The protective effect of L-leucine on muscle mass in female athletes

As a core component of branched-chain amino acids (BCAAs), L-Leucine is the only amino acid that can directly activate the mammalian target of rapamycin (mTOR) signaling pathway. It independently initiates the muscle protein synthesis (MPS) process while inhibiting muscle protein breakdown (MPB). For female athletes, in scenarios such as high-intensity training, energy deficit (fat loss phase), or recovery periods, L-Leucine can effectively preserve muscle mass by targeting and regulating muscle metabolic pathways, with its mechanism of action exhibiting certain gender-specific differences compared to male athletes.

I. Core Mechanisms of L-Leucine in Preserving Muscle Mass

Activating the mTOR Signaling Pathway to Drive Muscle Protein SynthesisThe initiation of muscle protein synthesis relies on the activation of the mTOR signaling pathway, and L-Leucine acts as the key "switch" for this pathway. Compared with males, female athletes have a relatively lower basal rate of muscle protein synthesis and exhibit differential sensitivity to amino acids. L-Leucine binds to intracellular leucine sensors (e.g., Sestrin2) to relieve the inhibitory effect on mTOR, which in turn phosphorylates downstream ribosomal protein S6 kinase 1 (S6K1) and eukaryotic translation initiation factor 4E-binding protein 1 (4E-BP1). This accelerates ribosome assembly and mRNA translation, promoting the synthesis of myofibrillar proteins (e.g., myosin heavy chain). Studies have shown that supplementation with L-Leucine increases the rate of muscle protein synthesis in female athletes by 15%–25%. Notably, this activation effect is more pronounced under conditions of low protein intake, making it possible to compensate for the limitations of insufficient protein intake on muscle synthesis.

Inhibiting Muscle Protein Breakdown Pathways to Reduce Myofibril LossDuring the fat loss phase or after high-intensity endurance training, female athletes experience elevated cortisol levels, which activate the ubiquitin-proteasome pathway (UPP) and autophagy-lysosome pathway, accelerating muscle protein breakdown. L-Leucine inhibits muscle breakdown through two pathways: first, it reduces the binding efficiency of cortisol to glucocorticoid receptors, thereby decreasing the expression of ubiquitin ligases in the UPP pathway; second, it increases the concentration of glutamine in muscle cells. As a "muscle protective factor", glutamine inhibits the expression of autophagy-related genes (e.g., Beclin-1), reducing myofibril degradation. In addition, L-Leucine promotes insulin secretion. Insulin not only synergizes with the mTOR pathway to enhance protein synthesis but also inhibits hepatic gluconeogenesis, reducing the mobilization of amino acids from muscle and further lowering the rate of muscle breakdown.

Regulating Energy Metabolism to Improve the Muscle Cell MicroenvironmentFemale athletes possess stronger muscle oxidative metabolism capacity, and high-intensity training often leads to energy depletion in muscle cells, which in turn induces muscle damage and breakdown. L-Leucine can directly serve as an energy substrate to supply energy, while also promoting the uptake and utilization of glucose by muscle cells, increasing muscle glycogen stores, and alleviating post-training energy stress. Meanwhile, it reduces the levels of pro-inflammatory cytokines (e.g., TNF-α, IL-6) in muscle tissue after exercise, mitigating inflammation-mediated muscle damage, accelerating myofibril repair, and maintaining the structural integrity of muscle.

II. Applicable Scenarios and Protective Effects for Female Athletes

Muscle Mass Preservation During the Fat Loss PhaseFemale athletes in the fat loss phase must maintain an energy deficit, and traditional dietary restriction tends to cause muscle protein breakdown to exceed synthesis, resulting in "muscle loss". Without increasing total caloric intake, L-Leucine can maintain the rate of muscle protein synthesis by activating the mTOR pathway while inhibiting breakdown pathways. Clinical data indicate that female athletes who supplement with 3–5 g of L-Leucine daily (administered in 2–3 divided doses) combined with resistance training can reduce muscle loss rate by 40%–60% without compromising the rate of body fat reduction, achieving the goal of "losing fat without losing muscle". Compared with males, females exhibit a milder dose-response to leucine; excessive supplementation (>8 g per day) does not further enhance efficacy but may instead increase gastrointestinal burden.

