with baking soda, another popular endurance-booster), questions about the appropriate dosage, and rising suspicion that beet juice only worked in untrained or recreational athletes but not in serious competitors. These days I rarely hear runners talking about beet juice, and the flow of new studies has tailed off. But a new review takes a fresh look at the accumulated evidence, and concludes that we shouldn鈥檛 be too quick to dismiss the potential benefits of the juice.

Why Beet Juice Might Help

The key ingredient in beet juice, from an endurance perspective, is nitrate. Once you eat it, bacteria in your mouth convert nitrate to nitrite. Then the acidity in your stomach helps convert the nitrite to nitric oxide. Nitric oxide plays a whole bunch of roles in the body. That includes cueing your blood vessels to dilate, or widen, delivering more oxygen to the muscles, faster.

In 2007, Swedish researchers that consuming nitrate鈥攖hat nitric oxide precursor鈥攎akes exercise more efficient, enabling you to burn less oxygen while sustaining a given pace. Two years later, a team led by Andrew Jones at the University of Exeter that you could get a similar effect by drinking nitrate-rich beet juice.

In subsequent years, researchers tested the effects of beet juice on various types of exercise. Crucially, Jones鈥檚 group figured out how to strip the nitrate from beet juice to create an undetectable placebo, and found that athletes improved their performance when given regular beet juice but not nitrate-free beet juice. That made the claims much more convincing. Meanwhile, a company called began selling beet juice with standardized nitrate levels, and eventually added to make the doses more palatable.

When the International Olympic Committee put together on sports supplements in 2018, they included beet juice as one of just five performance-boosting supplements with solid evidence. (The others were caffeine, creatine, baking soda, and beta-alanine.)

What the New Review Found

Over the years, scientists have made numerous attempts to sum up the evidence for and against beet juice. The latest attempt, by a group led by Eric Tsz鈥慍hun Poon of the Chinese University of Hong Kong, is an 鈥渦mbrella review鈥� of nitrate supplementation, mostly from beet juice. It pools the results of 20 previous reviews that themselves aggregated the data from 180 individual studies with a total of 2,672 participants.

The problem with lumping that many studies together is that they measure outcomes differently, use different dosing protocols, and have different study populations. Still, the broad conclusion is that beet juice works鈥攁t least for some outcomes. Most significantly, it improves time to exhaustion: if you鈥檙e asked to run or cycle at a given pace for as long as you can, beet juice helps you go for longer.

On the other hand, there was no statistically significant benefit for time trials, where you cover a given distance as quickly as possible. That鈥檚 the type of competition we care about in the real world, so this non-result is concerning. Time-to-exhaustion tests produce much bigger changes than time trials: a common rule of thumb is that a 15 percent change in time to exhaustion corresponds to about one percent in a time trial. So it may simply be that the studies were too small to detect subtle improvements in time trial performance.

Check out the relative effect sizes for time to exhaustion and time trial in these forest plots. Each dot represents an individual study with its error bar; the farther to the right of the vertical line it is, the greater the performance boost nitrate provided.

Taking the time trial data at face value, the results still look pretty encouraging. They鈥檙e all positive; they just need more participants so that the error bars will get smaller and no longer overlap zero. Of course, eyeballing the data like that is risky because it allows us to draw whatever conclusions we want. But I find it difficult to imagine a scenario where improving your time to exhaustion 诲辞别蝉苍鈥檛 also translate into an advantage in time trials. The two tasks are different psychologically, but they both rely on the same underlying physiological toolset.

Poon and his colleagues also run some further analysis to check whether the dose makes a difference. They conclude that the effects are biggest when you take at least 6 mmoL (just under 400 milligrams) of nitrate per day, which happens to be almost exactly how much a single concentrated shot of beet juice contains. The effects are also maximized when you supplement for at least three consecutive days rather than just taking some on the day of a race.

What We Still Don鈥檛 Know

The big open question that Poon鈥檚 review 诲辞别蝉苍鈥檛 address is whether beet juice works in highly trained athletes. Several studies have found that the effect is either diminished or eliminated entirely in elite subjects. This isn鈥檛 surprising. Pretty much every intervention you can think of, including training itself, will have a smaller effect on people who are already well-trained. This ceiling effect is presumably because elite athletes have already optimized their physiology so thoroughly that there鈥檚 less room to improve.

The flip side of that coin is that, for elite athletes, even minuscule improvements can be the difference between victory and defeat. The size of a worthwhile improvement at the highest level is a fraction of a percent, which is all but impossible to reliably detect in typical sports science studies. For top athletes, the decision of whether or not to use beet juice will have to remain an educated guess for now.

There are other unanswered questions, like whether beet juice is better than consuming nitrate straight. There have been several studies suggesting that this is indeed the case. The theory is that other ingredients in beet juice, like polyphenols鈥攚hich function as antioxidants鈥攎ight act synergistically with nitrate to produce a bigger effect. But as pointed out last year, the evidence for this claim is too shaky to draw any reliable conclusions either way.

Probably the biggest risk in the beet juice data is the preponderance of small studies, some with fewer than ten subjects. It鈥檚 easy to get a fluke result with small sample sizes, and it鈥檚 human nature to get unduly excited about positive results鈥攚hich is why positive flukes often get published more often than negative flukes. So we should remain cautious about our level of certainty.

Despite that caveat, my overall impression is positive. I sent the following summary to Andy Jones, the scientist most associated with beet juice research, to see whether he would agree:

鈥淚t works. It probably works less well in elites, like most things, but there may still be an effect. Higher doses taken for at least a few days in a row probably increase your chances of a positive effect.鈥�

Jones thought that sounded reasonable. He pointed out that there鈥檚 a 聽of evidence emerging that beet juice also enhances muscle strength and power in some circumstances, an effect that Poon鈥檚 review confirms. For endurance specifically, looking at the totality of evidence, Jones figures there鈥檚 a real effect. And he鈥檚 in good company. 鈥淓liud remains a big believer,鈥� he pointed out. That would be Eliud Kipchoge.

