Keto diet helps female athletes drop more fat, but not gain strength

By Hank Schultz

- Last updated on GMT

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Related tags ketogenic keto diet keto Sports nutrition

A new study has found that a ketogenic diet helped trained female subjects lose fat while maintaining muscle. But it concluded the diet was ‘suboptimal’ for building additional muscle during a training regimen.

The new research was published in the Journal of the International Society of Sports Nutrition​. The study was conducted by researchers from Spain, Colombia and Texas.

Looking at keto diets for female strength athletes

The authors said there is a lot of data on the effects of ketogenic diets on fat loss in relation to lean body mass.  But there is far less data on how this relates to female athletes.

The study notes that previous research has shown that ketogenic diets do not produce markedly different performance benefits over carbohydrate rich diets.  But they could have some specific sports nutrition applications, which could include long distance, lower intensity events that rely on fat oxidation as a primary fuel source or for bodybuilding or physique competitors who are looking to cut as much fat as possible before events.  Another potential application not mentioned by the researchers could be for competitors in weight restricted events like competitive weightlifting or combat sports like judo, wrestling or mixed martial arts.

The study noted that previous research had shown the ability of ketogenic diets to cut fat while preserving muscle in men. To test this hypothesis among females the researchers recruited 21 strength trained women whose ages ranged from about 23 to 31 years old. 

Diet compliance measured via test strips

The subjects were randomly assigned to a higher carb diet or a ketogenic diet. The participants were instructed what to eat but were not supplied with test diets.  Their compliance was measured with a urine reagent test strip to check on ketosis status. They completed a 3 week familiarization of the training protocol and then an 8-week, split-body, periodized strength training regime.

Body composition measurements were taken before and after the 8-week intervention. Strength gains were measured via a one time maximum bench press and back squat, and jump force measured with a reaction plate.

The researchers found that the ketogenic diet group lost about 1.1 kg of fat mass while maintaining their fat free mass within the range of statistical significance.  On the strength parameters, both groups improved in leg strength, while only the carbohydrate diet group improved in upper body strength.

Kleiner:  Study is worthwhile addition to body of knowledge

PhD nutritionist Susan Kleiner said the study was a worthwhile addition to the body of science on female athletes, something she has advocated for for a long time.  She also lauded the authors about being forthright regarding the limitations of their study. Those included the fact that the urine strips could be a less sensitive method of testing for diet compliance than blood tests would be, and the fact that the ketogenic diet group was showing signs of not being able to consume enough calories via this eating mode to keep up with their caloric needs during the training phase.

Another issue that Kleiner raised is that there were no measures of perceived exertion in this study. She said other research using ketogenic diets in strength training suggests that this eating mode makes training feel harder than it does for those eating carbohydrate rich diets.

“This is a nicely designed female-centric study by some of the top exercise scientists in the field. They timed their measurements to each subjects’ menstrual cycle, which demonstrates that once again that this is not impossible, or extremely difficult, to accomplish,”​ Kleiner told NutraIngredients-USA.

The results confirm several suspected outcomes: that following a ketogenic diet for 8 weeks along with a resistance training program will allow for slightly more body fat loss and body weight loss compared to a moderately-high carbohydrate-rich diet (significant decrease of fat mass of 1.1 kg vs. an insignificant increase of 0.3 kg). On the other hand, the subjects on the carb-rich diet gained 0.7 kg fat free mass and the ketogenic group lost 0.7 kg fat free mass, a significant difference between the groups. In all categories of measures (bench press, squat, countermovement jump) the carb-rich group outperformed the ketogenic group,”​ she added.

Overall, this is a study most applicable to female athletes that participate in weight class sports or physique competitions. The differences in outcomes in 8 weeks are too small to make a difference to most other fitness-minded adults for whom losing hard-earned muscle would hardly be desirable. We can see that the ketogenic diet will help you drop a fraction more fat mass and total body weight over a period of 8 weeks, but strength gains will be largely diminished or disappear and muscle will be lost.  . . . Certainly if strength gains are desired, a ketogenic diet is not the right dietary choice,” ​Kleiner concluded.

Source:Journal of the International Society of Sports Nutrition
17​, Article number: 19 (2020)
Effects of a ketogenic diet on body composition and strength in trained women
Authors: Vargas-Molina S, et al.

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