The growth-promoting effect of L-leucine in feed additives

As the core component of branched-chain amino acids (BCAAs), L-leucine, with its unique metabolic characteristics and physiological functions, comprehensively improves animal growth performance in feed additives by regulating protein metabolism, energy supply, and immune function. It is particularly suitable for fast-growing farm animals such as piglets, broilers, and aquatic species. Its growth-promoting mechanisms center on the dual cores of "nutritional utilization optimization + physiological function regulation," which are detailed as follows:

I. Core Mechanisms: Regulating Growth-Related Physiological Processes

1. Enhancing Protein Synthesis and Deposition, Reducing Degradation Loss

L-leucine is an essential amino acid for protein synthesis in animals and serves as a key regulatory factor for protein synthesis by activating the mammalian target of rapamycin (mTOR) signaling pathway:

As a raw material for protein synthesis, L-leucine directly participates in the synthesis of structural proteins such as actin and myosin, promoting muscle tissue growth and improving carcass lean meat percentage.

After activating the mTOR pathway, it accelerates the formation of the translation initiation complex by phosphorylating downstream ribosomal protein S6 kinase (S6K1) and eukaryotic initiation factor 4E-binding protein 1 (4E-BP1), significantly increasing protein synthesis rate.

Simultaneously, it inhibits the activity of the ubiquitin-proteasome system, reducing muscle protein degradation. Especially under animal stress conditions (e.g., weaning, transportation), it can alleviate growth stagnation caused by enhanced protein catabolism and maintain continuous growth.

Adding 0.8%–1.2% L-leucine to piglet feed increases muscle protein synthesis rate by 25%–35% and reduces the feed conversion ratio (FCR) by 10%–15%.

2. Optimizing Energy Metabolism, Improving Nutritional Utilization Efficiency

L-leucine’s metabolic characteristics make it an important energy source in animals. Especially when carbohydrate supply is insufficient or under stress, it can rapidly provide energy through the ketone body formation pathway:

In skeletal muscle and the liver, L-leucine undergoes transamination and decarboxylation to generate ketone bodies such as acetoacetate and β-hydroxybutyrate, providing energy for tissues such as the brain and muscles, reducing glucose consumption, and indirectly saving energy for growth.

α-Ketoisocaproate (KIC) produced during metabolism can participate in the tricarboxylic acid cycle, further improving energy conversion efficiency and reducing waste of energy substances in feed.

For animals with high energy demands such as broilers and aquatic species, L-leucine can increase feed digestibility (dry matter digestibility by 5%–8%) and reduce energy residues in feces, achieving a "low-consumption and high-efficiency" growth model.

3. Regulating Appetite and Feeding Behavior, Increasing Nutrient Intake

L-leucine enhances animal feed intake by regulating the appetite signaling pathway in the central nervous system, providing a sufficient nutritional basis for growth:

In the hypothalamus, L-leucine can activate the adenosine monophosphate-activated protein kinase (AMPK) pathway, inhibit the expression of appetite-suppressing factors (e.g., leptin), and promote the secretion of appetite-stimulating factors (e.g., neuropeptide Y, NPY), enhancing feeding desire.

Improves feed palatability. Especially for feed intake stress-sensitive animals such as weaned piglets, L-leucine can alleviate the problem of reduced feed intake after weaning, increasing feed intake by 15%–20% and avoiding growth retardation caused by insufficient nutrient intake.

4. Enhancing Immune Function, Reducing Growth Inhibition Caused by Diseases

L-leucine plays an important role in the development and functional regulation of the animal immune system, reducing growth loss caused by diseases by improving immune cell activity:

As an essential nutrient for the proliferation and activation of immune cells (lymphocytes, macrophages), L-leucine can promote the synthesis of immunoglobulins (IgG, IgM) and improve humoral immune levels.

Enhances macrophage phagocytic capacity and the secretion of cytokines (IL-2, TNF-α), strengthens cellular immune responses, and improves animal resistance to bacterial and viral infections.

Adding L-leucine to piglets during the weaning period reduces the incidence of diarrhea by 30%–40%, significantly increases the weight of immune organs (thymus, spleen), and avoids growth stagnation and weight loss caused by diseases.

II. Application Effects and Optimal Addition Levels for Different Farm Animals

1. Piglets

Growth Stage: 1–8 weeks after weaning (critical period for fast growth and stress sensitivity).

Optimal Addition Level: 0.6%–1.2% added to the basal diet (based on dietary dry matter), or combined with L-isoleucine and L-valine at a 2:1:1 ratio.

Application Effects: Average daily gain (ADG) increases by 18%–25%, FCR decreases by 10%–15%, diarrhea rate caused by weaning stress reduces by 30%–40%, and muscle lean meat percentage increases by 5%–8%.

