If you consider yourself a competitive athlete wouldn’t you do anything and everything you can to optimize your performance? As we all know, our performance is largely dictated by the fuel we put into our bodies. If we supply our bodies with inadequate nutrition, we will likely produce inadequate results that will not allow us to reach our full potential. The issue is that there is no one-size-fits-all approach, especially when it comes to optimizing performance. Decades of research have been dedicated to studying athletes who are carbohydrate adapted and/or using carbohydrate supplements that have produced remarkable results. However, in this article, we will dive into the possibility of using a different approach for endurance sports performance: a ketogenic diet.

Several professionals have supported the idea that we have essential amino acids. We have fatty acids that are essential. But there’s no such thing as an essential carbohydrate.  The present dogma in aerobic estimated-fuel-reservessport is that our bodies need ample amounts of carbohydrates in order to optimize the “fuel tank”. On paper this makes sense. Aerobic exercise depletes glycogen. We need to refill the glycogen in order to keep performing. Thus, we take in high-carbohydrate sports drinks, gels, etc. However, our bodies have a way of tapping into another larger fuel source. This presents us with an interesting question: what if we could maintain adequate glycogen stores without consuming large amounts of carbohydrates?  Could eating a low-carbohydrate diet somehow allow us to use an alternative energy source and still maintain sufficient glycogen stores?

The basic premise of a ketogenic diet consists of high fat, moderate protein, and low carbohydrate intake. A typical endurance athlete (not on a ketogenic diet) likely consumes anywhere from 40%-70% of their caloric intake from carbohydrates. Many successful athletes have used this type of dietary approach for decades; however, with this type of diet, your body becomes reliant on carbohydrates as a primary fuel source. The average endurance athlete carries about 2,000 stored calories in their metaphorical “carbohydrate tank” (i.e glycogen stores). When that tank becomes depleted, we often rely on oil-tankercarbohydrate-based gels and drinks to try and maintain energy and performance. Otherwise, the athlete can shut down, and shut down fast, or what is often known in endurance sports as “hitting the wall.” This conundrum is catastrophic and unfortunate because even the leanest of these athletes have ample amounts of fuel in their bodies (i.e fat); however, they just can’t tap into it as they are locked into a primarily glucose-based metabolism. The best way I’ve heard this described was by Drs. Jeff Volek and Steve Phinney when they say “An endurance athlete hitting the wall is like a fuel tanker running out of gas on the highway. You have ample amount of fuel, you just can’t utilize it effectively.” Alternatively, athletes following a ketogenic diet are adapted to utilizing this alternative fuel source to replace carbohydrates.

Why would fat potentially be a better fuel source for aerobic athletes? We know that one gram of carbohydrate contains four calories and one gram of fat yields nine calories; from that, we can derive that fat is more calorically dense than carbohydrates. One thing that people do not realize is how much ATP each yields. Adenosine triphosphate (ATP) is what our cells use for energy and is the driving force responsible for all cellular function. Without ATP our body would not be able to function! For every keto-vs-non-ketomolecule of glucose, which comes from carbohydrates, our bodies can produce 36-38 units of ATP through glycolysis.  This metabolic pathway can function anaerobically (without oxygen) or aerobically (with oxygen), hence the small variation in the amount of ATP we can yield from each molecule. On the other hand, fat is utilized through an aerobic metabolic pathway called lipolysis. This metabolic precursor breaks fat down into fatty acids known as triglycerides that can be turned into ATP (see Basic Fat overview for an explanation on how fatty acids are broken down). However, depending on the amount of carbon in each fatty acid tail, one triglyceride (fat molecule) can yield up to 496 ATP! This is crucial to know because fat metabolism is significantly higher in keto-adapted athletes. With this upregulation of FFA metabolism, our bodies also create more ketones which can be used as an additional fuel source rather than CHO. These ketones are utilized not only in skeletal muscle but in the heart and brain as well. This unique metabolic state has shown to improve muscular endurance by altering the fuel source used for oxidative respiration. Furthermore, the presence of ketone bodies has demonstrated an increase in performance even when co-ingested with carbohydrates in non-keto-adapted athletes (1).

