Should Endurance Athletes Go Keto? Ketosis and Ketogenic Diets for Endurance Athletes


When it comes to weight loss and endurance performance, dietary ketosis is the strategy everyone is asking about this year. On the surface, ketosis or a ketogenic diet offers everything an endurance athlete could dream of: endless energy, freedom from bonking, and an efficient pathway to weight loss. The diet has been all over mainstream magazines, it’s the subject of several new books, and the supplement companies have already jumped in with new products and a ton of marketing dollars. So, is it time for cyclists, triathletes, and runners to go Keto?

First, a refresher course on what a ketogenic diet is. To achieve dietary or nutritional ketosis you need to severely restrict carbohydrate intake (fewer than 50 grams of CHO/day) so the body transitions to using ketones for fueling muscles and the brain. Ketones are produced from fat, which is why nutritional ketosis is so appealing to sedentary people as a weight loss solution. It’s appealing to athletes because we have a virtually unlimited reserve of fat calories to pull from but can only store 1600-2000 calories worth of carbohydrate in muscles, blood, and the liver. An athlete fueled by ketones would be theoretically “bonk-proof”, since bonking is the result of running low on blood glucose.

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Dietary ketosis for athletes is one of the most hotly contested subjects right now. Proponents point to the metabolic advantage of relying on fat instead of carbohydrate, and critics point out the physiological limitations of eliminating carbohydrate as a fuel for performance. You’ll find bias in both groups, either because scientists and coaches (including me) have been in the high-carbohydrate camp for many years, or because there’s a lot of money to be made by creating a market for new media and supplements around a new high-fat dietary strategy. I recognize my historical bias toward carbohydrate, but have tried to look at the science objectively. Here’s the short conclusion I’ve come to:

Exogenous ketones may have promise as an additional fuel source for endurance athletes, but dietary ketosis has limitations that make it difficult to recommend to most athletes. Athletes are better served by periodizing carbohydrate availability in order to maximize training quality and performance outcomes.

And here’s how I arrived at that statement:

Ketosis doesn’t IMPROVE endurance performance

If you do everything right, you may be able to achieve similar performance levels during steady state endurance exercise following a high-carb (50-65%CHO) diet or a high-fat, low-carb (HFLC) diet (70-80% Fat, <5% CHO). This means you may be able to sustain a submaximal pace equally well using either strategy. The HFLC strategy has been shown to increase the utilization of fat for energy, especially in long-term (20 months) fat-adapted athletes (Volek, 2015). However, the oxygen cost of locomotion increases while exercising on a HFLC strategy (Burke, 2016). It takes approximately 20% more oxygen to liberate energy from fat compared to carbohydrate, which means relying primarily on fat reduces economy. This isn’t necessarily a problem, since you have a large supply of energy to burn, but these findings don’t indicate an IMPROVEMENT in endurance performance. Athletes don’t go faster on HFLC, which is why we see elite athletes utilize HFLC at specific times of the year and then complete high-intensity training and competitions with high carbohydrate availability (more on that later).

Ketosis is physiologically limiting

Without stored and exogenous carbohydrate during competition, you have very little fuel available for anaerobic glycolysis, the metabolic shortcut that rapidly produces energy by partially burning carbohydrate to meet elevated energy demands during short, high-intensity efforts. Ketones can be converted to acetyl-coA and metabolized aerobically in mitochondria, but you miss out on the turbocharged boost from anaerobic glycolysis. You also miss out on the lactate produced from anaerobic glycolysis, but lactate isn’t the enemy it was once thought to be. It is partially-burned carbohydrate that gets broken down to usable energy.

The reason I say you’ll have little carbohydrate available for anaerobic glycolysis instead of no carbohydrate is because an athlete in ketosis can still produce glucose in the liver via gluconeogenesis. In plain English this means athletes in ketosis have limited capacity for high-intensity efforts that rely on carbohydrate for fuel. (It is intriguing to note the amount of available glycogen increased in the long-term fat-adapted athletes in Volek’s study with elite ultrarunners.)

