Keto & Endurance Performance: Reviewing a Key Paper

Long time, no nerding. Ketogenic diets have grown in popularity in recent years. Literature is at odds as to the success of ketogenic diets for performance. So, what better way to discuss this than to get stuck into a key research paper?

The article in question comes from a 2017 article in Metabolism, and can be accessed here.

No Keto, No Problem: High-Carbohydrate for Endurance

Historically, high carbohydrates have been utilised for optimal endurance performance. The reason for this being that as exercise increases in duration, the body increasingly relies on glycogen. Carbohydrates are rapidly broken down to form glucose (and subsequently glycogen). The theory is that by maximising carbohydrate intake, we optimise our glycogen stores. This is well-established in sports nutrition – see any textbook!

However, many individuals are purporting that by becoming more “fat-adapted”, and drawing on energy from fat, we can enhance endurance performance. Let’s see what McSwiney et al. are talking about.

Why Should We Care About Keto Research?

I know what you’re thinking: I know what a calorie deficit is. I’ve tracked macros bro. And you can bash pretty much any keto zealot for their lack of understanding of some basic nutrition mechanisms. Now, keto zealots love to draw on research that has evidence for improved performance in cycling time trials etc. And of course, if you don’t question anything and take them at their scientific studies, you would be of course forgiven for assuming keto is the nations’ hero.

However, ketogenic diets are actually very poorly researched. You literally alluded to a litany of ketogenic studies Michelle, I hear you say. Enter Mr. Low-Carbohydrate. Ill-defined “low carbohydrate” studies are used often in reference to keto criticism. Low carbohydrate does not equal ketogenic. Before we go all #NotAllKeto, we must first delineate between the two. Many studies decrease carbohydrate in subjects, but don’t actually induce ketosis. Similarly, many studies don’t investigate the impact of ketogenic diets on performance beyond a couple of weeks.

This study aimed to address and investigate these issues to evaluate the true impact of ketogenic diets on performance.

Study Findings

This study compared traditional high-CHO diets with ketogenic diets amongst male endurance athletes.

Overall, the study concluded that ketogenic endurance athletes improved their endurance performance times, increased fat oxidation and optimised their body composition (read: fat down, muscle up) over a 12 week timeframe.

Sounds fab. Now, can we draw the same conclusions ourselves?

The Authors Doth Attribute Too Much

The authors are generally quite hyperbolic in their claims. We need to be careful about what we claim, and what data we use to back this up.

Claim #1: Keto Improves Performance, High CHO Can Suck It

This study concludes ketogenic diets improve performance, and their results clearly show this. However, performance outcomes are given relative to the athlete’s bodyweight. Given that the ketogenic group lost bodyweight, their performance would automatically increase (performance metric was linked with bodyweight). Whether this translates into real-time sport improvement is less obvious.

It may be more appropriate to measure endurance performance using metrics that are scaled using heart rate and not bodyweight. Jeukendrup et al. (1996) offer such an alternative. In this way, bodyweight does not directly affect the performance metrics.

Claim #2: Eat Keto, Get Jacked

It is also claimed by the authors that ketogenic diets improved body composition. However, protein intake was not controlled for between the groups. The ketogenic diet group had an elevated protein intake. In a hypocaloric state (calorie deficit bro), an elevated protein intake is associated with increased lean body mass (LBM) Mettler, Mitchell and Tipton, 2010).. Protein is linked with increased satiety and sparing of LBM (Westerterp-Plantenga, Lemmens and Westerterp, 2012).

An increased protein intake may have lead to these changes. The authors attribute these changes to the ketogenic diet, but I don’t think I agree with this. It is possible increased satiety from protein increases led to a reduced calorie intake, not the ketogenic diet itself.

Other Gripes: Experimental Design/Methodology

No Randomization, No Problem

Eep. Participants in this study were not randomized. Participants chose what diet to follow. In fairness, the researchers did this to promote dietary adherence. Adherence is obviously essential to this study. HOWEVER. This may have inadvertently created a placebo effect, whereby participants elected for the intervention and felt it improved performance.

The effect of placebo in ketogenic diets and endurance are not definitive ofc, but are certainly worth considering (Bailey and Hennessy, 2020).

