Ketogenic diets hit the mainstream in a big way last year after more than a decade of build-up in athletes in the CrossFit and ultra-endurance scene, and more than 30 years after initial research into the diet. The very low-carb, high-fat keto diet has shown some impressive results in people who are overweight, obese, struggling with type-2 diabetes or metabolic syndrome.(1,2,3) The reduction in carbohydrates in these groups - typically from elimination of processed foods, sugars, desserts and alcohol – results in a significant caloric reduction which is the key underlying principle of all successful weight loss regimes. Keto diets also increase your ability to burn body-fat as a fuel source, and perhaps the most important factor, helps your feel full and satiated for much longer periods throughout the day, new research highlighting this may be due to a reduction in triglycerides crossing the blood brain barrier.(4)
Okay, so keto diet can be a very useful tool for weight loss and improving metabolic health, but what about performance? The internet is loaded with anecdotes about transforming yourself into a ‘fat burning machine’ by shifting to a keto diet, as well as emphasizing to recreational athletes that ‘carbs are for suckers’, implying endurance performance is superior when you follow a keto diet compared to the traditional high carb diet. Are all of these anecdotes correct? Are sport scientists missing out on something? And is there any research to back up these claims? Let’s dig a little deeper and find out.
There is actually very little research on ketogenic diets and endurance performance. The original study on keto diets and performance was done by Stephen Phinney in the early 1980s on five cyclists. Among this group, two cyclists improved their endurance, one saw no change and two cyclists got worse.(5) Not exactly a home run in terms of evidence, however this study was ground-breaking because it showed fat-adapted athletes could burn far more fat (1.5g/min) compared to what had previously thought to be the maximum rate (1.0g/min). It’s an impressive finding. Advocates of a keto strategy for endurance performance highlight the fact that body-fat stores provide a massive fuel source, because even an athlete at 10% body-fat has approximately 30,000 calories of excess energy. Unfortunately, a major shortcoming of Phinney’s study, if you’re trying to win a race, is that cyclists trained at 62-64% of their Vo2max, not exactly a speed that will win a race.
Fast-forward to 2016 and Phinney and colleague Jeff Volek published the FASTER (Fat-Adapted Substrate use in Trained Elite Runners) study in elite ultra-endurance triathletes and runners. The long-term keto adapted athletes (almost 2 years adhering to the diet) were found to burn fat at a high rate (1.2g/min) during a 3-hour submaximal aerobic test at 64% VO2max.(6) Another study in 2016 by Tim Noakes group comparing keto-adapted endurance athletes – following a diet of less than 50g of carbs and 70% fat for over year – were pitted against a group of higher carb athletes in a 2-hour ride at 70% VO2max. Researchers found the keto group did indeed burn more fat (1.2g/min) compared to the high carb group (0.5g/min).(7)
All of this information is really interesting, but it still doesn’t answer the most important question; what happens on race day? When you’re trying to beat the competition, does keto help or hinder an endurance athlete.
Dr. Louise Burke PhD, Head of Sports Nutrition at the Australian Institute of Sport, put this question to the test last year. Her research group took a collection of elite and Olympic race-walkers, adapted them to a ketogenic diet, and simulated a race to examine the impacts on performance. To date, this is the best study on the impacts of keto diet on endurance performance. Let’s see what they found.
Twenty-one male racewalkers participated in the study at the Australian Institute of Sport (AIS), living in residences where all their meals and training was under strict supervision from the research team. These were not your run of the mill recreational race walkers. They were selected based on their performances and rankings by the IAAF (International Association of Athletics Federation) and made up of 7-time Olympic and World championship medalists to world-ranked juniors.
The study involved three different training groups; a high carb group, a periodized carb group and a keto group. All the participants underwent three weeks of intense training - race walking, lifting, and cross-training (running, cycling, or swimming) – and were tested pre- and post- competition to assess the impact of the three diets on performance.
What did Dr. Burke and her team uncover?
After the three weeks of training, all three groups of racewalkers improved their aerobic fitness regardless of which diet they were on.(8) (Not really surprising, considering they were all in a training camp setting. If they didn’t get fitter, something would really be off piste).