Almost all endurance sport are actually intermittent-intensity sports rather than steady state intensity activities. While long cycling event may have a moderate overall intensity, there are periods of high-intensity within it. Even ultramarathons and Ironman triathlons – long considered to be low-intensity, long-duration events – feature periods of intensity above lactate threshold. For competitors, hard efforts are required to drop rivals and build winning margins. Whether you are going for the win or trying to set a PR, you will achieve your best performances in events that feature intermittent high-intensity efforts by optimizing your ability to use all fuels and by providing your body with adequate supplies of all fuels.

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Ketosis may prevent gastric distress

Athletes in ketosis can perform well at a steady endurance pace, and can do so for many hours while consuming far fewer calories than carbohydrate-dependent competitors. As a result, ketosis may be a good solution for athletes who consistently struggle with gastric distress during ultradistance events. During exercise lasting 9-24+ hours, changes in blood volume, heat stress, and hydration status can slow or halt gut motility. This is a problem when you are consuming large amounts of energy and fluid because food that stays in the gut too long creates the gas, bloating, and nausea that make athletes drop out of races. In fact, GI problems are the leading cause of DNFs in ultramaraton events, so the prevention of gastric distress could potentially make dietary ketosis a reasonable solution for some ultradistance athletes.

For the record, CTS Coach Jason Koop, author of Training Essentials for Ultrarunning, disagrees with me on the paragraph above. He believes strongly that fat adaptation/dietary ketosis is not a good idea for ultrarunners because it is physiologically limiting and because the gut is trainable. He agrees ultraendurance athletes in ketosis might be less vulnerable to GI distress, but points out that GI distress is most often the result of poor planning and inadequate training (both physical and nutritional). In that context, ketosis is a complicated solution to a relatively simple problem, and an ultimately inferior solution in terms of maximizing physiological performance.

Ketosis is very disruptive to training

If carbohydrate is available it is the go-to fuel for muscles and the brain. Only in carbohydrate’s absence will the body transition to producing and using more ketone bodies for energy. This is evolutionary biology. When sugar from plants was available to our ancestors they could gorge on it, use some for energy and store the rest as fat. During times when there were no plants to eat, their carbohydrate stores ran out and they transitioned to ketosis to fuel themselves from their stored fat. To achieve ketosis voluntarily – instead of through inadequate insulin production – you have to essentially eliminate carbohydrate from your diet.

Initially, you will have neither enough carbohydrate nor ketones to fuel your brain. While you are always producing ketones, it takes time (up to 2-3 weeks) for your body to increase production to the point you are relying on them as a primary energy source. During this period, training performance will definitely suffer (and lifestyle performance may suffer as well). Your power output will be lower than normal. Your running pace will be slower than normal. Perceived exertion will go up, at all intensity levels. Recovery from training sessions will be hindered.

Once you are adapted to fueling yourself primarily on ketones for day-to-day living, you still need to adapt to performing optimally as an athlete fueled by ketones. This can take months, during which time your only progress will be in fat adaptation, not aerobic development, the ability to produce power, or the ability to achieve faster paces.

If you are going to try ketosis as an athlete, the best time to experiment would be a period of general aerobic endurance training. For summertime athletes in the Northern Hemisphere, this typically means fall or winter. It would be a mistake to try making this transition during a period of important, race-specific, high-intensity training.

Weight loss from dietary ketosis is primarily from caloric restriction

Exercise studies of athletes who have adapted to ketosis show they burn more fat at a given exercise intensity than when they were carbohydrate-fueled, but not that they can produce more work (go faster) (Zajac, 2014). When athletes get faster after adapting to ketosis, or even after a period of ketosis followed by a return to an “all fuels” strategy, weight loss is often a big contributing factor to the increase in speed. That’s not a bad outcome, but they lost weight primarily due to caloric restriction. Diets that severely restrict or eliminate food groups cause people to pay a lot of attention to all food choices. This increased focus dramatically reduces mindless eating, and the consumption of junk food, alcohol, and excess sugar. It typically leads to increased consumption of fresh, whole foods. In the case of ketosis, it leads to increased consumption of whole food sources of protein, fat, and vegetables. That’s a good outcome, too, but caloric restriction is still the primary reason people lose weight while following a ketogenic lifestyle.