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As headlines go, 鈥淪ocial Media is Bad鈥� 诲辞别蝉苍鈥檛 raise many eyebrows these days. TikTok and its ilk are said to be harming mental health, stifling creativity, eroding privacy, fueling disinformation, undermining national security, and so on. These are all big issues worthy of careful debate. But there鈥檚 a narrower and more tangible risk that Sweat Science readers might be concerned about. What if social media is making us slower?

A , from Carlos Freitas-Junior of the Federal University of Paraiba in Brazil and his colleagues, presents data on what happens when athletes scroll on their phones before training sessions. Surprisingly, it 诲辞别蝉苍鈥檛 just mess with that specific workout. Instead, over time, the athletes make smaller gains in performance. The findings tell us something about social media鈥攁nd they also suggest that the benefits of a workout may depend in part on the state of mind you鈥檙e in while doing it.

The Problem(s) With Social Media

Several studies over the years have examined social media use in athletes. Most famously, back in 2019 found an association between late-night tweeting (as it was then called) and next-day game performance in NBA players. If the players were tweeting after 11:00 P.M., the players tended to score fewer points, grab fewer rebounds, and shoot less accurately the next day.

You might argue鈥攃orrectly鈥攖hat the problem here is sleep deprivation rather than social media. But have found direct links between the usage of apps such as TikTok and sleep patterns in young athletes, suggesting that the root of the problem is the apps. Researchers have also linked social media use to mental well-being and even eating disorders in athletes, both of which impact performance.

These indirect impacts aren鈥檛 always straightforward: the TikTok-hurts-sleep study also found that Instagram usage was associated with greater calmness, for example. But there鈥檚 also a more immediate concern, which is that social media apps leave you mentally fatigued, which in turn directly compromises your endurance and decision-making abilities.

The Mental Fatigue Debate

The study that kicked off the modern conversation about mental fatigue in sport was a 2009 experiment from a researcher named Samuele Marcora. He showed that 90 minutes of doing a cognitively challenging computer task by about 15 percent compared to spending 90 minutes watching a documentary.

More studies followed, each investigating different types of mental fatigue and their effects on different types of athletic performance. Many of them echoed Marcora鈥檚 original results, but . One of the big unresolved questions is the extent to which the findings apply in real life. If you have to write an exam or do your taxes right before you run a marathon, that鈥檚 probably bad news. But what about the normal activities we engage in on a daily basis鈥攍ike scrolling through the social media apps on your phones? Do they induce sufficient mental fatigue to affect performance?

Back in 2021, found that 30 minutes of social media use hurt athletes鈥� times in 100- and 200-meter freestyle trials, but not in the 50 meters. found that boxers made worse decisions after using social media, but that their jumping performance was unaffected. found no effect of social media use on strength training performance. These results are consistent with the general pattern of research on mental fatigue and related stressors like sleep deprivation: with sufficient motivation, you can still exert maximal force, but your decision-making and endurance may be compromised.

What the New Data Shows

Freitas-Junior鈥檚 new study looks at volleyball players, testing their jumping performance and their 鈥渁ttack efficiency,鈥� a measure of how hard and how accurately they can hit the ball in a sequence of attacks. What鈥檚 different about the study is that it looked at long-term rather than immediate effects. Fourteen athletes spent half an hour before practice either using Facebook, WhatsApp, and Instagram on their phones, or watching documentaries about the history of the Olympics. After three weeks, their performance was assessed and then they switched groups and repeated the process for another three weeks.

At the end of the three-week period, jumping performance wasn鈥檛 affected under either condition, but athletes鈥� attack efficiency was worse following the three weeks of social media use. The difference was statistically significant, but to be honest the data isn鈥檛 very convincing.

For starters, take a look at the mental fatigue data. This shows how much, on average, mental fatigue (on the vertical axis) increased after watching the documentary (DOC) or using social media (SMA):

This is nice clean data. Watching the documentary increased the subjective perception of mental fatigue in almost every individual. Using social media increased it even more, again with uniform results in all the individuals. We can say with confidence that social media use increases mental fatigue compared to chilling with a doc.

Now take a look at the attack efficiency data, measured in arbitrary units where a higher number is better:

This time the individual data is all over the map. The statistical analysis tells us that, on average, the social media group got worse while the documentary group got better. This average effect may or may not be real鈥攐nly more and larger studies can confirm if it is. Based on the body of previous research, I鈥檇 guess that it鈥檚 probably real. But the pattern is so inconsistent on an individual level that I鈥檇 hesitate to use it as a basis for advice to athletes. Some athletes got better after social media use. That might be a fluke, or it might indicate that they have a healthier relationship with their apps such that a little phone time before practice gets them in a better headspace.

In the end, then, the narrative isn鈥檛 as tidy as we might like. It鈥檚 not that social media is uniformly bad, will leave you mentally fatigued, and will automatically rob you of training gains. There鈥檚 still a valuable message here, though. The things we do鈥攕ocial media, yes, but also real-world socializing, reading a book, listening to music, working, commuting, daydreaming, and so on鈥攁ffect our mental state and readiness to perform. We all respond to these things differently, so there鈥檚 no universal list of dos and don鈥檛s. But it鈥檚 worth figuring out what gets you in the right headspace and leaves you mentally energized, so that you can replicate it when it matters.

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The course has won the 2025 Barkley Marathons.

Or really, we should say the winner is race founder . For the first time since 2022, there are no finishers at the Barkley Marathons. Three-time finisher John Kelly completed loop three in 39 hours, 50 minutes, and 27 seconds in the wee hours of Thursday morning, Barkley Marathons Chief Resident Tweeter (or should we now say, Xer?) .

In doing so, Kelly dipped under the cut-off by 10 minutes to earn a 鈥渇un run鈥� before tapping himself out. The course, which some believe included a new nasty 45-minute section to this year, was just too hard. Only two other runners made it onto loop three, Tomokazu Ihara (Japan) and S茅bastien Raichon (France.) Raichon returned to camp five minutes later without completing the loop. Two hours later Ihara returned, well beyond the cut-off.

The 2025 Barkley Marathons is over. There are no finishers.