Next, the keto group of racewalkers did exhibit significantly higher rates of fat oxidation during the race walk competition over a wide range of intensities, averaging about 1.5/min.(8) (This is on par with Phinney’s earlier study in the 1980s).
Sounds great, but how did they the keto group perform in the 10km race and 25 km long walk event? In the 10km trial, both the high-carb and periodized-carb group improved their race time, by 6.6 and 5.3% respectively (over their pre-camp testing), following the three-week training camp.(8) Unfortunately for the keto group, their performance was marginally worse during the 10km race (see figure below).
In the 25km long walk, the keto group showed an increase in heart rate compared the both carb groups who experienced reductions after the long walk. The perceived exertion was also greater in the keto group compared to carbs race walkers.(8)
Why did the low-carb group struggle to perform as well as the carbohydrate fueled racers? Ketogenic diets have a negative impact your running economy.(8) Running economy is how much oxygen you used up at a given running pace. A runner with a superior running economy uses less oxygen at a given speed, which is a reliable metric for who will win a race. In Burke’s study, the fat-adapted elite keto racewalker did burn more fat, but at the expense of a reduce running economy (which means higher oxygen demand) at real-life race intensities. This wasn’t the case for either the high or periodized carb groups. (And don’t forget, the more fat-adapted you are, it comes at the expense of your ability to burn carbs effectively for fuel - pyruvate dehydrogenase enzyme levels fall – which is another performance roadblock).(9)
What does all this mean for you?
If you’re an elite athlete or performance-driven, you need carbs to win on race day. Keto diets reduce running economy (increasing your oxygen demand) as well as increasing heart rate and perceived exertion. At the moment, there just is not enough evidence to support going 100% keto, nobody has beat the competition using this strict dietary strategy. (Even renowned ultra-marathoners in the blogosphere who are “keto” athletes rely primarily on simple carbs when it comes to race day).
Of course, this isn’t a black or white situation. There are definite advantages to training in low-carb states (called low carb availability in the research) during your training blocks to trigger beneficial adaptations in the lead up to competition. Dr. Burke and her team acknowledge this in their research, stating … “a periodized programme that includes some training sessions deliberately undertaken with low… carbohydrate availability (‘train low’) or a delay in replacing muscle glycogen after a session (‘sleep low’) may promote greater cellular adaptations to training and enhance performance to a greater magnitude than undertaking all sessions with high carb availability.” This is a powerful statement.
If you’re a low-carb or keto athlete, it’s effectively saying you can absolutely get benefits by using these approaches during your training block. It can be a great training-nutrition strategy. Just don’t confuse that with your “race-day” nutrition strategy, where carbs are still king.
This doesn’t mean you commit to 100% keto or 100% high carb all of the time. As Dr. Burke highlight in her study… “the quantity and timing of carbohydrate intake should be personalized to the athlete and periodized within the various micro- and macro-cycles of training and competition.”
Just like you wouldn’t train the same way every day of the week, so why do you eat the same way? Regardless if it’s keto or high-carb. (There is a lot of nuance in different fueling strategies, and I’ll get into these in more detail in future blog posts).
Individualizing your carb intake to match your goals is a crucial piece of the puzzle.
Also, take into account your current fitness level and body composition.
If you’re a recreational exerciser training up for a half-marathon or marathon with the aim to lose weight and improve health, yet you’re guzzling back sports drinks every session and seeing not shift in body composition, then your fueling strategy is flawed. Re-assess and adjust accordingly.
Bottom Line: At the end of the day, what are you trying to accomplish? If it’s to win the race or perform your personal best then the research shows race-day nutrition should definitely have carbs (as well as in the 24-hours leading up to the race). If it’s to lose weight and improve metabolic health, and not about your personal best, then you’re fine to err more on the side of low-carb to reap more benefits on the weight loss and health front. But, don’t be afraid to add more carbs on days when training intensity is higher to maximize your adaptations to training.
There is a lot nuance and no single correct answer when it comes to fueling for sport. Just remember to always consider the context of your specific athlete, and their individual goals, when designing the right nutrition strategy for them.
Dr. Marc Bubbs ND, MSc, CISSN, CSCS
Ps. Wonder how exogenous ketone supplements would impact endurance performance? Stay tuned for an upcoming podcast episode on this topic… (Subscribe here!)