From a performance standpoint weight loss increases VO2 max (milliliters/kilogram/minute), improves power-to-weight ratio, and lowers the energy cost of locomotion. Even if your ability to produce power does not improve, you will go faster and be more economical when you lose weight. What doesn’t matter is whether you lost that weight through ketosis or through other ways of rebalancing caloric expenditure and caloric intake.

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Compliance is a major barrier to Sustaining ketosis

Advocates of dietary ketosis paint a picture of a glorious carbohydrate-free lifestyle where you’re not hungry as often and don’t suffer from energy fluctuations. From a health perspective, claims include decreased triglycerides, increased insulin sensitivity and reduced symptoms of Type II diabetes, lower blood pressure, slower growth in cancerous tumors, improved cognitive function, and many more.

I have been working with committed, goal-oriented athletes for more than 20 years. I have also witnessed countless diets rise and fall within the general population. We can barely get goal-oriented athletes to stick with an organized nutrition plan – inclusive of all macronutrients – for more than 6 months. In the general population, even people who experience great results with diets like Atkins (low-carb), Paleo (moderate-carb but no refined grains), The Zone (40-30-30) and Ornish (very high carb, extremely low fat) gradually move back toward 45-55% carbohydrate, 15-20% protein, and 25-30% fat within 12-24 months.

Dietary ketosis requires almost complete abstinence from carbohydrate, limiting intake to less than 50 grams (200 calories) per day for most people. And there are consequences for overconsumption, most notably that you kick yourself out of ketosis! Anecdotally, people who indulge in a slice of their kid’s birthday cake, a beer at the ballgame, or a banana in an aid station report feeling terrible afterward. For some, this negative feedback provides greater motivation to avoid temptations that knocked them out of previous diets. For the vast majority of athletes and sedentary people, even with good results the restrictive nature of the dietary strategy is too high a barrier for long-term compliance.

Ketosis is a Competitive Disadvantage

If eating a banana during a workout or competition will diminish your performance, there’s something wrong with your sports nutrition strategy. That’s not my bias toward sugar talking, but rather 40+ years of experience that has shown me that the best prepared athletes are those who are the most adaptable. To be a successful athlete you have to be able to perform using the fuel available and the equipment you have, in the environment provided. Courses change at the last minute, aid stations run out certain foods, your support crew can get lost, or your special food can fall out of your pocket. If you can’t immediately change your strategy to match the reality of your situation, you are at a competitive disadvantage.

Ketosis will be corrupted (already happening)

There may be some merit in a HFLC diet for disease prevention/mitigation, and it can be a lifestyle that promotes the consumption of whole foods and very little packaged food. But can the most vulnerable populations that would benefit most stick with it? If Atkins was too hard to stick with in its original form (more protein, fat, and fresh vegetables; less sugar and processed food) and was rapidly corrupted by packaged food manufacturers, how is it realistic to expect the overweight and/or chronic disease populations to adopt and stick with a much more intensive strategy?

Experience also tells me nutritional ketosis will be corrupted by supplement and packaged food industries the same way Atkins, Paleo, Zone, and other have been. The common pattern linking the rise and fall of popular named diets begins with a strategy that focuses on whole foods and somehow restricts energy intake. The strategy works, people feel great and lose weight. Foods and supplements are developed to make compliance more convenient, but these shift people back to old habits of consuming fewer whole foods. The packaged foods and supplements contribute to increased caloric intake, people regain weight, and once the positive results have disappeared their compliance diminishes and they return to their normal food choices and eating behaviors. As soon as you see “keto-cookies”, it’s over.

Exogenous Ketone Supplementation

From a sports science perspective, exogenous ketone supplementation is most promising development from this round of high-fat science (this isn’t the first time HFLC has come along). Ketone esters have made it possible to consume ketones in a drink or food and significantly reduce the time necessary to achieve dietary ketosis. They are also making the idea of “dual fueling” a compelling idea, in that an athlete could potentially supplement with exogenous ketones and thereby conserve limited carbohydrate stores for high-intensity efforts (Cox, 2014). This supports the fundamental sports science tenant of using training, nutrition, and recovery to maximize your body’s capacity to do work. I’m not sold on the science quite yet, but I think it’s an area that has promise.

