Should Endurance Athletes Go Keto? Ketosis and Ketogenic Diets for Endurance Athletes in 2020

Chris Carmichael shares his analysis

When it comes to weight loss and endurance performance, dietary ketosis is a strategy everyone asks about. 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 books, and the supplement companies have 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.

Dietary ketosis for athletes is a hotly contested subject. 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:

Both dietary ketosis and the use of exogenous ketone supplements have limitations that make them 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 low-carb, high-fat (LCHF) diet (70-80% Fat, <5% CHO). This means you may be able to sustain a submaximal pace equally well using either strategy. The LCHF 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 for exercise increases while exercising on a LCHF strategy (Burke, 2016 and 2020). It takes approximately 8% more oxygen to liberate energy from fat compared to carbohydrate, which means relying primarily on fat reduces economy.

To win races you have to do two things. You have to be able to go fast in training to induce the stress necessary to stimulate positive adaptation, and second, you have to be able to go fast at crucial times during competition. However, Burke’s findings in 2016 showed that despite nearly doubling the oxidation of fat for fuel during competition, a LCHF dietary strategy made elite racewalkers slower compared to racers using a high-carbohydrate strategy.

Updated Findings from Burke 2020

Twenty-six world-class racewalkers attended a 3.5-week training camp, during which a High Carb (HCHO) group consumed 60-65% carbohydrate and the Low Carb (LCHF) consume 75-80% fat and fewer than 50 grams of carbohydrate per day. Diets were energy matched and all athletes performed the same workouts. Athletes completed a 10k performance test at the beginning and end of the training camp, and the HCHO group improved their 10k performance by 4.8% (2:14 faster) while the LCHF group slowed by 1.6% (1:26 slower). The HCHO group increased their relative economy, meaning they could maintain a given pace at a lower percentage of their maximal aerobic capacity (VO2 max).

Read full text of the study here. Amby Burfoot also wrote a very good piece on the study that’s available on

Some critics of Burke’s 2016 study claimed that the benefits of adapting to a LCFH diet would allow an athlete to improve performance by returning to a diet with high carbohydrate availability during final preparation for competition, the idea being that the combination of high carbohydrate availability and improved fat oxidation would enhance performance. So, for the 2020 study, following the training camp all subjects consumed a high-carbohydrate diet during a two-week taper leading into a 20km racewalk competition.

Some athletes from both groups achieved personal bests in the 20km competition, indicating the training camp and taper were effective. For the HCHO group, performance level in the 20k was improved from the start-of-camp test and similar to their end-of-camp 10k level. The LCFH group, who trained for 3.5 weeks using a low-carb diet and then tapered and raced with high carbohydrate availability, bounced back from their poor end-of-camp 10k results to achieve a performance gain over their start-of-camp test that was similar to that of the HCHO group. In other words, the training worked and the performance detriment from the LCHF diet disappeared after the reintroduction of carbohydrate, but no additional performance benefit was gained.

One of the primary goals of endurance training is to increase the pace or power an athlete can sustain at any given percentage of his or her maximum aerobic capacity (VO2 max). Dietary ketosis or a LCFH dietary strategy makes an athlete utilize more oxygen to go the same pace, meaning that during steady state exercise you are operating at a higher percentage of your VO2 max for no performance improvement. When it comes time for high-intensity efforts that are necessary to win races (even for ultra-endurance cyclists, iron-distance triathletes, and ultramarathon runners), this reduction in economy lowers your maximum power/pace at VO2 max. Diet ketosis and LCHF will increase fat oxidation during exercise, but the ability to improve fitness and competitive outcomes depends on more than your ability to burn fat.

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.)

Adapting to a HCLF diet also has the effect of impairing glycolytic pathways by downregulating enzymes necessary for burning carbohydrate during high intensity efforts. As a result, the thought is that the oxidation of carbohydrate is limited even when there is plenty of carbohydrate available.

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.

Ketosis may prevent gastric distress

On the positive side, 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 the first week people often experience the “keto flu”, which is not an infection but rather a set of symptoms reminiscent of the flu: Headache, foggy brain, fatigue, irritability, nausea, difficulty sleeping, and constipation. 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 (this was noted in Burke 2020). 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. Some of the acute weight loss is also due to the fact you store 3 grams of water with every gram of glycogen stored in muscles. So, less muscle glycogen also means less stored water.

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 work does not improve, you will go faster and be more economical when you lose weight. (The LCHF group of Burke 2020 experienced a net loss of economy despite weight loss, but it is important to recognize they were elite race walkers with high VO2 max values and low bodyweight to start with. For the vast majority of athletes, modest weight loss will improve economy and increase VO2 max.) What doesn’t matter is whether you lost that weight through ketosis or through other ways of rebalancing caloric expenditure and caloric intake.

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 30 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 as an athlete and coach 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 LCHF 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 was the most promising development from this round of high-fat science (this isn’t the first time LCHF 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 also made 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.

Unfortunately, current research indicates exogenous ketone supplementation is rarely effective as an ergogenic aid. In an opinion published in the December 2019 issue of Sports Medicine, David Shaw et al. state: “No beneficial performance effects during high-intensity exercise have been demonstrated following the ingestion of exogenous ketone supplements that increase blood D-β-hydroxybutyrate concentrations up to ~ 1 mM. Racemic ketone salts and BD (1,3-butanediol) exert no effect on oxygen uptake, blood glucose concentration, lactate accumulation, or RPE at exercise intensities > 70% VO2max, and demonstrate no or negative effects on performance.”

That said, Tour de France teams and national teams have been purportedly utilized ketone supplements on and off over the past several years. examined the use and evidence for efficacy in early 2020. It seems the benefit, if there is one, may be related to recovery during long blocks of racing or training. The cost, however, is still prohibitive for most athletes.

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. In the aforementioned 2020 study of racewalkers, there was a third group that utilized a “Train Low” carbohydrate availability method, and their end-of-camp performances were better than the LCHF group and not as good as the HCHO group.

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 LCHF 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
Founder/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 et al. “Crisis of confidence averted: Impairment of exercise economy and performance in elite race walkers by ketogenic low carbohydrate, high fat (LCHF) diet is reproducible.” PloS one vol. 15,6 e0234027. 4 Jun. 2020, doi:10.1371/journal.pone.0234027

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).

Shaw, D.M., Merien, F., Braakhuis, A. et al. Exogenous Ketone Supplementation and Keto-Adaptation for Endurance Performance: Disentangling the Effects of Two Distinct Metabolic States. Sports Med 50, 641–656 (2020).

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.

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