How Low Carb Diets Jeopardize Muscle Growth and High-Intensity Exercise

Emerging research indicates that consuming a low carbohydrate diet can promote weight loss and reduce body fat⁽¹⁾.  

The reduction in liver and muscle glycogen (stored carbohydrate) levels promotes molecular adaptations within peripheral tissue that encourages the mobilization and oxidation of fatty acids⁽²⁾.  

These adaptations rapidly increase fat breakdown both at rest and during exercise. In addition to its extreme popularity in the public, this type of diet has gained tremendous popularity within the athletic community. One very popular aspect is that adhering to a low carbohydrate intake along with resistance training may help athletes achieve an optimal strength and power to mass ratio by reducing both total body and fat mass and potentially enhancing fat-free (lean) mass.

Although these diets sound promising in that they decrease fat mass, they also increase reliance on protein for energy⁽³⁾.

The issue with relying on dietary protein for energy production is that it restricts the availability of essential amino acids, namely the branched-chain amino acids (BCAA) leucine, isoleucine, and valine to preserve optimal protein synthesis⁽⁴⁾.  

The failure to support muscle anabolism following a low carbohydrate intake may limit muscle growth and jeopardize muscle remodeling and repair⁽⁵⁾.

Below I will review the current research examining the influence of dietary carbohydrate restriction on the molecular regulation of muscle mass and its ensuing effects on fat-free mass and anaerobic performance.

Cellular Adaptations in Skeletal Muscle

The reduction in glycogen availability due to low carbohydrate diets modulate intramuscular signaling pathways that control energy metabolism by decreasing carbohydrate oxidation, while enhancing both fat and protein oxidation.

Figure: Molecular adaptations regulating carbohydrate, fat, and protein metabolism in response to low carbohydrate diets within skeletal muscle⁽⁶⁾.

Essentially, the low carbohydrate availability increases fat oxidation via increased concentrations of mitochondria metabolites which shuttle fatty acids from the cytosol to the mitochondria of the cell and enhance beta-oxidation⁽³⁾. 

Beta-oxidation is a biochemical and metabolic process by which fatty acid molecules are broken down for energy.

As mentioned previously, the primary modification in energy utilization in response to low carbohydrate intake is increased fat oxidation, but there is also a notable increase in the reliance on dietary protein for oxidative and gluconeogenic purposes. The downside is that as one relies more heavily on protein oxidation, there is a lower rate of muscle protein synthesis upon exercise that is initiated with low carbohydrate availability⁽⁵⁾.  

Muscle building regulatory processes seem to be repressed during low carbohydrate availability.

In fact, increased BCAA oxidation contributed to reductions in the transcription of myogenic regulatory factors (e.g. MyoD, Myogenin) when glycogen availability is low⁽⁵⁾.  

In addition, there are regulatory steps of phosphorylation important to protein synthesis that are in part, regulated by insulin⁽⁷⁾.

Long-term low carbohydrate diets decrease upstream regulation of protein synthesis and myogenesis. Evidence indicates that reductions in myogenesis with low glycogen impairs muscle remodeling and repair of after exercise⁽⁶⁾.  

The current consensus is that when BCAAs are relied upon for fuel with low glycogen levels; anabolic and myogenic signaling is diminished. This, in turn, may contribute to reductions in skeletal muscle mass with chronic consumption of low carbohydrate diets.

Muscle Size, Strength & Power

The bullet points below are key takeaways from recent evidence in regards to the effects of low carbohydrate availability on fat-free mass, strength, and power.

A recent systematic review reported that when low carbohydrate diets are combined with exercise training; healthy, normal weight, active participants have greater decrements of total body and fat mass compared to participants consuming a mixed-macronutrient diet for 3 to 12 weeks⁽⁸⁾.

Participants consuming a mixed-macronutrient diet in combination with exercise training increased fat-free mas, whereas the low carbohydrate group loss fat-free mass. There was a 1kg difference in fat-free mass between the low-carbohydrate and mixed-macronutrient diet groups.

Figure: Changes in body mass (a), fat mass (b), and fat-free mass following a low carbohydrate or mixed-macronutrient diet for 21–84 days. Individual points represent a mean value from an individual study that were reported in the systematic review by Coleman et al.⁽⁸⁾

Key points to consider:

• Despite a blunted hypertrophic response to low carbohydrate diets, there does not appear to be any negative effect on muscle strength⁽⁹⁾.  
• Very low carbohydrate intake combined with daily resistance exercise for 8 weeks demonstrated no change in fat-free mass, decreased total testosterone and insulin-like growth factor-1 (IGF-1), and an increase in 1 repetition maximum (1RM) bench press and squat⁽¹⁰⁾.
• Conversely, those who consumed a mixed-macronutrient diet had no change in total testosterone or IGF-1 and a 2kg increase in fat-free mass along with enhanced 1RM bench press and squat⁽¹⁰⁾.
• This null effect of low carbohydrate availability on muscle strength appears to be consistent across studies for 2 to 84 days⁽⁹⁾.
• This data indicates that while carbohydrate availability may alter muscle growth, it does not impact muscle strength in response to resistance exercise training.
• The availability of carbohydrate is a crucial rate limiting fuel source to maintain muscle power during continuous or repeated bouts of high-intensity exercise⁽¹¹⁾.
• Glycogen availability plays an important role in its impact on muscle power. When glycogen availability is reduced, there is an associated negative impact on muscle power during high-intensity exercise (e.g., sprint performance)⁽¹²⁾.

Summary

There is a need to conduct more research in this area to elucidate the difference between low carbohydrate and mixed-macronutrient diets on body mass, fat mass, fat-free mas, strength, and power. 

Longer duration studies that are well controlled with dietary variables are needed.

The dietary component of research to date has not been well controlled so it is hard to delineate the difference because we cannot tell if the loss of total body mass, fat mass, and fat-free mass is due to low carbohydrate or just a negative energy balance.

Key takeaways from this research are that impaired muscle growth does not affect muscle strength, but persistently low glycogen following low carbohydrate intake diminishes anaerobic performance. Low carbohydrate diets decrease body fat mass and increase whole-body and skeletal muscle fat oxidation.

Increased amino acid oxidation stemming from low carbohydrate availability decreases muscle remodeling and repair after exercise, potentially contributing to blunted hypertrophic responses to resistance exercise training. New evidence indicates that low carbohydrate diets increase the concentrations of BCAA and muscle protein breakdown metabolites, due to increased reliance on essential amino acids for oxidation.

Now that you know how low-carb diets affect your muscle-builoding efforts, learn the difference between simple and complex carbs here. 


References:
    1.    Ludwig DS, Aronne LJ, Astrup A, et al: The carbohydrate-insulin model: a physiological perspective on the obesity pandemic. Am J Clin Nutr 114:1873-1885, 2021
    2.    Margolis LM, Wilson MA, Whitney CC, et al: Exercising with low muscle glycogen content increases fat oxidation and decreases endogenous, but not exogenous carbohydrate oxidation. Metabolism 97:1-8, 2019
    3.    Margolis LM, Karl JP, Wilson MA, et al: Serum Branched-Chain Amino Acid Metabolites Increase in Males When Aerobic Exercise Is Initiated with Low Muscle Glycogen. Metabolites 11, 2021
    4.    Gwin JA, Church DD, Hatch-McChesney A, et al: Essential amino acid-enriched whey enhances post-exercise whole-body protein balance during energy deficit more than iso-nitrogenous whey or a mixed-macronutrient meal: a randomized, crossover study. J Int Soc Sports Nutr 18:4, 2021
    5.    Margolis LM, Wilson MA, Whitney CC, et al: Initiating aerobic exercise with low glycogen content reduces markers of myogenesis but not mTORC1 signaling. J Int Soc Sports Nutr 18:56, 2021
    6.    Margolis LM, Pasiakos SM: Low carbohydrate availability impairs hypertrophy and anaerobic performance. Curr Opin Clin Nutr Metab Care 26:347-352, 2023
    7.    Litwiniuk A, Pijet B, Pijet-Kucicka M, et al: FOXO1 and GSK-3beta Are Main Targets of Insulin-Mediated Myogenesis in C2C12 Muscle Cells. PLoS One 11:e0146726, 2016
    8.    Coleman JL, Carrigan CT, Margolis LM: Body composition changes in physically active individuals consuming ketogenic diets: a systematic review. J Int Soc Sports Nutr 18:41, 2021
    9.    Henselmans M, Bjornsen T, Hedderman R, et al: The Effect of Carbohydrate Intake on Strength and Resistance Training Performance: A Systematic Review. Nutrients 14, 2022
    10.    Paoli A, Cenci L, Pompei P, et al: Effects of Two Months of Very Low Carbohydrate Ketogenic Diet on Body Composition, Muscle Strength, Muscle Area, and Blood Parameters in Competitive Natural Body Builders. Nutrients 13, 2021
    11.    Vigh-Larsen JF, Ortenblad N, Spriet LL, et al: Muscle Glycogen Metabolism and High-Intensity Exercise Performance: A Narrative Review. Sports Med 51:1855-1874, 2021
    12.    Hearris MA, Hammond KM, Seaborne RA, et al: Graded reductions in preexercise muscle glycogen impair exercise capacity but do not augment skeletal muscle cell signaling: implications for CHO periodization. J Appl Physiol (1985) 126:1587-1597, 2019