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Low-carbohydrate or Ketogenic Diet for Runners (Science explained)

ketogenic diet for runners

There’s a lot of continuing debate in the running community on whether or not a ketogenic diet or being “fat-adapted” helps or hinders the performance of long-distance runners such as marathoners and ultra-marathoners. Some will argue that a low-carb or ketogenic diet for runners and other endurance athletes is impractical and current sports nutrition dogma is that carbohydrates are necessary for athletic performance.

Ketogenic Diet for Runners

The scientific community has struggled on some aspects with good quality research on the keto diet for runners’ topic. Some of the published studies use a short window for fat-adaptation of a month or less.1 Other studies use different definitions of “low carb” diets, with some studies considering eating 100 grams of carbohydrate a day as a low carbohydrate diet. While the 100 grams a day is lower than a standard diet, it’s not low enough in many cases to get the body in a state of ketosis and develop the fat-adaptation pathways for improved fat burning during exercise.

Another problem with some of the ketogenic diet for athletes research is the wide variety of types of exercise studied but then attempted to extrapolate those results to other types of exercise intensities. Some studies that demonstrated no effect or decreased exercise on the ketogenic diet targeted high-intensity exercise which not not depend on fat utilization as its primary fuel source. 2

Fat Adapted Substrate use in Trained Elite Runners (FASTER) study

In 2016, Jeff Volek and Stephen Phinney published their FASTER (Fat Adapted Substrate use in Trained Elite Runners) study 3 which took 20 high-level ultra-endurance runners and triathletes. This study looked at potential benefits of the ketogenic diet for runners.

The researchers split the athletes into two groups, one that followed a traditional higher carbohydrate diet and a second group that followed a low carbohydrate, high fat (LCHF) diet similar to the ketogenic diet.

The LCHF athletes had all been on the low carbohydrate diet before the start of the study for a minimus of 9 months, and in some cases, had been on a low carb diet for up to 36 months prior to the start of the study.

Volek and Phinney were curious on studying what adaptations occurred in these elite low-carb athletes and what the impact would be on their VO2 max and carbohydrate and fat utilization during exercise.

The FASTER study measured the fat utilization of these athletes during a 180-minute treadmill run at 64% of their VO2max and also looked at blood and muscle biopsy results as well.

The researchers found that the fat-adapted athletes had about 2.3 times as much fat utilization when compared to the higher carbohydrate athletes. This translated into the fat-adapted athletes burning about 1.5 grams of fat per minute during their treadmill runs.

What Volek and Phinney also found is that the peak fat utilization in the fat-adapted runners occurred at a much higher percentage of their VO2 max than the high-carbohydrate runners (70.3% ± 6.3 vs 54.9% ± 7.8%; P = 0.000). That’s an important distinction because that higher fat utilization at a higher running speed means that the fat-adapted runners also relied on fewer carbohydrates at the same percentage of their VO2max than the high carb athletes. The fat-adapted runners were able to get about 88% of their calories from fat while the high carbohydrate diet runners could only tap about 56% of their fat stores for energy.

Glycogen storage in the ketogenic diet for runners

Why is that important? The liver and muscles can store only about 2000 calories or 500 grams of carbohydrates, so any longer endurance exercise over 2 hours typically will deplete most of the body’s carbohydrate stores. A high-carb and non-fat-adapted athlete will need to replace those carbohydrates with either a sports drink or sports gel and at a higher rate than the fat-adapted athlete that is using less carbohydrates at the same running intensity.

That means less demand on the stomach and intestines to absorb the carbohydrates and less gastrointestinal issues while exercising. That becomes a potential performance benefit since GI issues can negatively impact performance in marathons and long-distance triathlons.

The FASTER study also looked at how the athletes’ replenished their muscle glycogen after the 180-minute treadmill run and found there was relatively no difference in muscle glycogen recovery between the two groups. This was a surprising result to the researchers since a prior study by Phinney in endurance cyclists on 4 weeks of a ketogenic diet had shown a significant drop in muscle glycogen concentration in the low-carb athletes. 4

What’s also interesting when looking at post-exercise repletion of the muscle glycogen is that the low-carb/ ketogenic diet runners were able to increase their muscle glycogen stores with minimal post-exercise carbohydrates. Typically, rebuilding muscle glycogen stores after exercise was thought to require high amounts of a post-workout drink or meal. 5 But the ketogenic diet runners received a post-workout drink that only had 4 grams of carbohydrates (versus the high-carb runners that drank a high carb post-workout drink with 43 grams of carbohydrates. Despite the significant difference in the carbohydrate content of the two drinks, the low-carb/ketogenic diet runners were able to increase their percentage of muscle glycogen similar to the high-carb runners.

Jeff Volek discussing the FASTER study

One issue I think the scientific research has with studying the ketogenic diet for runners and other endurance athletes is that they only look at one aspect of the ketogenic diet and don’t look at the potential benefits. A fat-adapted runner on the ketogenic diet can still add carbohydrates back during exercise if needed to run at a higher intensity and they will still use more fat than carbohydrates that a runner on the high-carb diet. This optimized fat metabolism diet has been used successfully by ultrarunners like Zach Bitter to great success. The research seems to miss the advantages of becoming fat-adapted and then using carbohydrates as an adjunct to boost performance.

Read the full FASTER (Fat Adapted Substrate use in Trained Elite Runners) here to see how the ketogenic diet for runners can help improve fat utilization and decrease your dependence on carbohydrates.

If you’re interested in learning more about the ketogenic diet, you can see a list of the top keto documentaries here.


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