This lack of randomization also meant there were huge baseline differences between groups. A 10kg difference between groups is substantial. ESPECIALLY WHEN YOUR PERFORMANCE METRIC IS BODYWEIGHT BASED! This has huge implications on the conclusions we can draw from the study, imo.

Ketoschool Drop-Out

There was a huge drop out rate from the intervention group. Participants cited difficulty with adherence as one of their main reasons for dropping out.

This begs the question: if the diet is so great, why is it so hard to stick to?

Sneaky NDA of Author Affiliations

A major weakness is the conflict of interest of one of the lead researchers. Volek has co-authored several ketogenic diet books, and profits from same. In addition, a Google search reveals he is a shareholder in a ketogenic consultation firm. This may result in some implicit bias and was not disclosed. Pretty sneaky behaviour.

What Did They Do Well?

It’s not all hyperbole and non-disclosure.

A huge strength of this study was the duration of the intervention. As such, the study is one of the first to truly examine the impact of long-term ketogenic adaptation, and not merely acute effects. 12 weeks is one of the longest interventions in research, and nearly three times the length of the average intervention. Nice!

In addition, bloodwork and reporting carbohydrate intake ensured participants were in ketosis during the intervention. Self-reported carbohydrate intake can be misreported, but bloodwork is objective.

Where Do We Go From Here?

This study leaves us with some important lessons for future research.

Validity of future studies could be improved by randomizing participants into control/intervention groups. This may require more emphasis on ensuring adherence amongst participants.

It would also be important to control for protein intake, to ensure it did not influence body composition or appetite. It may have influenced the outcome of the current study.

This study found that the ketogenic diet led to increased fat oxidation during endurance activity. Similarly, Durkalec-Michalski, Nowaczyk and Siedzik (2019) found similar results amongst male CrossFit athletes following a ketogenic diet. Interestingly, this adaptation did not occur in female athletes. It may be beneficial to carry out a similar study on a female population.

Significant adaptations can occur in endurance athletes following a twelve-week ketogenic diet. Ketogenic diets may alter body composition, improve endurance performance, and increase fat utilization.

However, some individuals may find it difficult to adhere to, and so it should not be blanket prescribed to all endurance athletes.

With an increase in popularity of such a diet, it is important to familiarize ourselves with the difference between “low carbohydrate” and “ketogenic” diets, as the outcomes are not always the same!

I hope you enjoyed nerding out and ripping apart this study as much as I did. Let me know your thoughts down below!


Bailey, C.P., Hennessy, E. (2020) ‘A review of the ketogenic diet for endurance athletes: performance enhancer or placebo effect?’, Journal of the International Society of Sports Nutrition, 17(33). Citations: 2

Durkalec-Michalski, K., Nowaczyk, P.M., Siedzik, K. (2019) ‘Effect of a four-week ketogenic diet on exercise metabolism in CrossFit-trained athletes’, Journal of the International Society of Sports Nutrition, 16(16). Citations: 23.

Jeukendrup, A., Saris, W.H., Brouns, F.J., Kester, A.D. (1996) ‘A new validated endurance performance test’, Medicine & Science in Sports & Exercise, 28(2), pp. 266-270. Citations: 648.

McSwiney, F.T., Wardrop, B., Hyde, P.N., Lafountain, R.A., Volek, J.S., Doyle, L. (2017) ‘Keto-adaptation enhances exercise performance responses to training in endurance athletes’, Metabolism, 81(1), pp. 25-34.

Mettler, S., Mitchell, N., Tipton, K.D. (2010) ‘Increased protein intake reduces lean body mass loss during weight loss in athletes’, Medicine & Science in Sports & Exercise, 42(2), pp. 326-337. Citations: 396.

Westerterp-Plantenga, M., Lemmens, S., Westerterp, K. (2012) ‘Dietary protein – its role in satiety, energetics, weight loss and health’, British Journal of Nutrition, 108(2), pp. 105-112. Citations: 368.

Published by Michelle Carroll

I am an online coach (MSc Sports & Exercise Nutrition, EQF Level 4 Personal Trainer, PN Level 1) and radiographer (BSc). I believe in empowering others to make better choices for their health through education. I think that the fitness industry has created a disconnect between best practices and “evidence-based” practices. I hope by chronicling my experience as a healthcare professional and my education as a fitness professional I can assist others on the path to bettering themselves.

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