Is The Keto Diet Healthy For Endurance Athletes?

Is the Keto Diet Healthy for Endurance Athletes?

BY Dr. Jeff Sankoff

The ketogenic diet is particularly popular among athletes — but are its merits based on fact, or fiction? Doctor Jeff Sankoff debunks the myth.

As rates of obesity continue to rise in the western world, fad diets have become more and more numerous, but, alas, no more likely to accomplish any of the remarkable successes that they promise. One that’s gained significant traction among endurance athletes in recent years is the keto diet. This is the latest iteration of the low-carb Atkins diet that first appeared back in the early 70s. Though supporters will argue that this diet is based on science, the reality is that it doesn’t give athletes the nutrition they need to perform their best. Here’s what you need to know.

Our Relationship with Carbohydrates

The keto diet is based on a couple of fundamental principles rooted in observational medical science. The first of these is that obesity has risen in North America and the western world in conjunction with the intake of highly processed foods and refined sugars. The second is that repeated spikes in blood glucose, and the resultant insulin spike that accompanies it, is associated with long term health issues. Both of these observations are true but the misinterpretation of them has led to the development of the keto diet and everything that has followed since. 

The human body has evolved over hundreds of thousands of years to develop a complex endocrinologic response to food. Our cells can burn different types of fuels (like carbohydrates or sugars, fats, ketones, and proteins), but their preferred fuel is carbohydrates because that is the fuel that is burned most efficiently. When food is in scarce supply, our body will utilize other stored fuels to create carbohydrates while making use of the less efficient forms of fuel as well. 

In the past century, food has suddenly become plentiful beyond belief, and our body’s evolutionary food storage mechanisms have consequently become overwhelmed with this bounty. Designed for storage, our cells, when faced with more and more carbohydrates, continuously convert the excess into forms of storage (like fat), preparing for times of scarcity that never come.

Behind the Low Carb Diet

Enter Robert Atkins. His low carbohydrate, high fat, and high protein diet proved enormously popular, mostly because it did work to eliminate fat — at least initially. Many who adopted it lost weight, but soon found that the diet was too difficult to adhere to and then abandoned it completely, only to regain all the weight they had lost. In addition, the high fat component of the Atkins diet proved dangerous to those at risk of cardiac disease, who were developing problems related to the intake of saturated fats promoted by the diet.

Over time, the Atkins diet has reappeared in various forms, and most recently in the athlete-friendly keto diet. In this version, fats are not accentuated as much as protein, but the emphasis on extreme carbohydrate restriction remains. The theory for athletes is that by training the body to become more efficient at using alternative fuel sources like ketone bodies, they will have more success during a race.

Debunking the Keto Myth

Contrary to what keto supporters would have you believe, their claims do not hold up to the scrutiny of scientific research. First and foremost, research has demonstrated that the ketogenic diet is extremely difficult to adhere to. Because it is very restrictive and causes those who follow it to experience mood swings and increased irritability, more than half who start are unable to stick with it. Among those who do persevere, weight loss is seen but is transient. And as soon as dieters quit, all of the weight returns.

Celine Evans is a registered dietician and a nutrition coach with LifeSport Coaching in British Columbia, Canada. She is wary of the claims that the ketogenic diet has any positive effects on athletic performance. “In a recent review of eight different studies of the ketogenic diet in cyclists and runners, not only was there a very high failure rate of adherence to the diet (more than 50% in some studies), but among those who could complete the study, there was no benefit to performance seen in any discipline or any distance measured,” she said.

Evans also worries about the unintended consequences of strictly adhering to the ketogenic diet, namely with the restriction of foods and the impact on the gut microbiota. “By severely curtailing the kinds of foods that they eat, people on a ketogenic diet can experience micronutrient deficiencies, significant constipation from inadequate fiber intake, and significant alterations to their gut microbiome that can have an impact on their immune system as a whole,” she said. “This is especially significant for women who must be very wary of taking up this kind of diet.”

Why Keto Doesn’t Work for Athletes

There is also a claim by proponents of the keto diet that training in a ketogenic state and then racing using carbohydrates as a primary fuel allows them to efficiently make use of both sources of fuel, giving them the advantage of metabolic flexibility — but research says otherwise. Studies have shown that those who train in a ketogenic state and then switch to carbohydrates for events are actually less efficient at using carbohydrates than they were previously, and have also demonstrated a deterioration in their overall performance.

