The improvement effect of L-leucine on the muscle quality of animals

As the most potent component of branched-chain amino acids (BCAAs) in regulating muscle metabolism, L-leucine comprehensively improves animal muscle quality across three dimensions—physical properties, nutritional value, and sensory flavor—by modulating the balance between muscle protein synthesis and degradation, optimizing myofiber type transformation, improving muscle nutrient composition, and enhancing flavor substance accumulation. Its efficacy has been clearly verified in farm animals such as pigs, poultry, and aquatic species. The core mechanisms and application effects are as follows:

I. Core Improvement Mechanisms: Targeted Regulation of Key Processes in Muscle Quality Formation

1. Optimizing Myofiber Type and Morphology to Improve Meat Physical Properties

Myofiber type (fast-twitch type Ⅱ / slow-twitch type Ⅰ) and cross-sectional area are core determinants of meat tenderness and juiciness. L-leucine optimizes myofiber characteristics through signaling pathway regulation:

Activates the mTOR signaling pathway to promote satellite cell proliferation and differentiation, increasing the proportion of slow-twitch myofibers (type Ⅰ). Slow-twitch myofibers contain more mitochondria and myoglobin with a well-developed sarcoplasmic reticulum, resulting in tenderer and juicier meat. It also inhibits excessive proliferation of fast-twitch myofibers (type Ⅱb) to avoid rough texture caused by muscle fibrosis.

Regulates the homogenization of myofiber cross-sectional area, reducing the proportion of large myofibers and decreasing meat shear force (a key tenderness indicator). Adding 1.0% L-leucine to piglet diets increases the proportion of slow-twitch myofibers in the longissimus dorsi muscle by 20%–25%, reducing shear force from 4.5 kg to below 3.2 kg—meeting the high-quality meat tenderness standard (shear force < 3.5 kg).

Promotes the ordered polymerization of myofibrillar proteins (actin, myosin), reducing myofiber breakage and abnormal cross-linking, improving muscle elasticity and water-holding capacity (drip loss reduced by 15%–20%), and avoiding dry texture caused by excessive post-slaughter moisture loss.

2. Regulating Muscle Nutrient Composition to Enhance Nutritional Value

L-leucine improves muscle nutrient structure by optimizing the deposition of proteins, fats, and minerals:

Significantly increases crude protein content in muscle, especially the proportion of sarcoplasmic and myofibrillar proteins, while improving the balance of essential amino acids (lysine, methionine, threonine). This raises the amino acid score (AAS) by 10%–15%, making the nutritional value closer to the ideal protein pattern.

Inhibits excessive intramuscular fat (IMF) deposition while promoting the synthesis of unsaturated fatty acids (oleic acid, linoleic acid, linolenic acid) and reducing the proportion of saturated fatty acids. IMF content is controlled within 3%–5% (the optimal range for high-quality meat), and the proportion of unsaturated fatty acids increases by 25%–30%, meeting healthy dietary demands.

Enhances the deposition of minerals such as zinc, iron, and selenium in muscle. These minerals participate in myoglobin synthesis and antioxidant enzyme activity regulation, improving muscle color stability and nutritional value (e.g., iron content increased by 12%–18%, enhancing the iron-supplementing value of animal meat).

3. Promoting Flavor Substance Accumulation to Optimize Sensory Experience

Meat flavor is primarily determined by amino acids, nucleotides, fatty acid derivatives, and other substances. L-leucine enhances flavor substance production through metabolic regulation:

Increases the content of umami amino acids (glutamate, aspartate, glycine)—core components of meat umami—and promotes the synthesis of inosinic acid (IMP). IMP synergizes with umami amino acids to multiply umami intensity, improving meat flavor richness.

Regulates fatty acid metabolism to promote the production of volatile flavor substances (aldehydes, ketones, esters), such as increasing the content of characteristic meat flavor compounds like hexanal and octanal, while reducing the accumulation of off-odor substances (e.g., trimethylamine, unsaturated fatty acid oxidation products). This results in richer meat aroma without fishy or gamey odors.

Inhibits muscle glycogen glycolysis rate, reducing post-slaughter lactic acid accumulation and avoiding "PSE meat" (pale, soft, exudative) caused by rapid pH decline. It maintains normal meat color (stable lightness L* and redness a* values) and texture.

4. Enhancing Muscle Antioxidant Capacity to Extend Shelf Life

L-leucine reduces oxidative stress-induced damage to muscle quality by enhancing the activity of the muscle antioxidant system:

Promotes the synthesis of antioxidant substances in muscle, such as glutathione (GSH), superoxide dismutase (SOD), and catalase (CAT), while reducing malondialdehyde (MDA) content (a lipid oxidation product). This minimizes muscle oxidative browning and off-odor formation.

Delays post-slaughter oxidative rancidity of muscle, extending product shelf life. For example, broiler breast meat supplemented with L-leucine has a shelf life extended from 3–4 days to 6–7 days under 4℃ refrigeration, with no significant deterioration in color or odor during storage.

II. Muscle Quality Improvement Effects and Optimal Addition Levels for Different Farm Animals

1. Pigs (Finishing Pigs, Piglets)

Optimal Addition Level: 0.8%–1.2% added to finishing diets; 0.6%–1.0% added to post-weaning piglet diets (combined with L-isoleucine and L-valine at a 2:1:1 ratio).

