Low Carbohydrate Training

I’ve been running here in Colorado’s San Juan Mountains for three weeks. Most of my runs have been low-intensity hike up/run down efforts lasting from two and a half to five hours. I’ve consumed calories during only two runs – a cup of veggie broth while pacing at Hardrock and roughly ten gels as well as a good deal of sports drink during the Kendall Mountain Marathon. A small part of the reason behind this was to shed a couple extra ounces here and there before UTMB, but the main reason behind the move was to work on my body’s ability to utilize fat as an energy source during the same race. While this ability would greatly benefit me during any long ultra, it’ll be even more important at UTMB where I’ll only have one spot halfway through the race where I can restock my gels, which are normally the vast majority of my in-race calories.

Although much recent training advice has been toward runners fueling throughout their long training bouts, this sort of low-to-no carb training isn’t without precedent. I’m sure many ultrarunners and other endurance athletes have done this by default. I’ve much enjoyed this low-carb training article from First Endurance when I read it a few years ago and again now for its presentation of research on low-intensity, low-carb training.

While I’ve not been engaging in the practice, the article suggests not replenishing glycogen stores via carbohydrate consumption prior to some of these low-intensity, low-or-no card training sessions. Years ago, I heard of some top New England ultrarunners (if I remember correctly, Todd Walker and, maybe, Leigh Schmitt) occasionally running back-to-back “bonk runs” on the weekend in which they’d go long at a moderate-to-hard effort on Saturday with no carb intake, refraining from refueling with carbs afterward, and then running moderately long again on Sunday in an already glycogen-deleted state.

I will continue to ingest carbs, usually as GU energy gels, during races (I ate 30+ GUs during Western States) and higher intensity long runs. I’ll also continue to bring carb sources with me during all of my long runs as an emergency backup. However, I’ve enjoyed my no fueling experimentation this month as well as my low-carb long run training throughout this spring and summer. I think it’s too early to tell if it’ll result in a major performance boost on race day, but I don’t feel like my low-carb training plan has hampered me in the least the past few weeks.

Call for Comments
Do you always fuel during your long training runs? Do you ever restrict your carb intake during training runs? Do you routinely use such a restriction?

It’d be great to hear your reasons for whatever level of carbs you use during training.

There are 58 comments

  1. Schlarb

    Personal balance and timing are my opinions on this subject.

    That article spells out a specific process to train the body's different energy systems, which certainly is value added.

    Before running with 1st Endurance, I only used gels on runs over 2hrs and even then one 100 calorie gel per hour on long runs, because I was (am) cheap and my body could do it.

    During this being cheap phase and only taking a couple gels a week on a 80-90 mile week, yes, I was efficient, but I wasn't recovering well and thus I felt tired and wasn't meeting my potential for gains in my training.

    I went to a presentation last winter where Robert Kunz (1st Endurance) was asked about this topic of training the body to burn fat and being more efficient via limiting carbs/nutrition on a workout and if I remember correctly, Robert's response warned that there is a fine line between making the body more efficient at using fat and being able to recover fully and quickly enough to maintain high volume and intensity training.

    No nutrition on easy recovery days where I keep my heart rate average around or under 135 for 1-2hrs (fat burning), then taking in some EFS before or once during a hard effort workout and taking 100 calories of EFS per 40 minutes on long runs, really works best for me while I'm training to race trail ultras. This method allows me to get two hard running days, a long run and weekly mileage in the 100+ range, where I really couldn't without nutrition before.

    On the flip side, I DO believe a dedicated fat burning training block can/does work, as long as you have the time and patience. To really see the benefits (like most all training stimulus) you have to do it for at least 6 weeks and it works best when you don't mix anaerobic training (Dr. Mark Cuccuzzella has some great stuff on this!)

    This would entail a training block where you almost always keep your heart rate aerobic (under 135) and apply the tactics from the article for nutrition intake.

    The problem for me and I would assume most, is that this training block requires some serious patience… it is challenging to run slow enough to stay aerobic/under 130-135 depending on age (especially at altitude in the mountains). Secondly, during this training block you won't be in "race shape", well, at least not for anything hilly or fast!

    During the winter, after a big break from running due to injury, the end of your season etc… and where your goal race is months away would seem to be the best time to implement this block.