2. Broilers

Growth Stage: 0–6 weeks old (starter phase), 7–12 weeks old (finisher phase).

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

Application Effects: ADG increases by 12%–18%, FCR decreases by 8%–12%, breast muscle percentage increases by 4%–6%, dressing percentage improves by 3%–5%, and it can alleviate growth performance decline caused by heat stress.

3. Aquatic Animals (Fish, Shrimp)

Growth Stage: Larval to grow-out stage (critical period for fast growth and survival rate).

Optimal Addition Level: 1.0%–1.5% added to the diet (for fish), 1.2%–1.8% (for shrimp).

Application Effects: ADG increases by 20%–30%, feed conversion rate improves by 15%–20%, survival rate increases by 8%–12%, it can improve aquatic animals' utilization efficiency of plant proteins, and reduce the proportion of fish meal added.

4. Ruminants (Beef Cattle, Dairy Cows)

Growth Stage: Beef cattle finishing phase, dairy cow early lactation phase.

Optimal Addition Level: 0.5%–0.8% added to beef cattle diets, 0.3%–0.5% added to dairy cow diets.

Application Effects: Beef cattle ADG increases by 10%–15%, feed conversion rate improves by 8%–10%, carcass fat percentage decreases by 3%–5%; dairy cow milk yield increases by 5%–8% during early lactation, and milk protein content improves by 0.1%–0.2%.

III. Key Factors Influencing Growth-Promoting Effects

1. Dietary Amino Acid Balance

The growth-promoting effect of L-leucine depends on the balanced ratio with other amino acids, especially synergy with L-isoleucine and L-valine (intra-BCAA ratio) as well as lysine and methionine (overall essential amino acid ratio):

The recommended intra-BCAA ratio (L-leucine:L-isoleucine:L-valine) is 2:1:1; an imbalanced ratio will cause amino acid antagonism and reduce absorption and utilization efficiency.

The ratio of lysine to L-leucine in the diet should be controlled at 1:1.2–1.5 to avoid waste of L-leucine when lysine is insufficient.

2. Animal Growth Stage and Physiological Status

Animals in the fast-growing stage (e.g., post-weaning piglets, broiler starters) have higher demand for L-leucine, and the addition effect is more significant.Under stress conditions (weaning, transportation, heat, disease), L-leucine metabolism in animals accelerates, and the addition level needs to be appropriately increased (by 20%–30%) to maintain the growth-promoting effect.

3. Addition Form and Processing Technology

It is recommended to use high-purity (≥98%) L-leucine crystals to avoid impurities affecting absorption.The temperature should be controlled during feed processing (<120℃) to prevent high-temperature degradation of L-leucine.It can be mixed with carriers (e.g., corn starch, wheat bran) before uniform addition to ensure uniform dispersion in the diet, avoiding local excessive or insufficient concentrations.

IV. Application Advantages and Precautions

1. Core Advantages

Natural and Safe: As an essential amino acid in animals, it has no residues or side effects, meeting the requirements of green breeding.

Multifunctional Synergy: It has multiple effects such as promoting growth, enhancing immunity, and improving meat quality, which can reduce the use of additives such as antibiotics and hormones.

Cost-Effective: Low addition level (0.3%–1.8%), and it can improve feed digestion and utilization efficiency, reducing overall breeding costs.

Wide Adaptability: Suitable for various farm animals such as pigs, poultry, aquatic products, and ruminants, and compatible with different types of diets (corn-soybean meal type, plant protein replacement type).

2. Precautions

Avoid Excessive Addition: When the addition level exceeds 2.0%, it may cause amino acid antagonism, inhibit the absorption of isoleucine and valine, and instead reduce growth performance.

Adapt to Dietary Types: Diets with high plant protein content (e.g., soybean meal, cottonseed meal type) have relatively low L-leucine content, so the addition level needs to be appropriately increased; diets with high fish meal and meat and bone meal content can have the addition level reduced as appropriate.

Combine with Breeding Management: It is necessary to cooperate with suitable environmental conditions such as feeding density, temperature, and light to maximize the growth-promoting effect of L-leucine, avoiding environmental stress offsetting the nutritional regulation effect.

L-leucine comprehensively improves the growth performance of farm animals by activating protein synthesis pathways, optimizing energy metabolism, regulating appetite, and enhancing immune function, making it an efficient and safe growth-promoting component in feed additives. Its application effect depends on the precise matching of dietary amino acid balance, animal growth stage, and addition technology. The optimal addition level varies by animal species and growth stage (0.3%–1.8%). Under the background of green breeding policies, L-leucine can replace some antibiotics and hormones, not only improving breeding benefits but also ensuring the safety of animal products, with broad application prospects in piglets, broilers, aquatic products, and other fields. In the future, combining with probiotics, plant extracts, etc., can further expand its functional boundaries and provide technical support for efficient and ecological breeding.