鈥� Keith (@keithdunn)

While we鈥檙e disappointed that we鈥檙e robbed from 20 more hours of entertainment, really, we can all breathe a sigh of relief:

The Barkley Marathons can rightfully maintain it鈥檚 moniker as 鈥渢he world鈥檚 hardest race.鈥�

It鈥檚 the 25th time in 40 years that the Barkley Marathons has no finishers. Only 20 people total have ever done it. The 40th edition is the least 鈥渟uccessful鈥� (or should we say most successful?) since 2018, when similarly just one runner, Gary Robbins, completed a 鈥渇un run.鈥�

Kelly, who鈥檚 on a quest to tie Jared Campbell鈥檚 record of four finishes, will just have to come back again next year, armed with even more experience and grit.

Wondering what the Barkley Marathons is all about? Head over to our to learn about everything the rules, the course, the history and lore, and why an event that sounds an awful lot like orienteering actually isn鈥檛 orienteering at all.

Welcome to the 2025 Barkley Marathons

For those in the nichest of niche ultrarunning circles, Christmas came early. At 11:37 A.M. Eastern on Tuesday, March 18, the 2025 Barkley Marathons began.

The start date and time of this race that鈥檚 as fabled as it is mysterious change every year. But the third week of March is historically early. Perhaps race founder and his successor, Carl Laniak, were hopeful that pushing the event into the middle of March would bring colder, wetter, windier, and all-around grosser weather. After all, . And, ostensibly, they simply can鈥檛 let that happen again.

But if they were looking for miserable weather, the weather gods had their own tricks up their sleeves. A cold but clear Monday night gave way to a sunny Tuesday morning with temperatures projected to reach into the high 60s or even the low 70s, according to . That鈥檚 pretty damn perfect, for the runners anyway.

Of course, Laz and Laniak have other curve balls they can throw in the runners鈥� way. The biggest one, of course, is simply to make the course even harder. While the race is always five 20- to 26-mile loops and runners always have 13 hours and 20 minutes to complete each loop and 60 hours to complete the whole event, the course itself changes every year. But you can always expect a lot of nasty hills, prickly briars, and off-trail shwacking for a total of 120-130 miles and 60,000 feet of gain. No course markings, and no GPS devices allowed. Runners claimed last year鈥檚 edition was bramblier than ever, and yet more runners than ever rose to the challenge.

Here are our live updates from the 2025 Barkley Marathons, in reverse chronological order:

40 Hours Elapsed: The Course Wins

Kelly tapped himself out after earning a 鈥渇un run.鈥� Raichon returned to camp five minutes later in 39:55, but did not complete loop three, . Ihara made it back to camp two hours later.

Why did Kelly quit? With just 20 hours and 10 minutes left to run the final two loops after finishing loop three, time鈥�and any modicum of sanity鈥�was not on his side. When Kelly successfully finished the Barkley Marathons last year his splits on the final two loops were 14:10 and 13:30, respectively. We have 40 years of data showing the compounding effects of sleep deprivation, exhaustion, and delirium magnify exponentially over the final two loops at this race.

While in past years finishers have had some time to recoup in camp between loops, if they so chose, this year鈥檚 particularly brutal course forced runners fast enough to complete a loop in time to essentially head straight back out. Kelly spent less than 10 minutes in camp between loops two and three.

For the first time since 2022 and for the 25th time in 40 years, the Barkley Marathons has no finishers. The course (and let鈥檚 be honest, Laz) has won.

39:50 Elapsed: John Kelly Completes a 鈥淔un Run鈥�

Kelly was the first to return to camp. He arrived through the swirling wind and darkness at 3:28 A.M. on Thursday, . That鈥檚 just 10 minutes under the cut-off for earning a 鈥渇un run鈥� (completing three loops in under 40 hours).

Who鈥檚 Left in the 2025 Barkley Marathons:

• Tomokazu Ihara (Japan)鈥�on loop three. This is his sixth attempt. Ihara, 47, has run numerous ultras and is a coach and race director. In 2023 he won the 鈥淕rand Slam of Ultrarunning,鈥� meaning he had the fastest cumulative time at five 100-milers in one summer: the Old Dominion 100, Western States 100, Vermont 100, Leadville 100, and Wasatch 100.
• John Kelly (U.S.)鈥�on loop three. This is his eighth attempt. Kelly, 40, is a three-time Barkley Marathons finisher, making him the second-most finisher behind four-time finisher Jared Campbell. He also has a Ph.D. in electrical learning and machine learning and is the Chief Technology Officer at Envelop Risk and has set several high-profile fastest known times, including on the Pennine Way and the Long Trail.
• S茅bastien Raichon (France)鈥�on loop three. This is his second attempt. Raichon, 52, 聽has finished Tor des Geants (2019) and set the GR20 FKT last year.

27 Hours Elapsed: Maxime Gauduin Quits

The Frenchman called it quits on loop three with no pages in tow, at 2:56 P.M. Eastern. And then there were three.

25:29 Elapsed: 4 Runners Total Have Finished Loop 2

Frenchmen S茅bastien Raichon and Maxime Gauduin finished loop two together in 25:29, with an hour and 11 minutes to spare, .

Meanwhile, after just 10-ish minutes in camp John Kelly (U.S) began loop three about 90 minutes before the cutoff. Raichon and Gauduin followed suit about 30 minutes later. There are now four runners on loop three, with Tomokazu Ihara (Japan) in the lead.

Beginning Loop 3.

鈥� Keith (@keithdunn)

25 Hours Elapsed: We Have 2 Loop 2 Finishers (Phew)

Tomokazu Ihara (Japan) finished loop 2 in 24:32, and the collective Barkley community took a sigh of relief. He began loop three just 15 or so minutes later, . Three-time Barkley finisher John Kelly (U.S.) finished loop two on 25 hours on the dot (and 40 seconds, but who鈥檚 counting). That鈥檚 five and a half hours slower than Kelly鈥檚 overall split through loop two last year, when he went all the way.

Both Ihara and Kelly split over two hours slower for loop two than loop one, which is pretty in line with the discrepancy between loops one and two in previous years. However, their loop two splits of roughly 12:35 and 13 hours, respectively, are about two hours slower than Kelly鈥檚 loop two split last year.