  2. Jeff

    I only take gels with me if I'm running at least 2 or 2 1/2 hours. When I first tried this I would bonk after about ninety minutes, but found that my body adapted quickly when training like this.

    If I do take gels I try to "max out" on them so my stomach gets used to taking in fuel early and often – about two an hour. This is only on runs that last over two hours or on "marathon paced" training runs (14, 16 or 18 miles at marathon pace). This can be tough to do in hot weather when I need greater electrolyte and fluid intake – I typically only take one gel per hour this time of year.

  3. olga

    I've never taken fuel during runs under 2.5 hrs, and when was in OR and running 6-7 hrs in the Gorge, was the one going skimpy on 3-4 gels. I kind of started it as not knowing much (transitioning from road marathon training), then simply because I am cheap, then read on about adjusting body to fat burning – of which I have plenty. But – I DO always take carbs (gels in my case) during races, on the clock, not relying on anything "adjusted", although I've had stretches towards the end of either 50 or 100 when I stop eating because I was tired/stupid/sick/whatever.

  4. Ben Nephew

    I think you should train like you are going to race. If you plan on running without fuel during races, training like that will help. I'm wondering what happens to your blood sugar when you do low-carb training for weeks, and then take a gel every half hour during a race? 6 weeks without any quality workouts would definitely lead to a decrease in fitness for myself.

    I find that the best way to get faster is to get in as much quality work as possible while staying healthy, and this requires that I consistenly eat well. I'm not sure how exactly calorie depletion would affect injury rates, but I'm sure you are going to be at a higher risk for getting sick.

    For 50 milers, a hard long run is a major part of my training. While I think that being well-fed makes for happy and fast running, I usually don't take in any calories during my 2-3 hour long runs. Whether I eat breakfast before the run or not, I don't run into bonking issues.

  5. Cheryl

    Thanks for asking for comments about this very important topic of low carb use during training. I would encourage you to read Anthony Colpo's take on this. He is not an advocate of low carb training or the low carb diet for athletes. Fat metabolism requires a low level of carbohydrate intake and use. It is important to find the balance…


  6. John Clarke

    About 7 yrs ago, I wanted to make one last stab at breaking a 6 min pace for the half marathon before I hit 50. I came up with what I thought was a novel approach to training : Atkins. Anyone I shared my plan with considered it both dangerous and non-productive. My philosophy was to put myself into glycogen depletion early into the training run. After all, that’s a large part of the long marathon runs; get yourself into glycogen depletion, and then train yourself to burn and use fat.

    The early training runs were extremely difficult. There was just no energy. A simple 5 mile run was fatiguing right from the start. However, over the 3 month period, I was able to push the curve to where I was running not only longer, but also at a faster pace than when I began a few months prior. I was even able to do some long intervals on the track.

    2 weeks prior to the race, I started a gradual “super carbo load” as I thought of it. By race time, I was feeling very runnerish. I broke 1:18 for the race.

    I tried the same thing a few years later for a 10 mile race, only not so long a build-up. I also worked well.

  7. mtnrunner2

    I don't race or do the distances you and Jason Scharb do, but I eat low-carb and run some decent runs in the hills, and I just eat real food. It works to keep the energy up sufficiently. And I've lost the taste for anything sugary anway.

    Although I had a PBJ yesterday (not my typical snack) on a 22-miler with some good vert, lately I've had good success with jerky. It's substantive enough to feel like food and it has salt. Other than that, I go for as close to a real food bar as possible: nuts, fruit. Not sure how that would go in a race, where you're really pushing your stomach to its limits…

  8. Leigh Ann

    Low-carb during training can be a very difficult thing to master and might not be the best way to make your body efficient at burning fat for fuel. Instead, an overall low-carb eating EXCEPT while training can make your body more apt to burning fat most of the time. Simple sugars in gels and drinks are great for explosive training or immediate use. Timing can be critical though to ward off "bonking". If you want to recovery quickly to be able to either train the next day or continue a race it seems best to use high quality carbs (I am referring to non-grain, paleo carbs) to replenish glycogen stores. If recovering quickly isn't your main objective, this is less of a concern. By NO means am I an expert, just an avid reader and experimenter. Check out the following info:

    Paleo Diet for Atheletes by Loren Cordain & Joel Friel:

  9. Bobby Gill

    Great points being brought up by everyone. All endurance athletes can stand to benefit from becoming more efficient at fatty acid metabolization. In a racing scenario, there is less chance of bonking and energy levels stay more consistent. In terms of general health, great fat utilization equates to less food cravings, better hormonal signaling, consistent energy levels, lower insulin levels, and the list goes on.