Manipulating Carbohydrate Availability

Matching carbohydrate availability to training goals is a strategy that has been used successfully by amateur and professional athletes for a long time. There are various protocols for it, including “Sleep Low” (Marquet, 2016), but the basic idea is to complete high-intensity intervals and important competitions with high carbohydrate availability. During endurance or general fitness training, exercising with low carbohydrate availability can enhance weight loss and improve the body’s ability to metabolize fat for energy. I’ve written about Train Low before, and it’s a strategy supported by several of the sports scientists I respect, including Louise Burke.

So there you have it, at least for now. Sports science continues to develop and it is important to be open to new ideas and evaluate them on their merits. If you want to lose weight, ketosis or HFLC works. If you want to train effectively, a mixed diet with high carbohydrate availability for important workouts and competitions is your best bet.

Chris Carmichael
CEO/Head Coach of CTS

Jim Rutberg
Pro Coach and co-author The Time-Crunched Cyclist and Training Essentials for Ultrarunning


References and Suggested Reading:

Burke, Louise M., Megan L. Ross, Laura A. Garvican-Lewis, Marijke Welvaert, Ida A. Heikura, Sara G. Forbes, Joanne G. Mirtschin, Louise E. Cato, Nicki Strobel, Avish P. Sharma, and John A. Hawley. “Low Carbohydrate, High Fat Diet Impairs Exercise Economy and Negates the Performance Benefit from Intensified Training in Elite Race Walkers.” The Journal of Physiology (2016).

Burke, L. M. “”Fat Adaptation” for Athletic Performance: The Nail in the Coffin?” Journal of Applied Physiology 100.1 (2006): 7-8.

Burke, Louise M. “Re-Examining High-Fat Diets for Sports Performance: Did We Call the ‘Nail in the Coffin’ Too Soon?” Sports Medicine 45.S1 (2015): 33-49.

Cox, Pete J., and Kieran Clarke. “Acute Nutritional Ketosis: Implications for Exercise Performance and Metabolism.” Extreme Physiology & Medicine. BioMed Central, 2014.

Cox, Peter J., Tom Kirk, Tom Ashmore, Kristof Willerton, Rhys Evans, Alan Smith, Andrew J. Murray, Brianna Stubbs, James West, Stewart W. Mclure, M. Todd King, Michael S. Dodd, Cameron Holloway, Stefan Neubauer, Scott Drawer, Richard L. Veech, Julian L. Griffin, and Kieran Clarke. “Nutritional Ketosis Alters Fuel Preference and Thereby Endurance Performance in Athletes.” Cell Metabolism 24.2 (2016): 256-68.

Havemann, L. “Fat Adaptation Followed by Carbohydrate Loading Compromises High-intensity Sprint Performance.” Journal of Applied Physiology 100.1 (2006): 194-202.

Marquet, Laurie-Anne, Jeanick Brisswalter, Julien Louis, Eve Tiollier, Louise M. Burke, John A. Hawley, and Christophe Hausswirth. “Enhanced Endurance Performance by Periodization of Carbohydrate Intake.” Medicine & Science in Sports & Exercise 48.4 (2016): 663-72.

Pinckaers, Philippe J. M., Tyler A. Churchward-Venne, David Bailey, and Luc J. C. Van Loon. “Ketone Bodies and Exercise Performance: The Next Magic Bullet or Merely Hype?” Sports Medicine (2016).

Volek, Jeff S., Daniel J. Freidenreich, Catherine Saenz, Laura J. Kunces, Brent C. Creighton, Jenna M. Bartley, Patrick M. Davitt, Colleen X. Munoz, Jeffrey M. Anderson, Carl M. Maresh, Elaine C. Lee, Mark D. Schuenke, Giselle Aerni, William J. Kraemer, and Stephen D. Phinney. “Metabolic Characteristics of Keto-adapted Ultra-endurance Runners.” Metabolism 65.3 (2016): 100-10.