They have until 40 hours elapsed to make it back to the gate in time for a 鈥渇un run鈥� (three loops). That means they need to run faster than 14 hours for the third lap. Last year Kelly completed loop three in 12 hours, with the company of Ihor Verys and Damian Hall.

24 Hours In and No One Has Completed Loop 2

With the latest drop (Julien Chable from France), only six runners remain, . They have until 26:40 elapsed to return to the yellow gate and head back out on their merry (or should we say weary) way if they want to have any chance of completing a 鈥渇un run鈥� (three loops), much less the whole thing (five loops).

For context, 12 runners completed loop two in under 24 hours last year. The one runner to complete loop two in longer than that (Guillaume Calmettes, France, 26:25) made it to a fun run before tapping out. All five of last year鈥檚 finishers finished loop two in under 20 hours. And that includes Jasmin Paris, who finished the whole race with less than two minutes to spare.

Will the runners be able to pick up the pace now that they (theoretically) know the new course? Or will the alternating of loop directions, shifting from daylight to darkness, and compounding effects of sleep deprivation, fatigue, and frustration get the best of them?

We鈥檙e hoping for the former, but expecting the latter.

(And side note: the last time no one completed a 鈥渇un run鈥� was in 2006!)

21:00 Elapsed: 7 Runners Remain on Loop 2

Chris Fisher (U.S.) and Thomas Calmettes (France) both dropped from loop two at Bald Knob this morning, , noting, 鈥淭hough seemingly about a hundred yards apart, they did not see each other.鈥� This means at most 7 runners will successfully finish loop two within the time limit and move onto loop 3. That鈥檚 down from 13 who finished and 12 who moved onto loop three last year.

13:20 Elapsed: 10 Runners Complete Loop 1 Within Limit

That includes a final runner who finished the loop with under two minutes to spare, . They were able to do one of Barkley鈥檚 fastest-ever camp stops and turned it around in time to start loop two before the 13-hour-and-2o-minute cut-off.

Of those 10 runners, nine went on to loop 2, . Fourteen runners returned to camp, and a 16 remain out on the course. When they returned they will also receive a DNF and be ceremoniously dismissed from the race with the playing of 鈥淭aps鈥� on the bugle.

This means that there was just a 25 percent loop-one finish rate within the cutoff this year. That鈥檚 exactly half of what it was last year, when 20 of the 40 entrants finished loop one in time. What鈥檚 going on this year?

Some runners may have adopted the old 鈥渇ollow a veteran runner鈥� strategy, , maintaining that Laz was 鈥渘ot impressed鈥� by this game plan. We鈥檙e also hearing grumblings that Laz added a 鈥渧ery tough section鈥� that could add a whopping 45 minutes to each loop this year.

We鈥檙e also receiving reports that there are 16 books that runners must reach on every loop this year. If true, that鈥檚 at least one more than the typical 10 to 15.

11:00 Elapsed: Will It Be a Short Barkley This Year?

A third runner finished loop one in 10:20 elapsed, according to Dunn. Laz that 鈥渆very loop is easier than the loop before鈥� as he set out for loop two, which per tradition will be run in the opposite direction and also in the pitch black, about 20 minutes later. Ah, Laz, we鈥檝e missed your sense of humor!

snuck in under the 11-hour mark in 10:57:32 and 10:57:50, which means only five runners have finished loop one with one hour remaining before all runners left in the race must start loop two. The odds (and what seems to be an especially tough course, given the bluebird conditions today) do not appear to be in the runners鈥� favor this year.

Dunn that 鈥淭omo,鈥� otherwise known as Tomokazu Ihara (Japan), is one of the successful handful onto loop two.

9:45 Elapsed: Two Runners Complete Loop 1

They finished the loop in 9:44:55 and 9:44:57, . For reference, nine athletes completed loop one in well under nine hours last year. Given that we know of at least one (and we think more) Barkley finisher who was in that group of nine last year, signs are pointing to a very, very hard course this year.

Buckle up.

6:00 Elapsed: The DNFs Start Rolling In

Less than six hours into the 60-hour race and the 2025 Barkley Marathons already has its first drop. The first runner to quit the race returned back to camp on 鈥渜uitter鈥檚 road鈥� in the early evening, at 5:21 P.M. Eastern on Tuesday. They faced a rendition of 鈥淭aps鈥� played on the bugle horn, and with that their race ended. They made it about five miles in those six hours, Dunn said.

The second DNF followed just over an hour later. They made it about seven miles in seven hours, . An hour later and the DNF rate had doubled.

鈥淭he thing is, they are coming back to camp from all directions and in some cases cannot describe where they were,鈥� . 鈥淭his is old school Barkley.鈥�

鈥淔or as bad as you guys make it sound, it is worse.鈥� 鈥� a runner

鈥� Keith (@keithdunn)

For the first time, runners must return their race-issued analog watch when their race ends, .

11:37 A.M.: The 2025 Barkley Marathons Have Begun

After the classic 鈥渂rief memorial鈥� in which he cautioned runners to 鈥渕ake your peace with God,鈥� Laz lit the cigarette marking the start of the 2025 Barkley Marathons. Runners have 13 hours and 20 minutes to complete the loop and make it back to the yellow gate marking the entrance to Frozen Head State Park. That means we better see them all back there by 11:37 P.M. so they can begin their first night loop.

Who鈥檚 racing? We鈥檒l have to wait to find out until runners finish a loop鈥r two鈥r three, otherwise known as a 鈥渇un run.鈥� But Dunn outed three-time finisher John Kelly when the conch (eventually) was blown:

鈥淔inally,鈥� Kelly . 鈥淲e could have done a loop by now.鈥�

If Kelly, who was one of the five to finish last year, prevails again he will join the exceedingly rarified air of becoming just the second person ever along with Jared Campbell to finish the Barkley Marathons four times.

10:38: A.M.: The Conch Has Been Blown (Finally)

Technology鈥�namely online campsite reservation systems鈥�is making it increasingly challenging for Barkley custodians to keep the start date of this mystical event a secret. And sure enough, online grumblings came to fruition on Tuesday morning. At 10:38 A.M. Eastern after 鈥渕any failed attempts,鈥� Carl Laniak blew the conch marking one hour until the start of the 2025 Barkley Marathons.

