If you’ve ever seen cyclists chowing down on pastries or runners inhaling spaghetti, then you’ve noticed their preference for carbs. Sports nutrition has centered on carb-rich diets for decades with the intention of maximizing energy stores.

However, some athletes are ditching carbs all together. As part of what’s known as a “ketogenic diet,” it’s the practice of eating mainly high-fat, moderate-protein foods almost devoid of carbohydrates.

The objective of the diet for athletes is to ramp up cellular fat-burning machinery. However, besides the trouble of elimination of carbs being daunting enough, note that it can take several weeks for an athlete’s body to properly adapt.

For this reason, ketogenic diet proponents have contended that most research hasn’t shown performance benefits because the studies don’t allot enough time for the diet to work properly (1). But recently, sports nutrition researcher Louise Burke from the Australian Institute of Sport investigated the effects of adaption to a ketogenic diet over three weeks of intensified training (2).

Carbohydrate Metabolism Impairment

What she found was intriguing – the ketogenic diet did lead to increased fat metabolism, but it also reduced performance likely due to impaired carbohydrate metabolism. The study from Burke and her colleagues used 29 elite race walkers to better gauge application to performance. The study divided the race walkers into three groups that consumed a high-carb diet, a mixed diet using low- and high-carb availability, or a ketogenic diet.

As the subjects in the ketogenic group adapted to their diet, they markedly increased their fat-burning rates, yet this didn’t translate into any real-life performance improvements. In fact, they had impaired performance and higher oxygen requirements compared to the other groups.

From a simple chemistry perspective this makes sense: Carbs are able to produce greater amounts of energy per unit of oxygen consumption despite fat having more energy per unit of substrate (3). During exercise, this oxygen consumption difference can become quite important as the intake of oxygen can be limited, especially during high-intensity exercise.

Moreover, there’s a price to pay for becoming keto-adapted. While you may burn fat better, you partially “shut down” the cellular machinery needed to effectively use carbs (2). So while the ability to use fat in the ketogenic group was significantly increased, the ability to use carbohydrates actually decreased.

“A more holistic understanding of the ideal training program is that it should enhance an athlete’s capacity to utilize and integrate all of the body’s range of energy-producing pathways, with special focus on ways in which they become specifically limiting for performance,” the researchers wrote.

Metabolic Flexibility

Rather than abandoning a macronutrient like “carbohydrate” completely, it may make better sense to place them in your program more strategically. For instance, workouts that are easy to moderate can be done in a fasted or low-carb state to promote fat-burning capabilities, while those that require higher intensity can use carbs to increase tolerance and absorption (4).

This is a form of organizing your nutrition and training together to get the most out of both carbs and fat. It’s being seen more and more as a viable option for athletes to take advantage of fat-burning and high-performance output. Researchers call it metabolic flexibility.

As attractive as it may sound to paint an entire nutrient as bad, as an athlete it may be best to use every tool available for performance. Carbohydrates, just like every other nutrient, are a tool and can be misused. But as seen in athletes, carbs can also be used correctly and increase performance.


  1. Phinney SD, Bistrian BR, Evans WJ, Gervino E & Blackburn GL. The human metabolic response to chronic ketosis without caloric restriction: preservation of submaximal exercise capability with reduced carbohydrate oxidation. Metabolism. 1983 Aug; 32(8):769-76.
  2. Burke LM et al. Low Carbohydrate, High Fat diet impairs exercise economy and negates the performance benefit from intensified training in elite race walkers. J Physiol. 2016 Dec 23. [Epub ahead of print]
  3. Leverve X et al. Choosing the right substrate. Novartis Found Symp. 2007; 280: 108-21.
  4. Stellingwerff T. Contemporary nutrition approaches to optimize elite marathon performance. Int J Sports Physiol Perform. 2013 Sep; 8(5):573-8.

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