Zajac, Adam, Stanislaw Poprzecki, Adam Maszczyk, Milosz Czuba, Malgorzata Michalczyk, and Grzegorz Zydek. “The Effects of a Ketogenic Diet on Exercise Metabolism and Physical Performance in Off-Road Cyclists.” Nutrients 6.7 (2014): 2493-508.

Comments 31

  1. I disagree with you on this I have been running for 4 months after an 18 year break from back when I was airborne in the army. I am 43 and after 4 months back on the track I can run a 5k in the 29s and run 5 miles at a 10:40 pace and I have been on a low carb diet for about 4 months. I had 6 carbs the other day and did a 5k in 71 degree weather in 29:42 my personal best so far.

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  8. Interesting read, I’ve started looking into lchf recently after commiting to a 100mile foot race, my first over the distance, and first ultra for 2 years. I need to shift about 2 stone but the main draw was the reduced reliance on having to eat carbs, as it becomes a real chore over the 24-30hours I expect to be going. After a week of 25-50g carbs i feel good, and have lost the 7lbs of mainly carb-held water. Training has defo suffered as last week easily managed 13miles and this week struggled for 6.5. I understand that there is a difference between ketosis and fat adapted (I believe it can take a month or so).. SO all these articles are excellent in deciding whether it will my work for my very specific goal, or whether I need to up my carbs, or just abandon the experiment due to time constraints.

  9. I’m a retired physician. About 20 years ago I utilized the Atkins Diet to take off about 35 pounds that I really didn’t need. What I read in Dr. Atkins’ book and experienced myself was that there is first a rapid loss of 5-7 pounds–mostly water weight–as the ketosis has a diuretic effect. After that, the rate of weight loss slowed a bit.

    The single greatest help of the ketosis during the rapid weight loss is that the the ketones are appetite suppressants, so being in ketosis helps one not feel as hungry while one is not treating food, which in turn helps a lot in compliance with the diet. However, what actually happens is that it’s really just basic math–consume fewer calories than you burn, and you’ll lose weight. There is no substitute and no other way. When you stop eating one of the major food groups, your caloric consumption falls, thus the math shows the weight comes off. It’s still 3500 calories per pound of fat regardless of what fad diet one follows. This is true whether it’s Atkins, South Beach, Cabbage Soup, Paleo, or whatever moniker you attach to the diet.

    In the end, the ultimate weakness is that any diet that eliminates a major food group is not maintainable long-term for virtually anyone without tremendous self-discipline. This is true whether that food group is carbohydrates or fat or protein. Thus, at some point, the person will stop this “diet”, and will either resume the old dietary patterns (and regain the weight), or will have to change to a new lifestyle of eating to maintain the weight loss.

    The other issue is that it’s impossible to exercise enough to overcome poor dietary choices. (Jim Rutledge points this out quite well in another response.) This is all the harder when we’re bombarded with slick marketing of too-readily available energy-dense processsed foods and snacks–especially the tasty ones!

    Regardless, there’s no escaping the simple mathematics of the calorie balance. I totally agree with the statement to exercise for fitness and eat for weight management.

    1. You make some great points. The carbs that those in ketosis try to avoid or limit are the breads, pastas and anything that is white or can be white. The body is not a closed system. 1000cals of salmon vs 1000cals of cheese cake will effect the bodies systems differently. While I agree that those in dietary ketosis tend to consume few calories and thus it can lead to weight loss. It is not always the case. Dr. Peter Attia showed this in his personal journey of being in dietary ketosis. Chris Froome and Team Sky have been experimenting with ketosis and exogenous ketones now for 3+ years. The majority of their riding does not require rocket fuel. Fat has become their friend. When necessary they will consume sugar/gels and as they are already fat adaptive and insulin sensitive the sugar acts as the rocket fuel and surprisingly, the effects lasts much longer that the average pro rider, at least that’s what their testing showed. Froome will binge on carbs after a particular hard stage and will consume up to 200grams of carbs. This sugar will go right back to the muscles so he can fight another day.