After multiple failed attempts, the conch was blown at 10:37. The 2025 Barkley Marathons begins in one hour. .

鈥� Keith (@keithdunn)

Why didn鈥檛 race founder Gary 鈥淟azarus Lake鈥� Cantrell blow the conch?

鈥淏ecause he鈥檚 even worse than Carl at blowing it,鈥� Dunn (鈥渪鈥�-别诲?).

There鈥檚 probably at least some truth to that statement. For those who haven鈥檛 had the pleasure of trying, cajoling sound out of a big shell isn鈥檛 easy. But fans of this cult classic event are also wondering if it鈥檚 a sign of changing times. Laz, 69, has slated 聽Laniak as his successor to overseeing the Barkley Marathons.

We鈥檒l have the next 60 hours to find out. One hour until the race begins.

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Seven miles into the 2004 Cherry Blossom Ten Miler in Washington, D.C., a few months before that summer鈥檚 Olympic track trials, I felt a strange pop in my lower back. Hobbling gingerly to a halt, I realized that my race was over. As it turned out, so was my track career. I鈥檇 suffered a stress fracture in my sacrum, the bone that connects your lower back to your pelvis. It鈥檚 an unusual injury, and in the months that followed, I puzzled over my fate. Had I been wearing the wrong shoes, or logging too many miles, or not stretching enough? It wasn鈥檛 until a decade later that I began to consider another possibility: perhaps I hadn鈥檛 been eating enough.

, the International Olympic Committee unveiled something it called 鈥渞elative energy deficiency in sport,鈥� or REDs. The link between eating disorders, missed periods, and weakened bones was already widely known as the 鈥渇emale athlete triad.鈥� But REDs adopted a broader view. Failing to get enough calories to fuel both normal metabolism and the rigors of training were associated with a wide range of problems in 14 categories: not just poor bone health (it turns out that a stress fracture in the pelvis or sacrum is considered a primary indicator of REDs), but also impaired immune function, digestive issues, disrupted sleep, even urinary incontinence. The syndrome could afflict men as well as women, and it wasn鈥檛 limited to athletes with eating disorders. Some who鈥檇 been struck by it simply didn鈥檛 realize they weren鈥檛 getting enough calories to support their training.

The diagnosis caught on. The most recent , from 2023, pooled data from 178 studies involving more than 23,000 participants. It concluded that anywhere from 15 to 80 percent of athletes have REDs, depending on the sport. The problem is more common among women, and most prevalent in endurance sports like running, where weight affects performance. But not everyone is convinced that REDs should be applied so broadly. , penned by eight prominent sports scientists, poses a provocative question: Is REDs even real?

The piece鈥檚 lead author is Asker Jeukendrup, the Dutch Olympic Committee鈥檚 top nutritionist and a former head of the Gatorade Sports Science Institute. He and his colleagues take a deep look at the evidence underpinning REDs. The studies they examine are mostly short-term and observational, making it impossible to prove that calorie shortage causes the symptoms described. And in practice, measuring how many calories a person consumes and how many they burn is so error-prone that it鈥檚 impossible to say with confidence who is or isn鈥檛 coming up short. As a result, they conclude, estimates of REDs鈥檚 prevalence should be considered highly suspect.

Despite these shortcomings, 鈥淩EDs has become a much-discussed topic on social media and in mainstream media news outlets,鈥� the authors note 鈥攖o such a degree, they argue, that calorie shortage has become a convenient scapegoat for whatever problems athletes face. Jeukendrup and his colleagues suggest replacing the diagnosis with an alternative framework incorporating eight potential triggers: training, nutrition, disordered eating, sleep, infection, mental health, life/environmental, and undiagnosed clinical conditions. Any of these factors, alone or in combination, can cause a constellation of symptoms resembling REDs.

Meanwhile, out in the real world, some of the nuances of that message are getting lost. 鈥淭he title of the paper鈥斺�楧oes REDs Exist?鈥欌�攎akes for a shocking headline that can be easily shared but 诲辞别蝉苍鈥檛 fully reflect the content,鈥� says Megan Roche, a running coach whose doctoral work included research on low energy availability, hormones, and bone health in ultrarunners. Suddenly, she鈥檚 fielding tough questions from athletes and podcast listeners, few of whom have engaged with the details of the 11,000-word scientific review. Persuading athletes to seek help for REDs can be challenging at the best of times, she says; confusing them about whether a condition is even a thing only makes it harder.

The scientists who developed the original set of guidelines for REDs are also befuddled. They never intended to suggest that all training and health problems are the result of calorie shortage, or even that it should be the default assumption. Their are explicit that the signs and symptoms noted can be caused by other triggers. In other words, there鈥檚 nothing straightforward about any of this. 鈥淭here鈥檚 never going to be a 鈥榩regnancy test鈥� for REDs,鈥� says Trent Stellingwerff, a physiologist at the Canadian Sport Institute Pacific who helped author the IOC鈥檚 most recent REDs guidelines. 鈥淟ife is not binary. Disease is not binary. Clinicians have to make decisions based on incomplete information every day.鈥�

Much of the debate seems to be about messaging: Are the risks associated with calorie shortage getting too much attention? But there are some specific points of disagreement, like whether athletes who don鈥檛 otherwise show signs of disordered eating might sometimes slip into calorie shortage without realizing it. Meanwhile, Roche sees REDs crop up in athletes during times of stress or lifestyle transition鈥攆reshmen in college taking on a heavier training load, new mothers adjusting to altered schedules and the caloric demands of breastfeeding.

Which brings me back to that stress fracture. I was a hearty eater throughout my track career, but I increased my mileage substantially in 2003 and 2004, adding regular two-a-day runs for the first time. My weight decreased, and my BMI dropped below 18. It鈥檚 impossible to know exactly what led to my fracture, and it鈥檚 clear that REDs as a diagnosis will continue to evolve. We may eventually get a better understanding of how calorie supply interacts with risk factors like training load and stress. But I can鈥檛 help but wish that someone had been beating the drum about risks from inadequate fueling鈥攅ven in athletes who never turn down seconds, and even for men鈥攂efore I hit empty.

