The repair effect of L-leucine on muscle injuries after high-intensity exercise

The repair effect of L-leucine on muscle injuries after high-intensity exercise

L-leucine is the most physiologically active member among branched-chain amino acids (BCAAs, including leucine, isoleucine, and valine). Its repairing effect on muscle damage after high-intensity exercise is achieved primarily by activating protein synthesis pathways, inhibiting muscle protein degradation, alleviating inflammatory responses, and promoting the recovery of energy metabolism. These mechanisms accelerate the repair and regeneration of damaged muscle fibers while reducing the severity of delayed-onset muscle soreness (DOMS). The following elaborates on the damage mechanisms, action pathways, and application strategies in detail.

I. Core Mechanisms of Muscle Damage After High-Intensity Exercise

High-intensity exercise (such as resistance training, sprinting, and prolonged endurance exercise) causes dual damage to skeletal muscles and triggers the body’s repair response:

Muscle Fiber Structural DamageDuring exercise, eccentric muscle contractions generate mechanical tension, leading to the rupture of muscle cell membranes (sarcoplasmic reticulum) and myofibril fragmentation. Intracellular enzymes such as creatine kinase (CK) and lactate dehydrogenase (LDH) are released into the bloodstream, serving as biochemical markers of muscle damage.

Oxidative Stress and Inflammatory ResponseHigh-intensity exercise drastically increases oxygen consumption in muscle cells, producing a large amount of reactive oxygen species (ROS) that disrupt the lipid peroxidation balance of cell membranes. Meanwhile, damaged muscle fibers activate the recruitment of immune cells (e.g., macrophages), which release pro-inflammatory cytokines such as tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6), triggering local inflammation characterized by muscle soreness, swelling, and decreased muscle strength.

Protein Metabolism ImbalanceAfter exercise, the rate of muscle protein degradation temporarily exceeds that of synthesis. Without timely supplementation of nutritional substrates, net muscle protein loss occurs, delaying the repair process.

II. Key Action Pathways for Repairing Muscle Damage

Activating the mTOR Signaling Pathway to Accelerate Muscle Protein SynthesisMuscle protein synthesis is the core link in damage repair, and the mammalian target of rapamycin (mTOR) pathway is the key signaling pathway regulating protein synthesis.

L-leucine can directly bind to leucine sensors (e.g., Sestrin2) in skeletal muscle cells, relieving the inhibition of the mTOR pathway. This activates downstream ribosomal protein S6 kinase (p70S6K) and eukaryotic translation initiation factor 4E-binding protein 1 (4E-BP1), promoting ribosome assembly and mRNA translation. Consequently, the synthesis of structural proteins such as myosin and actin is accelerated, facilitating the repair of damaged myofibrils.

Compared with other BCAAs, the activation effect of L-leucine is specific and dose-dependent—when the blood L-leucine concentration reaches 0.5–1.0 mmol/L, the activation efficiency of the mTOR pathway is maximized, and this activation can last for several hours, providing sufficient driving force for protein synthesis during muscle repair.

Inhibiting Muscle Protein Degradation to Reduce Muscle Fiber LossAfter high-intensity exercise, the body accelerates the degradation of damaged proteins through the ubiquitin-proteasome system (UPS) and autophagy-lysosome system. However, excessive degradation can lead to muscle atrophy.

L-leucine inhibits the expression of ubiquitin ligases (e.g., MuRF1, MAFbx), blocking the activation of the UPS pathway and reducing the degradation of muscle fiber structural proteins. Meanwhile, L-leucine downregulates the expression of autophagy-related genes (e.g., Beclin1), preventing the excessive autophagic clearance of normal muscle cells.

In addition, L-leucine promotes insulin secretion. Insulin not only enhances the cellular uptake efficiency of amino acids but also further reduces the rate of muscle protein degradation by inhibiting the protein kinase A (PKA) pathway, maintaining the net balance of muscle protein synthesis.

Alleviating Oxidative Stress and Inflammatory Response to Reduce Muscle SorenessL-leucine indirectly regulates oxidative stress and inflammatory levels, creating a stable microenvironment for muscle repair:

Antioxidant Effect: L-leucine promotes the synthesis of glutathione (GSH) in skeletal muscle cells. As an important antioxidant in the body, GSH scavenges ROS generated during exercise, reduces lipid peroxidation damage, lowers the permeability of muscle cell membranes, and decreases the leakage of CK and LDH. Experiments show that L-leucine supplementation can reduce blood ROS levels by 20%–30% and CK activity by 15%–25% after exercise.