Now that we know a little bit about why fats have the potential to be a better fuel source for specific sport demands, we can discuss why a low-carb approach to endurance exercise has often been discouraged. When we look at the biochemistry as we did above, it appears that fat may be the preferred fuel source for an endurance athlete. Then why does research tell us that a low-carbohydrate diet is detrimental to performance? When it comes to performance and ketogenic dieting we know that there are many factors that contribute to perfecting the diet, with keto-adaptation being the most important. This key fundamental of ketogenic dieting is often overlooked in research, thereby resulting in the negative ideas formulated around low-carb dieting and endurance performance. There is undoubtedly more research out there saying that low-carbohydrate dieting results in decreased performance; however, most of these studies are short in duration and lack the adaptation period previously mentioned. For example, if you were to look up “endurance performance carbohydrates vs. fat”, the first study to pop up would demonstrate that performance is impaired as a result of low carbohydrate consumption (2). Under the circumstances in which this study was performed, these findings seem extremely accurate and useful. This study effectively established proper guidelines for pre/intra-workout nutrition for optimizing endurance performance in carb-adapted athletes. On the flip side, from a low-carb standpoint, one could argue that this means a ketogenic diet approach is benfits-of-keto-adapted-copyunfavorable for endurance performance. While this may be true with carb-adapted individuals, we see that when the athlete is keto-adapted, low-carb/high-fat dieting has shown to increase numerous endurance performance parameters (3). When athletes are given a short period of time to adapt to a ketogenic diet prior to the study intervention, we see that aerobic endurance performance is not compromised by nutritional ketosis but actually enhanced. After 4 weeks of low-carb ketogenic dieting, (given prior time for adaptation) Dr. Phinney found that performance was enhanced in endurance-trained cyclists likely due to a decrease in RQ (meaning the athlete metabolizes fat more effectively), a three-fold drop in glucose oxidation (the body is burning more fat) and a four-fold reduction in muscle glycogen use (meaning the body preferentially uses fat over carbohydrates). Keep in mind, this was only after a short adaptation period. It is possible that the results would be even more profound in athletes who have been following a ketogenic diet for substantially longer.  Once again, it is critical to observe the methods in a research study in order to see if the findings are applicable or not.             

These findings opened the door for further research looking at ketogenic dieting and endurance performance. Despite Phinney’s study being great, some may argue that the length was still too short and the sample size too limited. A great scientist and researcher, Dr. Jeff Volek, expanded on this work and performed a study in athletes who habitually consumed a low-carb/high-fat ketogenic diet compared to athletes consuming a high-carb/low-fat diet. These athletes had been on their respected diets for an average of 20 months (range 9-36 months). This allows for a much greater degree of adaptation to occur and therefore a better comparison to the two types of diets. The 20 athletes used for the study were elite ultra-marathoners and Ironman distance triathletes. He put them through a series of tests measuring their VO2max, resting muscle glycogen, level of glycogen depletion, fat oxidation levels, and other anthropometric data. Results from Dr. Volek’s study found that peak fat oxidation was 2.3 fold higher in the low-carb group and occurred at a much higher percentage of VO2max (4). This means that the low-carb athletes were able to utilize fat as an energy source much better than their high-carb counterparts. One of the most surprising results of the study was that despite the differences in nutrient utilization, there were no significant differences in resting muscle glycogen nor their level of depletion after exercise. These results were also seen by Roberts et al who found that resting glycogen levels weren’t different in animals adapted to a ketogenic diet compared to their Western diet counterparts. From all this information we can derive that being in a long-term keto-adapted state can alter fuel utilization AND create a unique state in which your body adapts by sparing and finding other ways to replenish glycogen stores. From this study, we can conclude that long duration aerobic performance is not hindered in a ketogenic state, and metabolic adaptations that occur as a result of the diet allow these athletes to possibly push the boundaries of human performance due to an increased lactate threshold and an improved ability to utilize fat as an energy source.


Ketogenic dieting is a powerful tool that can be utilized for aerobic performance whether you are a professional marathon runner, high school cross-country competitor, or a recreational endurance athlete. The physiological changes that come along with a keto-adapted state demonstrate a significant difference in how your body uses fuel, allowing you to use a much larger fuel source; >40,000 calories! Next time you get sick of “hitting the wall” or literally sick from consuming sugar-based gels and drinks throughout your race, consider experimenting with a ketogenic diet.

Keto Conclusions

  • Consuming a ketogenic diet allows the body to utilize fat as its primary source of fuel.
  • Ketones can provide more energy per molecule than carbohydrates.
  • Volek and others have demonstrated improved endurance performance in low carbohydrate athletes.
  • Volek and Roberts discovered that there were no differences in glycogen levels between keto-adapted and glucose adapted athletes/animals.
keto-conclusions-bar