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Everyone has a plan, Mike Tyson famously said, until they get punched in the face. The endurance athlete鈥檚 version of that dictum might be: everyone has a great VO2 max and an efficient running stride until they鈥檝e run 20 miles. How you fare in those final miles depends, in large part, on how steeply these factors have declined over the course of the race.

This is the fundamental premise of 鈥渇atigue resistance,鈥� an idea I first wrote about back in 2021 that is currently one of the hottest topics in endurance science. The old view was that you could run some lab tests to determine an athlete鈥檚 VO2 max, lactate threshold, and running economy (or an equivalent measure of efficiency for other sports) and calculate their predicted finishing time. The new insight is that these factors change as you fatigue鈥攁nd crucially, they change more in some people than others. Having good fatigue resistance, then, is the 鈥�fourth dimension鈥� of endurance.

So far, most of the research on fatigue resistance鈥攚hich is also called 鈥渄urability鈥� or 鈥減hysiological resilience鈥濃�攈as focused on demonstrating that it plays a role in determining who wins races. What we really want to know, of course, is how to improve it. That鈥檚 the question a pair of new papers tackles.

The Case for Strength Training

The first study, by Michele Zanini and his colleagues at Loughborough University in Britain, tests a twice-a-week strength training program in 28 well-trained runners with an average 10K best of 39 minutes. Half of them added the strength routine to their usual training for ten weeks, while the other half just carried on with their usual training.

The performance test was a 90-minute run at a pace near lactate threshold, followed by an all-out time-to-exhaustion test that lasted about five minutes. Every 15 minutes during the 90-minute run, they measured running economy, which quantifies how much energy you burn to sustain a given pace. They expected running economy to get worse as the runners fatigued, but wanted to find out whether strength training could counteract this deterioration.

The results were encouraging. Before strength training, running economy got 4.7 percent worse after 90 minutes of running; after strength training, it only declined by 2.1 percent over the same period of time. Here鈥檚 how running economy changed over the course of the run, with white circles showing the baseline test and black circles showing the post-strength-training test:

Note that a positive change (i.e. the line drifting upward) means that the runners were burning more energy to maintain the same pace as time went on. In the baseline test, running economy starts getting significantly worse after about an hour. After strength training, this drift is less pronounced.

The strength training program in the study consisted of a mix of heavy weights and explosive plyometrics. The resistance exercises were the back squat, single-leg press, and seated isometric calf raises, typically with around three sets of six reps. The plyometrics included vertical exercises (pogo jumps and drop jumps) and horizontal exercises (hops and bounding). It鈥檚 not clear from this study whether the heavy weights or the plyometrics provided the magic, though Zanini that both methods have produced similar results in previous studies of strength training and running economy.

Why does strength training improve fatigue resistance? Here we鈥檙e limited to speculation. It may have something to do with making fast-twitch muscle fibers more efficient, or making tendons stiffer and springer, or improving strength sufficiently to maintain good running form for longer.

Other Options for Boosting Fatigue Resistance

The other new paper is by Andy Jones of the University of Exeter in Britain and Brett Kirby of the Nike Sport Research Lab. Jones and Kirby played key roles in Nike鈥檚 Breaking2 Project in 2017, where they encountered what you might call the Zersenay Tadese Problem. Tadese had exceptional lab values, including the best running economy ever measured, but repeatedly struggled at the marathon distance, while his teammate Eliud Kipchoge had relatively modest lab values but turned out to be the dominant marathon runner of the decade. The difference, presumably, was that Kipchoge had better fatigue resistance.

The new paper sums up their thoughts on fatigue resistance, including some speculation on how to improve it. Strength training, they note, is one option鈥攖hough they point out that few of the East African runners who currently dominate international marathoning do structured strength training.

Overall, their view seems to be that the best ways of improving fatigue resistance are mostly the things that endurance athletes already do to get better: high mileage, especially accumulated over many years; long runs, including some sections at close to race pace; intense interval sessions; following a pyramidal training distribution. There may also be some more subtle effects from, for example, doing some fasted training or living at high altitude. None of these are uniquely targeted at fatigue resistance.

Jones and Kirby do mention one other possibility: put on some supershoes. There鈥檚 likely an instant effect, since the heavy cushioning reduces muscle damage and enables you to keep striding smoothly through the later stages of a marathon. And there may also be a chronic effect: the cushioning allows you to absorb and recover from higher levels of training, enabling you to safely rack up higher mileage and thus improving your fatigue resistance over time.

The overall impression, then, is more evolution than revolution. All these years, we鈥檝e been training to maximize VO2 max, running economy, and threshold. Now we鈥檝e got a new target鈥攆atigue resistance鈥攂ut so far the best ways of improving seem to be mostly the things we鈥檙e already doing. Even Zanini鈥檚 strength-training routine is the kind of thing coaches and scientists already recommend. But if you have the sense that fatigue resistance is one of your weaknesses, you now have extra motivation to move strength training and plyometrics from the 鈥淚 should do this鈥� column to 鈥淚鈥檓 doing it.鈥�

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When I started writing about sports science two decades ago, fueling advice for endurance athletes was simple. The goal was to take in roughly 60 grams of carbohydrate per hour, in order to preserve the limited supply of carbohydrate stored in your muscles and liver. More would theoretically be better, but studies had found that it simply wasn鈥檛 possible to absorb more than that from the stomach into the intestine.

The science has evolved since then, mainly with the realization that (like glucose and fructose) in specific ratios enables higher absorption rates. top out at 90 grams per hour, but recent studies have suggested that it鈥檚 possible to take in 120 grams per hour鈥攁nd top athletes in cycling, ultra running, and other sports are reportedly going even higher than that.

In contrast to all this, a new study gave its subjects just 10 grams of carbohydrate per hour, and argues that this is all you need. This is a surprising and contrarian take, and I鈥檓 not suggesting you should swallow it whole. But it鈥檚 a good opportunity to pause the carb mania for a moment and take a closer look at the evidence and assumptions underlying the 鈥渕ore is better鈥� view.