Allen Lim is a food scientist and the co-founder of Skratch Labs and also has concerns about athletes using ketogenic diets that go beyond the unsubstantiated claims of gaining a performance edge. “There is a basis of food that is about so much more than performance — it’s about culture, our social needs. Most of the people who embrace this diet do so to the extreme and it tends to be really isolating and I think that the isolation that this diet can cause brings with it more mental health issues than are worth it.”

Lim feels that with respect to keto, context matters. “In shorter, higher intensity events like triathlons, ketogenic diets don’t make a lot of sense. Our bodies use carbohydrates much more efficiently, and given today’s races, an athlete is never going to be faced with a time when they won’t have carbohydrate on them or readily available on the course. For ultra endurance events, in many ways if you are training properly for it, you are going to naturally fat adapt because the athlete is performing at a much lower intensity for a prolonged period of time, and here fat metabolism is an efficient means for the body to use.” Lim continues, “Our bodies are really well suited to adapt to starvation and these events in many ways are essentially starvation events.” His main point here is that even for these athletes, a keto diet is not necessary as the training alone will allow for their bodies to adapt seamlessly to using alternative metabolic strategies.

I know that this article will not persuade any adherents to the keto diet against it, and, in truth, that is not its purpose. Rather, it is intended for those who have heard the myriad claims about the diet and may have been considering it for themselves. If you are one of these people, the science is clear: the keto diet is not a panacea and in fact is likely detrimental to endurance performance. 

Now, have a cookie to celebrate. Just one, though!

Train hard, train healthy.


1. Vranceanu M, Pickering C, Filip L, et al. A comparison of a ketogenic diet with a LowGI/nutrigenetic diet over 6 months for weight loss and 18-month follow-up. doi:10.1186/s40795-020-00370-7

2. O’Neill B, Raggi P. The ketogenic diet: Pros and cons. Atherosclerosis. 2020;292(September 2019):119-126. doi:10.1016/j.atherosclerosis.2019.11.021

3. Mcswiney FT, Doyle L, Plews DJ, Zinn C. Impact Of Ketogenic Diet On Athletes: Current Insights. 2019. doi:10.2147/OAJSM.S180409

4. Shaw DM, Merien F, Braakhuis A, Maunder E, Dulson DK. Exogenous Ketone Supplementation and Keto-Adaptation for Endurance Performance: Disentangling the Effects of Two Distinct Metabolic States. Sport Med. 2020;50(4):641-656. doi:10.1007/s40279-019-01246-y

5. Burke LM, Whitfield J, Heikura IA, et al. Adaptation to a low carbohydrate high fat diet is rapid but impairs endurance exercise metabolism and performance despite enhanced glycogen availability. J Physiol. 2021;599(3):771-790. doi:10.1113/JP280221

6. Sherrier M, Li H. The impact of keto-adaptation on exercise performance and the role of metabolic-regulating cytokines. 2019. doi:10.1093/ajcn/nqz145

7. McSwiney FT, Wardrop B, Hyde PN, Lafountain RA, Volek JS, Doyle L. Keto-adaptation enhances exercise performance and body composition responses to training in endurance athletes. Metabolism. 2018;81:25-34. doi:10.1016/j.metabol.2017.10.010

8. Melby CL. High-fat versus high-carbohydrate diets for optimal higher intensity endurance exercise performance: and the winner is…. J Physiol. 2021;599(3):727-728. doi:10.1113/JP280552

9. Casazza GA, Tovar AP, Richardson CE, Cortez AN, Davis BA. Energy Availability, Macronutrient Intake, and Nutritional Supplementation for Improving Exercise Performance in Endurance Athletes. Curr Sfile///Users/Macsimus/Downloads/Energy_Availability,_Macronutrient_Intake,_and10.pdfports Med Reports. 2018;17(6):215-223. doi:10.1249/JSR.0000000000000494

10. O’brien LT, Powers S, Hamilton K. The Journal of Physiology Keto or carbo? Low-versus high-carbohydrate diets for fuelling elite endurance sport performance. J Physiol. 2021;599:731-732. doi:10.1113/JP280221

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About Dr. Jeff Sankoff

Jeff Sankoff is an Emergency Physician, long time triathlete and USAT/IRONMAN University Certified coach. He has completed 6 IRONMAN races (including Kona) and more than fifty races at the 70.3 distance including 5 World Championships. He produces the TriDoc Podcast that can be found on most popular podcast platforms and iWork’s with LifeSport Coaching. You can learn more about him at his website: TriDoc Coaching.

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