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Metabolic Changes in Athletes Following a Ketogenic Diet

Low-carb vs. high-carb approaches go head-to-head in ultra-endurance runners

by Bianca Garilli, ND

In the constant quest for improving performance, ultra-endurance athletes often alter exercise and dietary habits hoping to optimize peak physical and mental health. With this goal in mind, the ultra-endurance athlete community has turned its attention to the potential benefits of a low-carbohydrate diet. This is in stark contrast to the previously long-held approach of consuming a high-carbohydrate diet before, during and after exercise.1

Crucial to peak performance is the availability of adequate glycogen throughout the duration of exercise. Recommendations for consuming high-carbohydrate diets are aimed at providing sufficient glycogen to skeletal muscle during high intensity, endurance events. It has been shown in recent years, however, that following a diet low in carbohydrate does not necessarily jeopardize energy substrate availability as can be seen in studies which restrict carbohydrate for an extended period of time.1 Instead, after several weeks of reduced carbohydrate intake, the body’s hepatic ketone production increases dramatically leading to the replacement of glucose, the brain’s major energy source, with ketone bodies.1 Similarly, fatty acids replace glucose as the major energy source in skeletal muscle.1

The underlying metabolic changes occurring in ultra-endurance athletes following a low-carbohydrate (LC) diet when compared to the consumption of a high-carbohydrate (HC) diet are not well known at this point. The FASTER (Fat Adapted Substrate use in Trained Elite Runners) cross-sectional study was designed to learn more about the mechanisms involved in both dietary approaches.1

In the FASTER study, 20 male ultra-endurance runners between the ages of 21-45 were placed into either the LC (n=10) or HC (n=10) group based on their habitual macronutrient profile. Eligibility guidelines included:1

  • LC diet: <20% energy from carbohydrate and >60% from fat, consistently for at least 6 months prior to the study
  • HC diet: >55% energy from carbohydrate, consistently for at least 6 months prior to the study

On average, participants had been following their respective habitual diets for 20 months.

Over a two day period, participants engaged in an on-site exercise protocol designed to better understand the metabolic changes occurring in ultra-endurance athletes following a LC vs HC diet.1 The results from the exercise protocol (180 minute run at 64% VO2max followed by a 120 minutes of recovery) found an average 2.3-fold higher peak fat oxidation in the LC group.1 Notably, every participant in the LC group exceeded the highest value achieved in the HC group. The average contribution of fat during the exercise protocol in the LC and HC groups were 88% and 56% respectively.1

Lipid metabolism markers in the LC group indicated a significantly higher level 1 Serum non-esterified fatty acid peak levels at the end of exercise were not significantly different between the two groups.1 Plasma triglyceride levels also did not differ between the diet groups.1 Plasma glucose and serum insulin levels were comparable between the two groups at rest and during exercise.1 Muscle glycogen level changes were similar in both groups with a significant decrease immediately post-exercise (66% in LC and 62% HC) and at 2 hours post-exercise (34% in LC and 38% in HC) when compared to their respective baselines.1

The authors of this study conclude that “chronic keto-adaptation in elite ultra-endurance athletes is associated with a robust capacity to increase fat oxidation during exercise while maintaining normal skeletal muscle glycogen concentrations.”1 Additionally, the authors concluded that long-term consumption of low-carbohydrate diets resulted in metabolic mechanisms which regulated glycogen levels similar to those following a high carbohydrate diet.1 

Why is this Clinically Relevant?

  • Fat oxidation is increased in highly trained athletes who are keto-adapted
  • Muscle glycogen availability in athletes following a LC diet is similar to that found in athletes following a HC diet
  • HC recommendations for endurance athletes is no longer the only option for optimizing energy availability during exercise

Link to article


  1. Volek JS, Freidenreich DJ, Saenz C, et al. Metabolic characteristics of keto-adapted ultra-endurance runners. Metabolism. 2016;65(3):100-110.

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