  10. Hi, as a coach with 5 Spartathlon victories and also course record from this 246Km long running event, I can tell my runners diet is high in carbs. Of course most of the runs are with empty stomach, and some of the long ones with no or minimal carb intake to increase fat burning ability, but specially after training, we are eating carbs.
    So periodize training and carb intake as well.

  11. I am interested in the possible inclusion and benefit of exogenous ketone fluids as an additional fuel source for my endurance running training and racing. I have previously made the decision that i neither have the need or the desire to achieve full ketosis for the reasons well articulated in the above CTS article. So, two questions:
    1. recommended guidelines/protocols for the inclusion of exogenous ketone fluids?
    2. recommended producers/sellers of ketone fluids? Please disclose conflicts/affiliations if you are a producer/seller. THX

  12. A couple years ago I started doing a variation on the “Train Low” idea. On training rides, I also train my metabolism simply by riding as long as is comfortable without eating. My time limit has gradually increased until I can now do a 3 hour hard ride with no food and no discomfort. I do the same when I day hike or snowshoe.

    What changes is increased fat use and decreased glycogen use. This is about the same result claimed by the LC advocates. However I eat a HC diet which is also rich in protein. On “serious” or event rides when I want a good result, I eat my usual HC foods right from the start. Actually, I drink a 7:1 maltodextrin/flavored whey protein homebrew and eat little solid food on event rides, brevets, etc. I find I need less of this beverage now than I did before I became “fat-adapted.”

    After every ride or workout, I have the usual recommended ~4:1 carb/protein recovery drink.

    This adaptive process has not hurt my anaerobic power or anaerobic endurance.

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  14. sorry but I disagree. I am a coach and currently have several clients that were carb a holics like I was. They were also FAT. They are now slim lean and much faster and have less health problems. I have also moved from 25 in my AG to 13th in the country all from being lean and racing on strategic carbs but training largely on a LCHF approach. In the end you can’t say it does not work. If athletes are insulin Resistant and they are FAT then it will help them easily lose excess weight once they adapt they will be healthier and because they have lost excess kgs they will have better pwr to weight ratios and be faster. If people can tolerate a heavy carb approach then great good on them. There is no one size fits all. So maybe go and ask a few people like Dave Scott and other people how they eat… LCHF

    1. Post

      To summarize, you eliminated a food group and ate primarily whole foods and lost 22 pounds while continuing to train. Of course you got faster. You used diet to lose weight, but when you want to be competitive you still recognize the benefit of carbohydrate for high intensity. I think you agree with the article more than you disagree with it. – Jim Rutberg, CTS

  15. I transitioned into a keto diet November 2015 while drinking exogenous ketones and it has been very easy to stay with it all Year! I initially started Drinking ketones (and gaining ketosis in less than an hour) because I wanted to get rid of the extra winter weight that comes from not riding much during the off-season. When I dropped all of my gained weight in just a matter of weeks back in December 2015, I started realizing that there were also performance benefits from tapping into my own body fat for fuel!

    All last year I fueled my rides that ranged from 40-115 miles (completed 10+ centuries Last year) completely on body fat and Ketones! Absolutely no carbs! My energy during rides is high and consistent with no “bonk”!! On longer rides (80-100mi) I put Ketones in my water bottle and I don’t even get hungry! I also find that I can maintain a higher level of exertion for a longer period of time with little fatigue (because fat fueling requires less oxygen). I notice that during these harder efforts I can maintain a higher heart rate for longer which translates to better performance!! I also notice I recover faster during my rides between hard efforts as well as between workouts! It’s amazing! And I’ve seen the same thing on every ride since starting to use this! Keto diet AND exogenous ketones is AMAZING! It took several months to get used to fueling only on body fat but because I was drinking ketones, my performance never suffered and it only improved after fat-adaption happened!

    It sounds like this article was written by someone with no personal experience with fat adaption and using ketones and fat for fuel! I would love to hear you interview Chris Froome and Romain Bardet who are both keto experts!

    Regards! Love your other training tips!