    Carb-depleted long runs are one method for attaining this, but just like barefoot running should be a "tool" in one's training toolkit and there should not be 100% reliance on it, so too should carb-depleted runs fit into one's schedule. Use them to get the training benefit, but also get in training runs that mimic race-day scenarios and include consistent carb fueling.

    Day-to-day nutrition can also greatly influence what type of energy your body burns for fuel. Eating more fat from natural sources, staying generally low carb on non-training days and consuming non-gut-irritating carbs from sources like tubers [sweet potato, yams, etc.] or white rice surrounding workouts will help accomplish this.

    1. Bryon Powell

      Well, that'd be an entirely different issue.

      Personally, I skip it. Yeah, it might aid performance, but I don't eat meat and the protein-fortified sports drinks make me want to puke from the get go.

      1. Martin

        Thanks for the answer.

        But I was more wondering about protein intake in terms of recovery. If I see my food plate as a mix of carbohydrate, protein and fat; should I lower the overall amount of food I eat, or would lowering carbohydrate and fat(?) and leaving protein at usual amount suffice?

  10. Gustav Skaar

    No suger for me! I run 3-5 hours in the mountains on the west-coast of Norway just on water. I do this 4-5 days a week and eat just normal food afterwards and a couple of hours before.

  11. bob

    I use jells when I run over 2hours and if I am not near any water sites to replenish my electrolytes during the runs. I prefer to use them 15 to 20 minutes before the hills to give me a bit of a boost.

    I find that if I don't use the jells, then I hit a hard walls about 20k into a long run and slow down to a really slow pace.

    My long runs were in the 5 to 6 hour range and after the 3 jell, it got difficult to swallow the jells. So I use a combination of jells and chews during a long run and it makes it easier.

  12. CraigR

    Well, you are taking a page from the "Tony Krupicka Fueling on the Run Method". Train one way as far as nutrition is concerned and race a totally different way.

  13. Speed

    Perhaps training on what they will have for you at the Aid Sataions would be more productive. They will have cheese, and they will have sausage! Pooping becomes the real issue at UTMB –just my experience I suppose!


    1. Bryon Powell

      I can't say that I'll eat any sausage or much cheese during UTMB! Maybe I'll eat some bread, but that's about it. I'm pretty sure my digestional abilities for both bread and cheese are pretty well primed as it is. :-)

    1. Bryon Powell

      I can't imagine a scenario in which I'd eat during a run of two hours or less … unless it was for the pure enjoyment of the food. We've got plenty of glycogen stores to get through two hours. From two to three hours, I'd say it totally depends what the purpose is. If you're busting out a 24 mile run with 14 miles at marathon pace, then by all means, eat! If I were simply going out for an easy 3 hour training run for a long ultra, I might bring a couple gels with me but wouldn't plan on eating them.

  14. Annette Bednosky

    I think fueling depends all on what you are used to,and the race day conditions. Most of my regular life would be "carbo load" to some people: bread, fig bars, pb & j, fruit, hummus, red wine (generous amounts), some chocolate on stressful work days, jolly ranchers,yogurt, etc.

    I generally don't ingest calories (maybe watered down sports drink) for runs under 2 hours…yet I almost always carry a back up gel or fig bar w/ me in case my blood sugar goes to my toes. I have experienced toe-level blood sugar too much in the past to allow for it to happen when I run ultras and need it or not, I am knocking down 3 Cliff Shot Blocks after 90 minutes of running,and if going 4 hours plus and aim to keep it to 200 calories (sometimes more depending on temps -cold = more…

    Maybe low carbo training is good, yet I am too much of a whimp to give it a try (on purpose anyway!).

  15. Myles Smythe

    I am sure you are consuming large amounts of beautiful scenery while running in the San Juan mountains!!

