Can a ketogenic diet support athlete health and performance? by Matthew Carpenter

Ketogenic diets are a hot, yet controversial topic, in the world of health and exercise performance. Whilst originally studied as a method of treating drug-resistant epilepsy in the early 1900s, today’s advocates promote the numerous health and performance benefits of the diet. 


Controversy surrounds the impact of the diet on health, with a historic fear of the effects of high levels of fat consumption. In spite of the contrary nature of the diet in this sense, many health markers actually tend to improve. For example, weight loss is often greater in ketogenic diets when food is consumed ad libitum (1). A key mechanism for this is the shift to fat oxidation, which increases satiety, potentially leading to fewer calories consumed (2). Further to this, circulating ketone bodies are being studied for, amongst other things, a hunger-reducing effect, which may be a further mechanism for the spontaneous reduction in calorie consumption. The ketogenic diet has also been shown to improve several metabolic disease markers and conditions such as diabetes, in which insulin requirements can be dramatically reduced, and in many cases eliminated.

An important caveat to this, often concerning medical professionals, is the volatility of LDL cholesterol when following a ketogenic diet. While a meta-analysis has shown no significant difference in LDL levels (3), the high degree of variability suggests that caution should be taken when embarking on the diet. Thus, while research suggests that a ketogenic diet is a safe and effective tool to elicit weight loss and improve health, the testing of health markers is recommended to ensure continued safety of the diet.

Exercise performance

When it comes to exercise performance, little else can spark such heated conversations like the impact of ketogenic diets. With less research in this area, there is a great deal of hysteria around the potential use of ketogenic diets to enhance exercise performance and, at the other end of the spectrum, individuals who are vehemently ‘anti-keto’ for exercise performance. The key rationale for ketogenic diets in exercise performance are as follows:

  •  Human fat stores are vast (>40,000 calories in lean individuals), compared to carbohydrate stores, which are finite (~ 2000 calories).
    • Ketogenic diets lead to dramatic increases in fat oxidation, with a two-fold increase in fat oxidation in keto-adapted ultra-runners compared to non-adapted athletes (4).
    • - The ability to tap into these fat stores is clearly attractive to distance athletes, as it can allow for this essentially unlimited fuel source to be used, rather than having to continuously ‘top up’ carbohydrate stores.
    • - This carbohydrate dependence can lead to extreme dietary requirements for carbohydrate during exercise, which likely contributes to GI distress in ultra-endurance competition.
  •  Training with low carbohydrate availability induces numerous metabolic adaptations, such as increased mitochondrial biogenesis (5).
  •  Power-to-weight ratio may improve if the diet causes weight loss without a subsequent loss of power output

The above factors could point to potential performance improvements, particularly in endurance-based activities.

Yet, the research to date has been equivocal, with most studies showing no overall effect of a ketogenic diet on most exercise modalities. This is interesting in itself, particularly in research on high-intensity exercise, with a growing volume of studies showing no performance impairment following keto-adaptation in high-intensity aerobic exercise (6) and HIIT (7,8).

This is particularly noteworthy, since one would expect exercise conducted at higher intensities to be impaired by ketogenic diets because carbohydrate remains the key fuel at high intensity. Thus, in theory, high-intensity exercise should be impaired by ketogenic diets because skeletal muscle adaptations culminate in reduced activity of pyruvate dehydrogenase during exercise.

The finding that keto-adaptation seems not to impair high-intensity exercise directly contradicts sport nutrition guidelines, making it a particularly hot topic. However, there may be a degree of nuance to this, as there is a lack of research into elite athletes, who are by nature, outliers. Only one study that tested the performance effects of a ketogenic diet in elite athletes has shown a clear performance impairment (9). While these athletes may not have been given sufficient time for keto-adaptation to occur, it is plausible that ketogenic diets do not represent an effective nutrition strategy for many elite athletes.

Events in which ketogenic diets seem to be particularly popular are endurance-based. Long-duration activities such as ultra-endurance events have triggered great interest due to the desire to spare carbohydrate and access the virtually unlimited fat stores, and to reduce the need for in-race feeding. And with research showing no overall reduction in performance, ketogenic diets may be an attractive option to those looking to manage body weight and increase fat oxidation, while maintaining performance.

Does that mean you don’t need ANY carbohydrate?

Short answer: If you want to perform optimally, carbohydrate is still king. Acute carbohydrate feeding before exercise is better than none at all, even when keto-adapted.

While the research to date highlights the fact that a ketogenic diet is unlikely to impair performance following keto-adaptation, at least in sub-elite athletes, it’s an interesting topic that will no doubt be studied further in depth. It’s important to note that high-profile athletes/teams known to have utilised ketogenic diets so far, such as ultra-runner Zach Bitter, and MLS football team Columbus Crew, under the guidance of former high-performance director Steve Tashjian, have reintroduced carbohydrates prior to performance in almost all cases.  

Thus, whilst ketogenic diets remain an exciting tool, predominantly in increasing fat oxidation and improving body composition, it seems very likely that acute carbohydrate consumption will boost performance output in sporting individuals and therefore is still required to optimise performance.

The question then comes down to whether living without carbohydrate and adding it before competition can enhance exercise performance. This has not been explored much in research yet, but a ‘train keto, compete carb’ strategy may be the next area in which the ketogenic diet can be used as a performance-enhancing tool.

To find out more about Matthew Carpenter’s research, you can email him here.

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  1. Bueno NB et al (2013). Very-low-carbohydrate ketogenic diet v. low-fat diet for long-term weight loss: a meta-analysis of randomised controlled trials. Br J Nutr. 110(7):1178-1187.
  2. Waldman HS et al (2018). A shift toward a high-fat diet in the current metabolic paradigm: A new perspective. Nutrition.46:33-35.
  3. Gjuladin-Hellon T et al (2019). Effects of carbohydrate-restricted diets on low-density lipoprotein cholesterol levels in overweight and obese adults: a systematic review and meta-analysis. Nutr Rev.77(3):161-180.
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  9. Burke LM et al (2017). Low carbohydrate, high fat diet impairs exercise economy and negates the performance benefit from intensified training in elite race walkers. J Physiol.595(9):2785-2807.