The combination of resistance training with a solid dietary regimen is acknowledged as an effective strategy to improve body composition (1).
Existing sports nutrition guidelines propose carbohydrate-based or periodized carbohydrate-based diets to augment athletic performance and muscular adaptations to resistance training (2).
However, in the last decades, there has been a surge in the popularity of low-carbohydrate and high-fat approaches such as the ketogenic diet, largely due to purported benefits for body composition (3).
A ketogenic diet is commonly characterized by decreasing carbohydrate intake to a maximum of approximately 50 grams/day, or 5% of daily energy intake, while protein consumption is moderate or high (e.g. ~1.2 to 1.5 g/kg/d).
The remaining energy intake is predominantly from fats (~–80%), with relative fat intake depending on the degree of displacement of carbohydrates and proteins (4).
This macronutrient distribution leads to an increase in the production of ketone bodies, such as acetoacetate, β-hydroxybutyrate, and acetone, and consequently to the state of physiological ketosis (i.e., elevated ketone bodies concentrations in the plasma as compared to a mixed diet) (5).
In addition to the investigation as a dietary intervention to combat overweight and obesity, ketogenic diets have received increasing attention among normal-weight adults and those with aesthetic goals, including resistance-trained individuals (6).
It is recognized that resistance training is partially dependent upon muscle glycogen (stored carbohydrate) as a form of energy supply.
Carbohydrate intake and consequent intramuscular glycogen concentrations play a critical role in the energy status by influencing the activity of a key cellular protein which limits muscle size and capacity for muscle growth and promotes muscle loss (7).
Moreover, it has been demonstrated that ketogenic diets and low-glycogen concentrations may result in up-regulate this catabolic cellular protein, which essentially decreases muscle protein synthesis rates (8).
How does a keto affect muscle mass?
Ketogenic diets can be considered rapid body mass loss diets and there isn’t much evidence indicating that fat-free mass loss may be a consequence of the rapid loss of body mass that often occurs on keto.
It is recognized that the early, rapid body mass loss with a ketogenic diet is related to loss of glycogen stores and associated water weight (3).
Keto can contain a moderate to high protein content (approximately 30-35% of daily calorie intake) which is a crucial factor for resistance training based muscular hypertrophic adaptations. In addition, it’s been recently shown that a high protein diet could preserve muscle mass during a body mass and/or fat loss phase (9).
Consequently, the question as to whether and how ketogenic diet could affect fat-free mass in those performing resistance training remains unclear.
It’s been thought that ketogenic diets lead to greater body mass and fat mass loss while maintaining fat-free mass, and therefore these diets may effectively improve body composition in individuals who participate in resistance training.
However, the general impact of keto on body composition changes in resistance-trained individuals is uncertain and has varied in prior investigations.
However, a recent systematic review and meta-analysis from randomized controlled trials was conducted to compare the efficacy of ketogenic diets in individuals performing resistance training on body composition variables, including body mass, body mass index, fat mass, fat-free mass, and body fat percentage (8).
This is unique because historically, research utilizing a keto diet was done in obese people who don’t exercise or just do aerobic exercise.
Characteristics of this meta-analysis:
Key findings from this research:
The novelty of this research was that the effects of ketogenic diets on body composition in individuals participating in resistance training was not previously investigated.
According to the results derived from this study, a combination of resistance training with ketogenic diet was associated with decrements in all body composition parameters including body mass, body mass index, fat mass, body fat percentage, and fat-free mass.
Body mass, fat mass, and fat-free mass tended to decrease more following longer ketogenic interventions (> 8 weeks) and in participants with obesity (BMI>30 kg.m-2). The loss of body mass on keto could be influences by the differences in stored glycogen (carbs) and the associated water content.
Both keto and non-keto diets have previously been shown to have similar effects on body mass and fat mass loss when calories are decreased the same amount (5).
Greater body mass loss observed following keto diets in the present analysis could likely be due to lower calorie intake. This is because in ad libitum (able to eat freely) settings, physiological ketosis elicited by ketogenic diets may result in suppressing appetite (10), therefore decreasing daily calorie intake.
For people that can adhere to this regimen, the ketogenic diet is a feasible option for enhancing body mass and fat mass loss in ad libitum settings (8).
Unfortunately, there is no scientific evidence on the effects of long-term ketogenic diets on body composition changes in resistance-trained populations. All studies included in the current analysis were short-term (3 weeks to 3 months).
Similarly, to the loss of body mass, a significant difference in fat mas was observed only in ad libitum studies.
Thus, it seems that even in short-term interventions, the fat loss benefit of keto diets is associated to calorie intake reduction. Therefore, these findings suggest that in resistance-trained athletes, when calories are equal, ketogenic diets don’t demonstrate an advantage on fat loss (8).
A major concern with ketogenic diets is the potential loss of fat-free mass. It’s apparent in untrained populations that the amount of fat-free mass loss is slightly higher following ketogenic diets compared to non-ketogenic diets.
The current analysis showed that in resistance trained populations, fat-free mass decreased significantly in people assigned to a ketogenic diet as compared to non-ketogenic diet.
This catabolic or anti-anabolic effect of ketogenic diets may be due to inhibition of protein synthesis and an increase in protein breakdown. The underlying cellular and molecular mechanisms that are activated by a ketogenic diet are important in regulating muscle mass gains (11).
Another possible mechanism is the positive effect of carbohydrates on muscle protein balance in regards to the higher fat free mass loss in ketogenic diets. Although carbs are not generally known for affecting muscle protein synthesis, previous research shows carb consumption can improve net muscle protein balance by reducing muscle protein breakdown; potentially mediated by insulin (12).
Most of the studies included in the current analysis showed that participants adhering to a ketogenic diet consumed significantly higher protein than those adhering to a non-ketogenic diet.
However, keto diets produced greater fat-free mass loss in resistance trained individuals compared to non-keto diets, essentially indicating higher protein intake of keto diets was insufficient to preserve fat-free mass despite concurrent resistance training.
Ketogenic diets may potentially result in significant loss of muscle mass as the body converts amino acids from skeletal muscle protein in order to maintain blood glucose.
In support of the importance of carbohydrate intake for muscle mass maintenance in people participating in resistance training, it’s been shown that non-ketogenic hypocaloric diets (1250 calories= 55–60% carbs, 20–25% protein) combined with resistance training have led to fat-free mass preservation (13) potentially due to a lesser degree of amino acid utilized for making new glucose due to a higher carbohydrate intake.
Some limitations of this analysis are that most of these trials examined lasted less than three months, therefore there is no evidence on the long-term differences in keto diets and non-keto diets on body composition in resistance trained athletes. Another limitation was the difficulty in establishing isocaloric diets in free-living conditions and a lack of resistance training supervision or standardization in several of the investigations included in this analysis (8).
Summary and my take on this:
This was a really well conducted meta-analysis that included solid inclusion criteria for the randomized controlled trials that were investigated.
The evidence indicates that on the positive side, individuals on a keto diet did lose weight, the negative side is that a good portion of this weight loss was from lean body mass; which is not what we want. The loss in lean body mass could be from loss of body water, as we know keto has lower carbs, therefore if you have lower carbs stored as glycogen, you will have less water inside the muscle.
I think it’s safe to say from this evidence that individuals on keto are potentially building less muscle tissue. I don’t think people will get a maximum resistance training performance without carbs and they won’t get as strong or muscular on a keto diet.
If your goal is to be as big and strong as possible; a keto diet is not an ideal diet. If you decide to utilize keto to lose weight; just understand the drawbacks of this type of diet.
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