Study: Effect of a four-week ketogenic diet on exercise metabolism in CrossFit-trained athletes

What is This Study About?

Carbohydrates and fat are the main fuel sources used during exercise; however, the body cannot store as many carbohydrates as it can fat. Therefore, it is suggested that a ketogenic diet may be advantageous for exercise because it can improve the body’s ability to tap into stored body fat, which is a much bigger fuel supply. 

The purpose of this study was to put the above theory to the test by assessing the effects of a four-week ketogenic diet on fat and carbohydrate utilization during an incremental cycling test in CrossFit-trained female (11) and male (11) athletes.

How was the Study was Conducted?

For this study, participants continued following their customary diet for 14 days while recording their daily food intake. The average carbohydrate consumption during the customary diet was 285 grams for the women and 294 grams for the men.

After the 14 days, the participants completed a cycling test to see how which fuel source (carbohydrates or fat) was predominantly used at various exercise intensities. 

After the baseline exercise test, subjects started a ketogenic diet for four weeks. During the ketogenic diet phase, dietary adherence was monitored. On the keto diet, both men and women consumed around 30 grams of total carbs and got nearly 80 percent of their calories from fat.

After four weeks on a keto diet, participants returned to the lab to complete the same cycling tests to compare to their baseline. 

Study Results

Of the 30 participants that started the study, only 22 finished. The other eight were removed from either injury or non-adherence to the dietary protocol.

Results from this study showed that for both genders, there was an increase in fat oxidation during exercise while following the ketogenic diet. However, there were some notable differences between genders.

For males, there was an increase in fat oxidation throughout the whole testing, even up to 80 percent of Vo2 max, (a numerical measurement of your body’s ability to consume oxygen).  However, in females, it wasn’t until over 60 percent of Vo2 max that an increase in fat utilization was noted.

Conclusions and Insights

A great feature of this study is its length of time. Many keto and exercise studies are too short in duration and don’t allow for keto-adaptation to occur. Thus, shorter-term studies skew the results. In this study, subjects followed keto for four weeks, enough for most people to get keto-adapted. 

Another notable element of this study was that total calories and protein intake were not significantly different between the subjects’ customary diet and their ketogenic diet. This is important because it prevents more variables from skewing results. 

Results from this study demonstrate that ketosis can effectively alter the type of fuel we use during exercise, and since our bodies can store more fuel as fat, this could lead to a greater capacity to complete endurance exercises for longer duration and at higher intensities.

Results from this study also indicate that males may be better able to transition to burning fat for fuel at lower exercise intensities compared to women. However, this should be studied in much greater detail before determining conclusions.

These findings are significant because they demonstrate why a ketogenic lifestyle should be seriously considered for athletes and people who exercise in general, especially since keto can improve exercise through several additional mechanisms, including stimulating mitochondrial biogenesis (the process by which cells increase mitochondrial mass) and limiting lactate production.

Besides being able to tap into stored fat, keto also leads to the production of ketone bodies and research has shown that these ketone bodies can also be used as energy providing the body with a “dual-fuel” for exercise.

All of this together suggests that whether your goal is fat loss or improving your exercise endurance performance, keto is a strategy that should be considered!



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