    1. I’m sorry to say that your comment seems heavily misinformed.

      You say that you ‘ wanted to get rid of the extra winter weight that comes from not riding much during the off-season’. Presumably the fact that you actually started training more helped you lose the additional body fat?

      You also say ‘I can maintain a higher level of exertion for a longer period of time with little fatigue (because fat fueling requires less oxygen)’. This is simply incorrect as fat actually requires more oxygen to breakdown than carbohydrate. There are multiple studies demonstrating that VO2 is higher as fat utilisation increases.

      Finally, your proposal at the end that cyclists such as Chris Froome are keto experts is again simply untrue. I attended a lecture by James Morton (head of performance for team Sky) less than two weeks ago who confirmed that all members of the team follow a periodised carbohydrate rich diet.

    2. Post

      We talked about this on FB. I respect the fact you sell Pruvit ketone supplements and you use and believe in the products you sell, but you should include that in your comments. And like Ryan, I have a little trouble with the science behind your statements.

      “I notice that during these harder efforts I can maintain a higher heart rate for longer which translates to better performance!!”: How? Heart rate response is affected by hydration status, core temperature, stimulants, fatigue, stress, etc. If anything, producing more power at the same or lower heart rate would be an indication of improved performance. Maintaining a high heart rate for longer doesn’t mean your power output remained high as well, it just means your cardiovascular system was working harder.

      I’m glad your performance has improved, but if you lost weight, increased training volume, maybe included some structure, and have been enjoying riding because you feel better, of course your performance (power-to-weight, VO2 max in ml/kg/min) improved. And because of the weight loss you are also better at dissipating heat and have a lower oxygen cost for locomotion, meaning you can go further on less fuel than you consumed before. The exogenous ketones may be helping, but my point is you may be giving the elimination of carbohydrate too much credit for your performance. – Jim Rutberg, CTS

    3. Your comments and they way you CAPITALISE everything make you sound like an infomercial for ketone supplements. Disclosure!

  16. GI has always been my issue. As a hobby nutritionist and scientist I have tried every strategy, being a millimeter man I have worked on percentages amounts per hour bodyweight fluid loss etc. etc. Going LFHC was the single best thing I did to prepare for event over 3 hours. Like so many others I can race and compete all day with maybe 200 calories for an entire 4 hour ride. No doubt, you’re high and will suffer initially until you adapt but then he recovers fine. My power output, heart rate etc. all showed improvement on LFHC. Everyone is different and my gut was always my achilles heal despite tedious attention to detail. No more. I am 56 year old mtn and road rider. My performance and results mostly improved on LFHC but more importantly I felt great and recovered well after events. Once you are keto adapted, you can add carbs back into your fueling for races and events by the way.

  17. Everyone is different metabolically. Laziness with canned workouts and “eat this way” theory are insane at best. Being a type 1 diabetic and listening to the so called experts on what’s best for me always left me struggling to improve. Once I took a step back and looked at my situation, I realized that the LCHF diet was not only a performance enhancer for me but I was able to get off an insulin pump and improve my metabolic function. Now I have been very good at keeping records, to include AIC, Blood pressure, Lactate levels and all I see now Is a slow, consistent improvement. I sleep better, I recover faster, cognitive function has increased, I’m not always wondering have I eaten enough to perform a workout, will I get low while training. All these were of great concern when I showed up at a race with a bar of this, a bar of that, loaded up in my jersey pockets, and whatever was the “craze” for quick energy drink in my bottles. Now I only carry water and just in case flask of energy gel. I can now ride for 3 hours solo, no carbs. I’m 188 lbs, and I’m climbing faster now at endurance then anything I was doing before. I have actually put on weight, what gives? Heart rate overall is lower for any exertion. All I can see now is continually improvement on the horizon. Full disclosure I’m on a 80/15/5 LCHF. Started 12 may 2016. And yes you will see a definite drop off in power and energy level in the first month but I can tell you if your are patient and you figure out “how” much carbohydrates you can tolerate then I believe you are well on your way to make consistent improvements both in health and performance. There is definitely a place for carbohydrate’s in cycling like a crit (1 hr). But lets look at American sports that promote carb’s. How long do they last. Football 1 hour, 4 quarters, 15 min each, Hockey 1 hour, 3 periods, 20 minutes each, Basketball 1 hour, 4 quarters. You ever watch a overtime game, everyone is in slow motion, I thought these were the most highly trained specimen’s in our midst? And how much glycogen stores does “everyone” have? You guessed it enough for 1 hour +/-. So that’s how you are trained into believing a one hour turbo session requires carbs, before during and after. All the best in the new year!!