The new study is published (and ) in the American Journal of Physiology 鈥� Cell Physiology by a group of researchers at several universities led by Philip Prins and Andrew Koutnik. Its main purpose is to compare endurance performance in ten well-trained triathletes following either standard carb-heavy diets or low-carbohydrate ketogenic diets. That鈥檚 a complex and long-running debate (which I wrote about most recently in 2020) that I鈥檓 not going to get into here, other than to note that they didn鈥檛 see any significant differences either way in an endurance test lasting about 90 minutes following six weeks on either diet.

What鈥檚 more interesting here is their test of in-race carbohydrate supplementation. All the subjects did two rounds of endurance tests on each diet: one with a carb drink every 20 minutes totaling 10 grams of carbs per hour, the other with a placebo drink with no carbs. On average, the athletes lasted 22 percent longer with the carb drink, regardless of which diet they were on. That鈥檚 a big improvement. Time-to-exhaustion tests, where you hang on as long as possible at a predetermined pace, are different than races, but an improvement like that likely corresponds to going one to two percent faster in a race.

The reason they chose such a small dose of carbohydrates is that one of the study authors, South African scientist Tim Noakes, that we鈥檝e badly misunderstood the role of in-race carbohydrates. The traditional view is that we drink carbs to prevent our muscles from running out of glycogen, the form in which muscles store carbs. Noakes鈥檚 view is that glycogen 诲辞别蝉苍鈥檛 matter, and that the real benefit is preventing a blood sugar crash. This is a brain-centered view of endurance: keeping blood sugar high convinces the brain that everything is OK, so the muscles鈥攚hich were never truly in danger of running out of carbs鈥攌eep on pumping.

If blood sugar is what matters, then we don鈥檛 need to choke down such large quantities of carbohydrate after all: at any given moment, there鈥檚 only about a teaspoon of glucose circulating in your bloodstream. What鈥檚 missing from Prins and Koutnik鈥檚 study is an explicit test of higher carb doses. We see that 10 grams per hour helps, whether by maintaining blood sugar or simply by tricking the brain into thinking that fuel is coming (as has been demonstrated with studies of ). But we don鈥檛 know whether, say, 30 grams per hour would have been better or worse.

On the other hand, you might imagine that the conventional view of carbohydrate needs鈥攖he more the better鈥攊s backed by plenty of evidence. And you鈥檇 be right. But Noakes argues that in all the studies showing that the depletion of muscle glycogen corresponds to a drop-off in performance, the subjects also had low blood sugar. We鈥檝e been watching the wrong variable, in his view, and drawn the wrong conclusions. This argument echoes Noakes鈥檚 critique of hydration research, which was that studies didn鈥檛 distinguish between being dehydrated and feeling thirsty. In his view, being dehydrated only matters if you feel thirsty, since it鈥檚 your brain that decides when to slow down.

The debate gets pretty complicated at this point, with dueling interpretations of the minute details of decades of research. Rather than getting lost in the physiology, though, I think the simplest test is to ask about the outcome we really care about: Does taking higher loads of carbohydrate lead to better performance? When you dig into this dose-response literature, the findings aren鈥檛 as clear as I might have expected.

Here鈥檚 a graph from one of the key papers, from the Gatorade Sports Science Institute (who, I鈥檓 sure I don鈥檛 need to point out, like the idea that more carbs are better). Fifty-one cyclists and triathletes completed a series of tests consisting of two hours of moderately hard cycling followed by a 20-kilometer all-out time trial, while consuming anywhere from 0 to 120 grams of carbohydrate per hour, in 10-gram increments. The results:

The paper describes this relationship as a 鈥渃urvilinear dose-response relationship鈥�: more carbs are better initially, but at the highest doses more carbs hurt performance. The sweet spot where performance is optimized, in this data, is 78 grams of carbohydrate per hour, consistent with the idea that 60 to 90 grams is the right range.

But take another look at that data. Performance is worst at 0 or 10 grams; it鈥檚 a little better at 20 grams. Take those three data points out, and it鈥檚 hard to see any evidence of a dose-response relationship above 30 grams. It鈥檚 certainly not a very strong demonstration that 60 grams is better than 30 grams, let alone that there are benefits from upping to 90 or 120 grams.

The case for 90 grams over 60 grams, using a more modern mix of carbohydrates, is made in from researchers at Leeds Beckett University. Ten subjects tested 0, 60, 75, 90, and 112.5 grams of carbohydrate per hour for two hours of cycling followed by a 30-minute time trial. Here鈥檚 the average power outputs in the time trial:

From this, you might conclude that 90 grams is indeed the best bet. It鈥檚 hardly definitive, though. The placebo option, with no carbs at all, is clearly the worst option, but it鈥檚 not that far from the 75-gram result, and there鈥檚 no data to compare with for lower doses. How would the cyclists have fared with, say, 20 grams an hour鈥攅nough, as Noakes would argue, to keep blood sugar constant but not to conserve muscle glycogen in the legs?

Personally, I find it hard to believe that muscle glycogen 诲辞别蝉苍鈥檛 matter. Even if we don鈥檛 grind to a halt because our glycogen tanks are empty, there鈥檚 evidence that we begin slowing down when our muscles are partly depleted. It could even be that the brain monitors glycogen levels and dials back performance as fuel levels drop, just as Noakes proposes for blood sugar.

Whether that means big carb doses like 120 grams per hour are a good idea is a different question, though. The scientific data that I posted above 诲辞别蝉苍鈥檛 seem overwhelmingly convincing. The real-world experiences of elite athletes are much more compelling, and that evidence should be taken seriously. I鈥檇 love to see better dose-response data showing more clearly what happens across the whole range of intakes between 0 and 120 grams per hour. But those are hard studies to do, so in the meantime we鈥檙e stuck with the golden rule of training and sports science: try a few different approaches, and see what works best for you.

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Consider this age-old dilemma: you鈥檙e at the bottom of a hill, and you want to get to the top. Should you head straight up the steepest slope or switchback back and forth at a gentler incline? The answer depends on the context. If you鈥檙e on a marked trail, for example, you should definitely stick to the prescribed switchbacks. But a more general answer involves digging into the physics.