Muscle Repair During the Recovery Period After High-Intensity TrainingAfter strength training or interval training, female athletes experience micro-damage to myofibrils. Failure to repair this damage promptly can lead to decreased muscle function. L-Leucine accelerates the repair process of damaged myofibrils: supplementing with L-Leucine within 30 minutes after training rapidly increases blood leucine concentration, activates the mTOR pathway, and initiates the repair program. Studies have shown that female athletes who supplement with 2–3 g of L-Leucine after training (paired with a small amount of carbohydrates) experience a 30% reduction in muscle soreness, a 20% increase in muscle strength recovery speed, and demonstrate better muscle endurance performance during subsequent training sessions.

Muscle Maintenance for Female Athletes on Vegetarian/Low-Protein DietsSome female athletes follow a vegetarian diet due to dietary preferences or have insufficient protein intake (<1.2 g per kg of body weight daily) due to training schedules, leading to a lack of raw materials for muscle synthesis. As a "synthesis trigger", L-Leucine maximizes the activation of muscle protein synthesis under low-protein conditions, compensating for insufficient total protein intake. For vegetarian female athletes, daily supplementation with 3 g of L-Leucine combined with plant-based proteins (e.g., soy protein, pea protein) can achieve a muscle protein synthesis rate equivalent to that of the animal protein intake group, effectively preventing the decline of muscle mass.

III. Key Precautions for L-Leucine Supplementation in Female Athletes

Dosage and Timing of Intake

Recommended Dosage: 2–3 g per day for daily muscle mass maintenance; 3–5 g per day during the fat loss phase or high-intensity training period, administered in 2–3 divided doses to avoid single large-dose intake.

Optimal Timing: The 30-minute window immediately after training is the golden period for supplementation, as L-Leucine can be rapidly absorbed and utilized by muscle. During the fat loss phase, supplementation can be divided into three times: breakfast, post-training, and before bedtime. Pre-sleep supplementation can delay nighttime muscle protein breakdown.

Combination StrategiesL-Leucine yields better results when combined with carbohydrates, other BCAAs (isoleucine, valine), or whey protein. Carbohydrates promote insulin secretion, synergistically enhancing the synthetic effect of leucine. Isoleucine and valine work in synergy with leucine to maintain the balance of BCAAs in muscle, avoiding the loss of other amino acids caused by single-component leucine supplementation.

Gender-Specific DifferencesFemale athletes differ from males in muscle mass and hormone levels (e.g., estrogen, progesterone). Estrogen enhances the sensitivity of muscle to insulin, thereby improving the efficacy of leucine. Therefore, the leucine dosage required by females is lower than that of males (males are recommended to take 4–6 g per day). In addition, female hormone levels fluctuate significantly during the menstrual cycle; leucine supplementation can be appropriately increased by 1 g per day to counteract the impact of hormonal changes on muscle metabolism.

Avoiding Excessive SupplementationExcessive L-Leucine supplementation (>8 g per day) may cause gastrointestinal discomfort (e.g., bloating, diarrhea), interfere with the absorption of other amino acids, and affect the overall efficiency of protein synthesis. Long-term excessive supplementation may also increase the metabolic burden on the kidneys, requiring rational adjustment based on individual training intensity and dietary conditions.

L-Leucine exerts a significant protective effect on the muscle mass of female athletes by activating muscle protein synthesis pathways, inhibiting breakdown pathways, and improving energy metabolism. It is particularly suitable for scenarios such as the fat loss phase, recovery period after high-intensity training, and low-protein diets. Its efficacy exhibits distinct gender-specific characteristics, and female athletes need to develop personalized supplementation plans based on their physiological characteristics and training requirements.