    1. Dear Jason,
      I’m not type 1 diabetic but i’m on a LCHF “rehab” about a year – not as low as yours, with a 20hs intermitent fasting 2 times per week. I swim, cycling and do resistance training 4/5 times per week and I can say undoubtly that all the results you described happens with me too.
      It’s amazing become a non “carb-addicted” and experiences all its benefits – an overall improvement in life and sport performance.
      When a I read the article below – an iroman legend sharing his LCHF nutrition daily strategy, it was amazing!!! It gave me strengh to keep punching (going) and stay on focus.
      We definitly need much less CHO we think we need.

  18. As an avid reader of Dr Attia and Phinney and Volek I experimented with essentially no carbs for about two months.
    From a performance standpoint my ability to do long endurance rides at the top of my endurance zone showed a marked improvement. I could power along for hours with little need for food and much lower water intake, but my top end power definitely suffered. Sprinting and out of the saddle climbing suffered.
    Physically I dropped weight even though I was already thin.
    Mentally the diet is hard to stay on for more than a few months.
    It did force me to rethink everything I eat from a sugar standpoint. I eat far less now.

  19. We at First Endurance came to these exact same conclusions as Chris in this article. Remembering that we are talking about the benefits to performance in endurance athletes, not the ‘potential’ health benefits to the general public. For endurance athletes the vast majority will seen no benefit from a keto diet. Ultra long distance athletes ‘may’ find that they can perform at an aerobic pace with fewer calories, however if they need to push above threshold levels they will suffer. We feel this benefit may be best reserved for athletes going 20+ hours.
    We do feel that spending a few months on a modified Keto diet may benefit the long distance athlete by creating even a slight shift towards using fat as a fuel. Benefits are likely minimal and may only be worthwhile for athletes that have trouble consuming enough calories during long distance exercise. Ultimately glucose/glycogen will be the prefered fuel source when racing and athletes should be prepared by consuming the right sugars at the right times.

    1. Also, keep in mind (didn’t see it mentioned in the article above), that keto diets improve fat utilization *at the cost* of carbohydrate utilization. We can adjust our metabolic profile easily through nutrition, but we need to understand the underlying mechanisms at work. The reason you can’t go hard on a keto diet is not only because of impaired CHO storage, but also because you cannot *access* the CHO due to the PFK reaction in glycolysis being inhibited through high fat oxidation.

  20. I agree with Chris BeHanna’s comment above. LC in the range of 150-200 grams is adequate to fuel most workouts and shift away from the fumes of a high CHO diet. Keto is the extreme low end that has drawbacks that get confused with LC.

  21. One major topic that you did not address was the contribution of a high-carbohydrate diet to heart disease via high levels of insulin. As Jim Fixx unfortunately demonstrated (and as I personally experienced by failing a stress test a few years ago and winning a trip to the cath lab), endurance exercise does NOT offer adequate protection when you are shoveling the wrong things into your mouth.

    I’ve adopted a hybrid approach: no grain, very little sugar, and almost never eating starchy vegetables (corn, potatoes, etc.). On an average day, I eat about 100-150 grams of carbohydrate. My lipid profile has become astonishingly good, and my LDL-P (particle concentration, a much better predictor of coronary events than LDL-C), is now just over the lowest-risk threshold. My power at threshold is up dramatically, and I can complete fast, long rides without having to eat along the way (most recently, I finished the Hotter Than Hell Hundred in 4h 47m).

    Dr. Peter Attia, himself a triathlete, discusses this at length in his “The Truth about Cholesterol” series on his website, complete with citations to the literature.

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