That鈥檚 the goal of , from a research team led by David Looney and Adam Potter of the U.S. Army Research Institute of Environmental Medicine. Previous researchers have found that 鈥渟teeper is cheaper鈥� for runners, meaning that it takes less energy to ascend directly up steeper slopes. But it wasn鈥檛 clear whether the same is true for walkers and backpackers, or whether the answers change depending on how hot or cold it is.

The Best Slope for Trail Runners

For starters, it鈥檚 worth looking back at the trail-running data. In 2016, researchers at the University of Colorado decided to the increasingly popular world of . The total elevation gain in these races is set at 1,000 meters, or 3,281 feet, but every course is different. A steep slope will have a shorter course distance but be harder to run up. A gentle slope will be easier to run up but cover a longer total distance. For a given finishing time, what鈥檚 the sweet spot?

The Colorado researchers built the world鈥檚 steepest treadmill (video ), capable of reaching a slope of 45 degrees鈥攁 100-percent grade, in other words. To put that into perspective, a black diamond ski run is typically about 25 degrees, and gym treadmills rarely go more than 9 degrees. They had to line the treadmill belt with sandpaper for grip, and even then runners couldn鈥檛 stay balanced beyond 40 degrees.

They tested runners at a variety of slopes, with the treadmill speed adjusted so that they were always gaining elevation at the same rate, equivalent to a vertical kilometer in a very respectable time of 48 minutes (the world record is just under 30 minutes). Here鈥檚 what the results looked like for walking (black circles) and running (white circles), with metabolic rate (basically how quickly they were burning calories) on the vertical axis:

At gentle slopes like 10 degrees, it takes a lot of energy to climb, because the treadmill is moving really fast to gain the required elevation. At steeper slopes, the calorie burn decreases: steeper is indeed cheaper, at least up to a point. Beyond about 30 degrees, calorie burn starts increasing again, presumably because the incline is now so steep that it鈥檚 hard climb efficiently. The sweet spot, then, is between 20 and 30 degrees鈥攚hich, as it turns out, corresponds to the average slopes of the courses where the fast vertical kilometers are held.

(You might also notice that walking burns less energy than running for most of the steeper slopes. That鈥檚 a truth that most mountain and trail runners eventually discover for themselves. However, it 诲辞别蝉苍鈥檛 necessarily mean that you should only walk up hills, as I explored in this article on the walk/run dilemma in trail running.)

The Best Slope for Hikers

Climbing a kilometer in 48 minutes is really fast, the aerobic equivalent of running as hard as you can for 10 kilometers, so it鈥檚 not clear that the Colorado results have much relevance for backpackers or military personnel. Looney and his colleagues decided to run similar experiments at a range of much slower climbing speeds. The Colorado study had a climbing rate of 21 vertical meters per minute; Looney鈥檚 study looks at four different climbing rates of between 1.9 and 7.8 meters per minute, a much more realistic range for hikers.

The overall results are similar to the running results: steeper was once again cheaper. For each climbing rate, choosing a steeper slope corresponded to burning fewer calories. As with the running data, there鈥檚 probably a point where getting too steep becomes counterproductive. But the steepest slope in Looney鈥檚 study was only about 13 degrees, and in that range steeper was always better.

There was an additional wrinkle in Looney鈥檚 protocol: the military is on Arctic operations, so they ran the same protocol at three different temperatures: 32, 50, and 68 degrees Fahrenheit. The two warmer temperatures were basically the same, but the data at 32 degrees was slightly different.

At slower vertical climbing rates, calorie burn rates were higher than normal at 32 degrees, because the subjects were spending extra energy keeping themselves warm by shivering and activating their . At higher vertical climbing rates, calorie burn rates were roughly the same regardless of temperature, presumably because they were working hard enough to stay warm even at 32 degrees. In cold temperatures, in other words, pushing harder can sometimes be more efficient because it saves you the energetic cost of keeping yourself warm. (Conversely, you might imagine that the steepest slopes would cause problems in really hot conditions because you鈥檙e more likely to overheat.)

The Takeaway

The most important caveat to keep in mind when interpreting these results is that the comparisons are based on a fixed climbing rate. If you鈥檙e at the bottom of a hill and want to get to the top in a given amount of time, then choosing a steeper route will generally save you energy. If you鈥檝e got all the time in the world and don鈥檛 care how long it takes you to reach the summit, then you might well choose to take a gentler route that will feel easier as you climb.

Most of us, though, live in a world where time is scarce. Even if we鈥檙e not racing vertical kilometers, we鈥檙e hoping to make it to the summit and back, or to the next campsite, before dark. In that situation, if you鈥檙e choosing between two routes, remember this: If one route is twice as steep as the other, you鈥檇 have to walk twice as fast up the gentler route to reach the top in the same time. Counterintuitive though it may sound, that data shows that under most circumstances, twice as steep is easier than twice as fast.

For more Sweat Science, join me on and , sign up for the , and check out my forthcoming book .

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As athletes look to eat healthy while saving money, Costco has become the place for scoring mammoth deals on groceries. After all, who 诲辞别蝉苍鈥檛 love affordable groceries?

And yes, Costco has plenty of affordable healthy groceries. It might seem like they鈥檙e hard to find amid the colossal tubs of M&Ms and lifetime-supply jugs of ranch dressing, but nutritious options at lower price points are plentiful. You only need to be strategic about what you buy.

I worked my way through the Costco product lineup to round up the healthiest Costco foods for athletes. Add these to your shopping list the next time you are ready to push around that oversized shopping cart.

The best healthy Costco foods for athletes

Kirkland Signature Organic Peanut Butter聽聽

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When John Orth, a violin maker from Colorado, set out to break his own world record for the most pull-ups in 24 hours, he had no idea he was competing against a college kid from Virginia. And that kid, Andrew Shapiro, didn’t know Orth had his eyes set on the same number鈥�10,000 pull-ups. No one had previously thought such a feat was possible, and as the two men grabbed their respective bars and started to pull, they would find a new limit to human endurance.

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