Anti-inflammatory Effect: L-leucine inhibits the polarization of macrophages toward the pro-inflammatory phenotype (M1 type), reducing the release of pro-inflammatory cytokines such as TNF-α and IL-6. It also promotes the secretion of anti-inflammatory cytokines (e.g., IL-10), alleviating local inflammatory responses and shortening the duration of DOMS. Studies have demonstrated that supplementing with L-leucine after high-intensity resistance training reduces the visual analog scale (VAS) score of muscle soreness by more than 30% and accelerates muscle strength recovery by 2–3 days in subjects.

Promoting Energy Metabolism Recovery to Improve Muscle FunctionAfter high-intensity exercise, muscle glycogen stores are largely depleted, and ATP levels decrease, affecting the contractile function and repair efficiency of muscle fibers.

L-leucine provides substrates for the liver through gluconeogenesis, promoting hepatic glycogen synthesis and indirectly replenishing muscle glycogen reserves. Meanwhile, metabolites of L-leucine (e.g., α-ketoisocaproic acid) participate in the tricarboxylic acid cycle, supplying energy to muscle cells and accelerating ATP regeneration.

Furthermore, L-leucine enhances the glucose uptake capacity of muscle cells and improves insulin sensitivity, further promoting the synthesis and utilization of energy substances and facilitating the rapid recovery of muscle function.

III. Application Strategies for Post-Exercise Muscle Repair

Supplementation Dosage and Timing

Dosage: For individuals engaging in high-intensity exercise, the recommended single dose is 2–5 g, with a maximum daily total dose not exceeding 10 g. Low doses (<2 g) cannot effectively activate the mTOR pathway; high doses (>10 g) may increase renal metabolic burden and cause antagonistic effects with other BCAAs.

Timing: The optimal supplementation window is within 30–60 minutes after exercise, when muscle cells have the highest amino acid uptake efficiency, enabling the rapid initiation of protein synthesis. It can also be supplemented 30 minutes before exercise to provide substrates for muscles in advance and reduce protein degradation during exercise. For prolonged exercise (e.g., marathon), fractional supplementation during exercise (1–2 g every 1–2 hours) is recommended to maintain stable blood L-leucine concentrations.

Compound Supplementation to Enhance EfficacyCombining L-leucine with other nutrients exerts a synergistic effect:

Combination with other BCAAs: Formulated at a ratio of leucine: isoleucine: valine = 2:1:1, isoleucine and valine can assist L-leucine metabolism and avoid amino acid imbalance caused by single supplementation.

Combination with carbohydrates: Carbohydrates promote insulin secretion, further enhancing the mTOR pathway activation effect of L-leucine. A recommended ratio is carbohydrates: leucine = 3–4:1 (e.g., 5 g of leucine plus 20 g of glucose).

Combination with proteins: Combined with high-quality protein supplements such as whey protein and casein, whey protein’s high leucine content directly provides repair substrates, while casein’s sustained-release property maintains long-term amino acid supply.

Precautions

Avoid high-dose supplementation on an empty stomach to prevent elevated blood ammonia levels, which may cause dizziness, nausea, and other discomfort.

Individuals with renal insufficiency should supplement with caution, as excessive L-leucine may increase renal excretion pressure.

Muscle repair requires sufficient rest and moderate recovery training; L-leucine supplementation alone cannot replace post-exercise recovery measures.

IV. Research Evidence and Application Value

Multiple clinical studies have confirmed the muscle repair effect of L-leucine. In a randomized controlled trial involving athletes, continuous supplementation with 5 g/d of L-leucine for 7 days after high-intensity resistance training reduced blood CK levels by 28%, muscle soreness by 35%, and accelerated muscle strength recovery by 2 days compared with the control group.

In practical applications, L-leucine is not only suitable for muscle damage repair in professional athletes but also for relieving DOMS in fitness enthusiasts and preventing sarcopenia in the elderly, serving as a safe and effective nutritional supplement.