    Like some have said, I want to eat healthy and maintain proper energy/nutrition levels. But training your body to burn a long-burning fuel such as fat is an awesome idea. I will always replenish after runs and during my normal daily diet routines. I sometimes will skip beakfast before a slow run, but am more likely to run on minimum fuel intake which usually happens during a race. Two weeks ago at mile forty my stomach felt like it was all twisted up and I didn't want to eat anything so I relied on fat to get me to the finish line of a 50-miler. So glad I practiced!

    I really don't have much fat left and I am normally trying to actually gain weight while running 50-60 miles per week.

    Last thing, a good RD (registered dietician) friend of mine commented to me that the brain still requires a small amount of glycogen to function. She said the ginger "boost" candy I chew should be sufficient.

  16. Christian

    Hi all,

    i´am trying to eat less carbs overall in my life, but when i am going to a race i eat lot´s of them, train low compete high. I think this is a good way to learn your body to use fat as fuel faster and more in races.And if i fuel up on a long run i rarly use sugar, some gel och drink with carbs but without pure suger.

  17. Jacob

    I would suggest that the scientific research quoted by the article shows correlation, but not causation. Statements like "it is well known that reducing carbohydrate intake during training increases the body’s ability to oxidize fats" are merely assertions and arguments from authority. No scientifically defensible basis is presented for them. The cynic in me would contend that if the research were clear, they would quote said research and they wouldn't have to rely on tangentially related work.

    As for anecdotes, I can cherry pick whatever personal stories I want to support one side of the argument or the other. But an anecdote is neither a statistically significant sample size nor a scientifically defensible experiment. It's an anecdote. It might be interesting, but taken by itself it doesn't prove or disprove anything. For example when someone is able to run longer with less fuel, how do we know whether their body is more efficient at fat burning or just has developed larger muscle glycogen stores? or both? We don't.

    None of this is to say that their hypothesis is wrong, only that there is not a convincing case that it is right. It's quite possible that if someone did an actual experiment looking at fuel intake and fat burning percentages there would be something there.

    One question I have not seen raised is the extent to which the body will burn muscle/protein if it does not have enough carbs.

    Regardless, I'm a believer in training how I fight. And in races I usually eat every 20-30 minutes starting an hour or so into it. So that's how I train.

  18. roger

    O yeah – as for your questions:

    I mostly fuel during training runs but mix it up. Some days I set my insulin level low so I'll specifically have to run hard and carb low-moderate to keep blood sugars in a good range. If I get cotton mouth it means I'm going too slow. This doesn't do exactly the same as running low, but on a continuum it gives the benefits of training on the lo-carb side of normal. Other days I specifically chug carb at a higher rate than I'll want to at a higher intensity in a race, because it's the best combination of variables for training my gut to deal with it.

    80kg bodyweight, in race going through about 70g CHO/hr, on a low day training 40-50g CHO/hr.

    Sometimes just chugging a heap of carb and running at decent effort as long as I can until blood sugar goes below 4mmol/L.

  19. StephenJ

    I've only been running for 2 years, but I've been under the impression that running mountain trail ultras is mostly and eating and drinking contest with some running involved. Maybe when I've got the stomach trained really, really well I'll start trying to train the fat burners, but for now I try to consume as many calories as I can without puking. Ginger candy is my stomachs new best friend.

    1. Bryon Powell

      Stephen, ultras are an eating and drinking contest… tuning in the fat burning might just mean a couple dozen extra calories of fueling on top of what I eat. In no way am I advocating no eating during ultra. For any long ultra, I'm certainly aiming to eat at least 300 calories an hour!

  20. Bryon Powell

    As with you Myles, I'm quick to replenish after the vast majority of my runs and if I don't it's usually due to circumstances or laziness. Today, I went 11 hours with one Stinger Waffle on the trail when my stomach was rumbling and no other calories on trail…. though I had a huge lunch after 5 hours. When I got to the car, I had a Mexican Coke (actually, meant for the penultimate pass, but a hail storm stopped that idea) and had a couple burritos within an hour and a half of finishing.

  21. Kristen

    Sticking with the same number of calories with a higher percentage of protein and fat would be beneficial, as long as the amount of protein isn't too high(max 20-25grams), as it becomes hard on your body to digest all at once

  22. Bryon Powell

    Depends why you are considering lowing your overall food intake. I'm no nutritionist… but I'd guess most folks (endurance runners included) routinely take in significantly more protein than they need for recovery on a day to day basis. Surely, there are a small number of runners and runs that are exceptions.

