Road cyclists racing.
Road cyclists racing.

Going on a Ketogenic Diet May Not Make You a Competitive Endurance Athlete

Last year I reviewed some scientific research papers evaluating a ketogenic (keto) diet on both strength and endurance performance and the conclusion was that neither strength or endurance performance improves when on a keto diet.

To see if there have been any updates since that time, I was browsing the literature again and found a series of publications from Louise Burke’s lab down under at the Australian Catholic University.

If you don’t know Dr. Burke, she is one of the most preeminent sports dietitian and nutrition researchers around. She has authored over 250 research papers on sports nutrition and is the Chief of Nutrition Strategy at the Australian Institute of Sport, which is Australia’s premiere training center for professional and Olympic athletes. I’ve been a long-time fan of her work along with her husband, Dr. John Hawley, and fondly remember reading their work as I conducted my Master’s research on carbohydrate supplementation. I’m such a fan that I even scrounged up enough money during grad school to buy Dr. Burke’s textbook on clinical sports nutrition to read for fun and still have it to this day!

Anyways, Dr. Burke’s lab published three articles this year, and in this post, I will be summarizing the first article. If you want to dig into the details, which I highly recommend since they’re freely available online, I’ll provide the links at the end of this post. With that said, let’s dive into the research!

The first article is titled “Ketogenic low-CHO, high-fat diet: the future of elite endurance sport?” in which Dr. Burke mentions how there are social media influencers attempting to push the benefits of a keto diet — namely, improving fat burning capability during endurance exercise with little scientific evidence as support. She describes that to be competitive for both endurance and ultraendurance events requires high power output for extended periods of time and what she finds is there seems to be a throttling effect on high-intensity endurance exercise when on a keto diet.

To understand this concept better, think about the time you tried lighting a fire — if you were successful, you should have been warmed up by the fire as you absorb the thermal energy produced from the interaction of wood, air, and a spark. A similar interaction occurs within your muscles where oxygen from the air interacts with the muscle’s mitochondria and a fuel source whether it be carbohydrate, fat, or ketone bodies to produce chemical energy that allows your muscles to contract and produce movement.

Now, researchers use this knowledge about how energy is produced in the human body to indirectly measure an athlete’s maximum energy production by measuring the amount of oxygen an athlete consumes — this measurement is called the VO2max or the maximum volume of oxygen uptake. By measuring the amount of oxygen an athlete consumes, scientists are able to determine what proportions of carbohydrate and fat are burnt for a given exercise intensity and how much energy an athlete can produce. An athlete’s VO2max is also highly limited to one’s genetic inheritance, so training to improve one’s VO2max can only go so far.

Going back to Dr. Burke’s article describing how a keto diet can throttle high-intensity endurance exercise, the transformation of the vast supply of energy from fat depends on the ability of an athlete to extract sufficient oxygen for a given time period into their muscle mitochondria to match the energy demands of a race. While the energy demands can be met mainly by fat and ketone bodies at a low- to moderate-exercise intensity, Dr. Burke and other researchers have discovered that a keto diet limits an athlete’s ability to produce sufficient energy for high-intensity endurance performance. That is an athlete on a keto diet has a VO2max that is throttled or limited compared with being on a high carb diet. The reason this happens is partly attributed to the lengthier time it takes for fat to oxidize into energy and the greater amount of oxygen required to produce the same amount of energy compared to carbohydrates. At low- to moderate-exercise intensities, there is sufficient time for the muscle cells to extract and use the oxygen being delivered, however, at higher intensities, there isn’t enough time for the muscle cells to convert fat into sufficient energy. On the other hand, carbohydrates can quickly convert into energy for high-intensity performance. This describes the crossover effect whereby as exercise intensity increases, the proportion of fat utilized for energy decreases. While a keto diet does shift the crossover to allow more fat to be used at a relatively greater exercise intensity than on a high carb diet, the trade-off is the muscle mitochondria are unable to generate sufficient energy in time for high-intensity endurance performance, and thus, the athlete’s VO2max is blunted on a keto diet. This may be problematic for some endurance athletes and the goals they may be targeting.

For example, if you are a Boston Marathon or Kona Ironman qualifier hopeful and your genetics dealt you a VO2max that is just enough to be competitive to qualify, you would want to optimize your training, dial in your equipment, hope for ideal race day conditions, and rebuild your body with sound nutrition. By going on a keto diet, you may be limiting your ability to perform at your peak during the qualifying race, because you would be unable to maintain a high absolute performance for both short and long periods of time resulting in possibly not qualifying. By going keto is akin to you purposely giving yourself a handicap going into a race.

While certain social media influencers show amazing athletic ability on a keto diet, we must all consider our own genetic capabilities and whether a keto diet can really improve our endurance performance relative to what we hope to achieve whether that is getting a podium finish or meeting a qualification. Just because there are social media influencers who qualify for or win certain races on a keto diet does not necessarily mean the same for us as we don’t get to really see what nature has gifted them with — and for the purpose of this post — what their VO2max is when on a carb vs keto diet? If such athletes are already genetically gifted with elite level physiological parameters like a large VO2max, then they would have more room to experiment with their diet and still outcompete us in local, regional, or global races.

As Dr. Louise Burke concludes in her article:

The availability and capacity to use all muscle fuels to support the specific demands of exercise (‘metabolic flexibility’) is the Holy Grail for high‐performance endurance athletes, explaining continued fascination with strategies to better utilise the body’s relatively unlimited fat stores…Athletes who are contemplating the use of ketogenic low‐carbohydrate high‐fat diets should undertake an audit of their event and their personal experiences to balance the risk of impaired performance of higher intensity exercise with the potential benefits of replacing an unavoidable depletion of carbohydrate stores with greater reliance on muscle fat use.

She also points out that elite Ironman triathletes operate at higher absolute energy expenditures than slower/less competitive triathletes implying that she does not recommend a keto diet at least for endurance and ultraendurance athletes who hope to be competitive.

I hope this brief post helps you better understand the limitations of a keto diet on being a competitive endurance athlete. Since, I was unable to review Dr. Burke’s other two articles in this post, be on the look out for those reviews in the near future.


Burke LM. Ketogenic low‐CHO, high‐fat diet: the future of elite endurance sport? J Physiology 02 May 2020. (PubMed)

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