Improvement Effects: Longissimus dorsi muscle tenderness (shear force) reduced by 20%–25%, water-holding capacity increased by 15%–20%, IMF content controlled at 3.5%–4.5%, umami amino acid (glutamate + aspartate) content increased by 18%–22%, meat redness (a* value) increased by 10%–15%, and PSE meat incidence reduced by 40%–50%.

2. Poultry (Broilers—Starter and Finisher Phases)

Optimal Addition Level: 0.7%–1.0% during the starter phase, 0.5%–0.8% during the finisher phase.

Improvement Effects: Breast muscle percentage increased by 5%–8%, shear force reduced by 15%–20%, drip loss decreased from 8%–10% to 5%–6%, IMP content increased by 25%–30%, unsaturated fatty acid proportion increased by 20%–25%, and oxidative rancidity rate reduced by 30%–35% during refrigeration.

3. Aquatic Animals (Fish, Shrimp)

Optimal Addition Level: 1.0%–1.5% added to fish diets, 1.2%–1.8% added to shrimp diets.

Improvement Effects: Muscle crude protein content increased by 8%–12%, moisture content reduced by 3%–5% (firmer texture), total umami amino acid content increased by 15%–20%, and muscle elasticity and chewiness significantly improved. It also reduces muscle fat deposition in fish, avoiding greasy texture and enhancing the sweet flavor of fish meat.

4. Ruminants (Beef Cattle, Sheep)

Optimal Addition Level: 0.5%–0.8% added to beef cattle finishing diets, 0.6%–0.9% added to sheep finishing diets.

Improvement Effects: Muscle marbling score increased by 1–2 grades (meeting high-quality beef standards), tenderness improved by 15%–20%, myoglobin content increased by 10%–15% (brighter red color), and unsaturated fatty acid (e.g., conjugated linoleic acid, CLA) content increased by 25%–30%, resulting in richer meat flavor.

III. Key Factors Influencing Improvement Effects

1. Dietary Amino Acid Balance

L-leucine requires an appropriate ratio with other BCAAs (isoleucine, valine) and essential amino acids (lysine, methionine): an intra-BCAA ratio of 2:1:1 and a lysine-to-L-leucine ratio of 1:1.2–1.5. Imbalances cause amino acid antagonism, reducing muscle protein synthesis efficiency and weakening quality improvement effects.

2. Addition Phase and Duration

The most significant effects are achieved when added in the late finishing phase (30–45 days before pig slaughter, 2–3 weeks before broiler slaughter)—a critical period for muscle quality formation. Continuous supplementation for ≥21 days is required to achieve long-term regulatory effects such as myofiber type transformation and flavor substance accumulation. Short-term supplementation (<14 days) only slightly improves water-holding capacity, failing to achieve comprehensive quality improvement.

3. Animal Breed and Physiological Status

Lean-type breeds (e.g., Duroc pigs, white-feathered broilers) are more responsive to L-leucine, showing more prominent quality improvement effects. Stress conditions (transportation, pre-slaughter stress) can offset some effects; thus, the addition level should be appropriately increased (by 20%–30%) during stress periods, combined with anti-stress measures (e.g., adding vitamin C, electrolytes).

4. Feeding Management and Slaughter Technology

Appropriate feeding density, temperature, and sufficient exercise enhance L-leucine’s regulatory effects. Post-slaughter low-temperature aging (0–4℃ for 24–48 hours) further improves meat tenderness, synergizing with L-leucine’s improvement effects.

IV. Application Advantages and Precautions

1. Core Advantages

Natural and Safe: As an essential amino acid, it has no residues or side effects, complying with green breeding and food safety requirements.

Multifunctional Synergy: Simultaneously improves meat tenderness, juiciness, nutritional value, and flavor, achieving "quality enhancement + value addition."

Strong Compatibility: Can be combined with probiotics, plant extracts, vitamins, and other additives without antagonistic effects.

Cost-Effective: Low addition level (0.5%–1.8%) significantly improves meat grade and increases breeding economic benefits (e.g., 10%–20% premium for high-quality pork).

2. Precautions

Avoid Excessive Addition: Addition levels exceeding 2.0% inhibit the absorption of isoleucine and valine, hindering muscle protein synthesis and reducing meat quality.

Adapt to Dietary Types: Diets with high plant protein content (e.g., soybean meal-based diets) require higher addition levels, while diets rich in fish meal or meat and bone meal can have reduced levels.

Combine with Post-Slaughter Processing: L-leucine improves intrinsic muscle quality; standardized slaughter, cooling, and storage processes are required to maximize the presentation of improvement effects.

L-leucine comprehensively improves animal muscle quality across three core dimensions—physical properties, nutritional value, and sensory experience—by targeting myofiber types, muscle nutrients, flavor substance accumulation, and the antioxidant system. Suitable for pigs, poultry, aquatic species, and ruminants, it is a key nutritional regulator for enhancing meat competitiveness under green breeding policies. Its efficacy depends on the synergy of dietary amino acid balance, appropriate addition phases, and feeding management. By precisely matching animal needs, it achieves the dual goals of improving meat grade and increasing breeding benefits. In the future, combining with functional ingredients such as creatine and betaine, or integrating molecular breeding technology, is expected to further expand the boundaries of muscle quality improvement, providing more efficient technical solutions for high-quality meat production.