  23. Michael Owen

    Anyone who knows me would say I don't know much about the science side of running – they are right, mostly. Like in all aspects of running I go with what feels right to me in my mind. On this subject I feel that I am training myself best when I train without calories on runs. Even my biggest weeks at 150+ miles and my biggest runs at 30+ miles are done without gels. Granted, I go through many bonk periods but I always feel like I can recover with eating "real" food before and after runs. A good daily diet, in my mind, will support what gels do for runners. When it comes to the weeks leading up to a race, and tapering mileage, I feel the body will have calloused, per-say, to the feeling of bonking and then gels during races will give the body some unexpected positive energy.

  24. Candice

    Your body will tell you what it needs. Seems to me, the more real food (fruit, cheese, etc) the better. I only use gels for races and emergency as well. I also like doing less calories on long training runs and find that as long as the intensity is low, it's fine. Listening to tour body's cues regardless of your dietary ideas seems smart though. But-we probably need less food than we think.

  25. Mats from Sweden

    It depends on your ability, I would say. Bryon is well trained and personally I would have the same approach as him. But if you are new at running you might crash totally in those two hours. That's because your system is not trained so well yet. My advice is to carry some gel or a bar just in case also for those "shorter" runs. Take care!

  26. Martin

    Phil Maffetone has done some research with low HR/fat burning training in the 70's when HRMs were introduced as with low carb diet to supplement this type of training. The Maffetone method is more popular around triathlon circles.

    Not so sure around ultras, but I think many ultra runners have been doing it unknowingly –training at a low HR to get higher miles. Otherwise it wont allow you to recover week after week of high mileage training and be consistent with it.

  27. Oscar

    I follow a permanent low-carb diet. I find I only need some fuel after one and a half hours running, and in fact I feel there is a barrier after half an hour where I start to perform better. I think this is when I switch to mainly using fat as energy source, after depleting the little glycogen I might have. For fuel I use a beverage made of coconut oil, water, salt and a little sugar. Have not tried yet to add some BCAA but it could be interesting to avoid muscle depletion due to gluconeogenesis.

    Sometimes if I feel a bit low, I have a piece of fruit or some chocolate or almond milk just 10 minutes before starting exercise, not before, otherwise you get the insulin spike and afterwards the rebound effect and low sugar.

  28. Allan H

    I never ingest any food before or during my training runs. I always run in the morning. All my training runs are 1-3 hours in length, except for an occasional 4-5 hour effort. Most of my training runs are on a treadmill, wherein I consume water every quarter mile. During races (last year at age 61 I finished 4 100-mile races and nearly finished a couple more), I consume a home made blend of by weight 95% maltodextrin, 5% soy or pea protein, 5 S-Cap tablets per 2000 calorie batch, a heaping teaspoon of cinnamon all blended as a syrup 500 grams of powder to 300 grams of water. I consume some this blend at the rate of about 200 calories/hour. I eat a meal of oatmeal, fruit, nuts and soy milk within an hour of any training run and then an evening meal of fruit, vegetables and lean meat. This seems to work OK for me. I never feel a bonk during my training runs. Sometimes if I happen to be low on glycogen I will feel a hard effort the first 5 minutes of my training run if I start out too fast, then I feel OK the rest of the run.

  29. George

    I am like you Oscar. I started eating low-carb 6 months ago. I dropped some weight and my running improved a lot. I do not hit the wall any more. And, I feel better the longer the race goes on, exactly as if I switch to burning fat almost exclusively. In long trail races I eat cheese, nuts and a bit of fruit. I am actually amazed by the improvement. It only took 2-3 weeks to get adjusted to eating low carb and start seeing the good results.

  30. Scoobey

    I wouldn't worry about the brain needing glycogen to function. The liver easily converts protein to sugars for brain functioning when needed; that's why the Inuit (and the woman who just skied across Antarctica on only olive oil, etc.) can subsist on almost nothing but fats and a little protein without any cognitive problem. And the amount of carb used by the brain is very small — ie, the candy your friend mentioned is more than enough.

  31. chris h

    I'm with you Gustav, no sugar, low carbs, natural fat and water. I like to run slow and long and find the primal blueprint fills my dietry needs. Check out marksdailyapple.com for more info.

  32. AllanHoltz

    I never consume anything but water during training runs. Then I consume a home made maltodextrin, vege protein, water and electrolyte mix every 15 minutes during races of a marathon through 100-mile length. Works for me. I also do not eat anything in the morning before my training runs. Often my last food consumption was complete at noon from the day before. I have been doing this for 10 years now. I am 62 and I've averaged 3000 miles a year for the past 15 years. I've finished over 125 marathons, 100 ultramarathons and 33 races of 100 or more miles. I do usually eat a couple sandwiches and a banana in the morning before these races.

  33. Chris

    I switched from eating high-carb (followed a complex carb eating plan by top nutritionists for athletes) to eating low-carb, and I found Volek/Phinney's book: The Art and Science of Low Carbohydrate Performance — http://www.amazon.com/The-Art-Science-Carbohydrat… to work really well for me. For me, that means <=50 grams of carbs per day. I test my bohb levels and they are right in the right range for being fully keno adapted. It's a myth that high-carb is the "only" way for long-distance running. A friend of mine who has been fully low-carb/keno adapted just completed a 50 mile trail race (finished in the top 8 I think) and is getting ready to run his first 100 mile strictly low-carb. According to Phinney/Volek, the ability for the body to use fat as the primary energy source is tied to the amount of carbs ingested, when they go up, the ability to utilize fat can quickly go down. I'm sure everyone's body reacts differently, hence why I test my blood bohb levels to ensure I'm in the right zone. That said, I've only been running for 1 year, have only run 2 half marathons and am going to attempt my first marathon in a few months. I have found I have much more energy and less fatigue in my long runs (only 14-16 miles so far — i know, not far given this website) once I became keno-adapted versus my prior high-carb.

    All that said, I don't believe low-carb is the "only" way to go, just that it's a viable option and there is a ton of science behind it when you really study the history (which Volek/Phinney do a fabulous job at) and carefully implement it, track every single carb you consume down to the minute amount, so you know exactly at the end of the day how many carbs you consume, etc.

    I've benefitted also from losing weight, not getting the shakes/tired/lows that you sometimes get, etc. I can go out and run 8-10 miles on an empty stomach, no fatigue, and I'm not hungry afterwards and may eat 2-3 hours after a run, with no shakiness, etc., because I consume the majority of my diet from fats and moderate protein.

  34. Laura

    Hey, just have a quick question. I am on day 9 of low carbing, and am finding running VERY hard! I used to do a 7mile trail training run everyday & now I can't even do half that without stopping!! Is this normal? Please tell me it'll get better soon as I love running & if I don't start to enjoy it again soon I'll have to quit this way of life! Any advice gratefully received.

    1. George T

      Yes, it is normal. It took me two weeks with some bad runs to get adjusted. It took my wife an entire month. Not only it gets better, it gets a lot better (running gets a lot more enjoyable)

      1. Laura

        Thank you!!!! That's exactly what I wanted to hear!!! Do you think if I had a gel just before running at the moment it would help? I don't want to undo any good I've done but I am itching to get out there!! I really appreciate you responding

        1. George T

          I think you should stick with your low carb diet. Eating gels will just delay your adjustment. There are a few suggestions recommended to speed up the adjustment: 1) Eat more fat. 2) Get some extra salt (like a broth) before exercise.

  35. Allan Holtz

    I would NOT low carb. I do not think it is conducive to a high run lifestyle. I would just be careful regarding the nature of the "carbs" I ate, i.e. I would go light on the pastries, pastas and desserts and heavy on the whole grains, fruit and vegetables.

    Happy living,

    Allan Holtz

    1. George T

      Have you tried it? In my 2 years of eating low-carb I have completed 2×100 mile races, 3×50 mile races and about 15x50Ks, all with excellent results. I have more energy than ever, especially towards the end of the race. I can run a 50K without any food. I have never hit the wall. Works great for me.

  36. Allan Holtz

    I do consume home made gel during marathon to 100-mile races to fuel those (long) efforts. But that is the only time I use them. In a blender I blend 475 grams of maltodextrin (Maltrin M180) with 25 grams of soy protein (protein optional) with a heaping teaspoon of cinnamon powder with the contents of 5 Succeed S-Caps and 300 grams of water. This creates 800 grams (2000 calories) of gel (about 7/8 quart of gel). I consume about 200 calories every 15 minutes of long races.

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