Modern broilers are either marketed at a younger age or as heavier broilers competing with turkey meat products. They have higher breast meat potential and different feed intake regulation compared to previous strains. Therefore, it is crucial to refine both their energy and amino acid requirements. These refinements are essential not only for growing poultry and swine with rapidly evolving genotypes but also for reproducing animals with increased productivity. However, this review will not cover that aspect.

Nutritionists have long aimed to optimize growth and tissue accretion by increasing nutrient density, such as amino acids. The question remains about the potential benefits of amino acids beyond protein synthesis for muscle or reproductive organ development. The first step is to consider the impact of dietary amino acids on feed intake and its regulation. Amino acids are absorbed in the intestines and can affect intestinal physiology, from absorption mechanisms to interactions with gut microflora. They are metabolically active in protein synthesis and may also influence immune function from the gut to the whole body. Additionally, amino acids can impact muscle composition, meat quality, and the final product’s stability and conservation.

 Amino acids and feed intake

Amino acids play a significant role in promoting feed intake in broilers and layers by influencing various physiological and metabolic processes. Here’s an in-depth look at how amino acids enhance feed intake in poultry:

Nutrient Sensing and Appetite Regulation:

• Tryptophan and Serotonin Production: Tryptophan is a precursor to serotonin, a neurotransmitter that influences appetite and mood. Higher serotonin levels can stimulate feed intake.
• Leucine and mTOR Pathway: Leucine activates the mTOR (mechanistic target of rapamycin) pathway, which plays a role in energy balance and appetite regulation.

Enhanced Gut Health:

• Glutamine: Supports the health and integrity of the intestinal lining, improving nutrient absorption and overall gut function, which can enhance appetite.
• Threonine: Essential for mucin production, which protects the gut lining and promotes efficient nutrient absorption, indirectly promoting feed intake.

Improved Growth and Metabolism:

• Lysine and Methionine: These amino acids are critical for protein synthesis and muscle growth. As the birds grow and their metabolic demands increase, their appetite and feed intake naturally increase to meet these demands.

Hormonal Regulation:

• Arginine: Influences the production of growth hormone and insulin-like growth factors, which can enhance growth rates and feed intake.
• Insulin Sensitivity: Amino acids like leucine and arginine can improve insulin sensitivity, promoting better glucose uptake and utilization, which can increase feed intake.

Immune Function Support:

• Glutamine and Arginine: These amino acids support the immune system, reducing the impact of diseases and stress on feed intake. Healthy birds are more likely to maintain consistent feed intake.

Stress Reduction:

• Tryptophan: Helps reduce stress by increasing serotonin levels, which can lead to better feed intake under stressful conditions.
• Antioxidant Support: Amino acids like methionine and cysteine (precursors to glutathione) reduce oxidative stress, helping birds cope better with environmental stressors and maintaining their feed intake.

Amino acids and intestinal physiology

Amino acids significantly impact the intestinal physiology of broilers and layers, influencing gut health, nutrient absorption, immune function, and overall performance. Here’s a closer look at how specific amino acids contribute to intestinal health in poultry:

Key Amino Acids and Their Roles

Glutamine:

• Gut Health: Glutamine is crucial for the maintenance and repair of the intestinal mucosa. It serves as a primary energy source for enterocytes (intestinal cells).
• Immune Function: Supports the immune system by promoting the production of cytokines and other immune cells.

Threonine:

• Mucin Production: Threonine is a key component of mucin, the main protein in the mucus layer that protects the gut lining from pathogens and physical damage.
• Barrier Function: Helps maintain the integrity of the intestinal barrier, preventing leaky gut syndrome.

Arginine:

• Cell Proliferation: Stimulates cell proliferation and differentiation, promoting intestinal growth and development.
• Nitric Oxide Production: Enhances blood flow and nutrient transport to the gut through the production of nitric oxide.

Cysteine and Methionine:

• Antioxidant Production: Precursor for glutathione, a major antioxidant that protects intestinal cells from oxidative stress.
• Detoxification: Supports the detoxification processes in the liver, indirectly benefiting intestinal health.

Tryptophan:

• Serotonin Production: A precursor to serotonin, which can influence gut motility and the enteric nervous system.
• Immune Modulation: Plays a role in immune regulation within the gut.

Effects on Intestinal Physiology

Gut Morphology:

• Adequate amino acid intake promotes healthy villus height and crypt depth in the intestines, which enhances nutrient absorption.
• Improves the surface area for nutrient absorption, leading to better feed efficiency and growth in broilers and layers.

Nutrient Absorption:

• Amino acids support the production of digestive enzymes and transport proteins, facilitating efficient nutrient absorption.
• Healthy gut morphology and function enhance the overall absorption of nutrients, contributing to optimal growth and production.

Microbial Balance:

• Amino acids influence the gut microbiota, promoting the growth of beneficial bacteria and suppressing pathogenic microbes.
• Glutamine and threonine, in particular, help maintain a healthy microbial balance, reducing the risk of intestinal infections.

Amino acids and immunity

Amino acids play a crucial role in the immune function of broilers and layers, affecting their ability to resist infections and maintain overall health. Here’s how specific amino acids contribute to immunity in poultry:

Key Amino Acids and Their Immune Functions

Glutamine:

• Immune Cell Proliferation: Serves as a primary fuel source for rapidly dividing cells, including lymphocytes and macrophages.
• Cytokine Production: Enhances the production of cytokines, which are essential for immune signalling and response.

Arginine:

• Nitric Oxide Production: Promotes the synthesis of nitric oxide, a critical molecule for immune defence against pathogens.
• Lymphocyte Function: Supports the proliferation and activity of T-cells and B-cells, key components of the adaptive immune response.

Methionine:

• Antioxidant Defence: Acts as a precursor for glutathione, a potent antioxidant that protects immune cells from oxidative damage.
• Protein Synthesis: Essential for the synthesis of proteins involved in immune responses.

Threonine:

• Mucin Production: Vital for the production of mucin in the gut, which forms a protective barrier against pathogens.
• Immune Support: Contributes to the structural integrity of immune-related tissues.

Tryptophan:

• Serotonin and Melatonin Production: Modulates immune responses through the production of serotonin and melatonin, which have immunomodulatory effects.
• Kynurenine Pathway: Metabolized into kynurenine, which can influence immune tolerance and inflammation.

Cysteine:

• Glutathione Synthesis: As a precursor to glutathione, cysteine plays a crucial role in maintaining the antioxidant status of immune cells.
• Detoxification: Supports detoxification processes that can protect immune cells from toxins.

Mechanisms of Immune Enhancement

Antioxidant Protection:

• Amino acids like methionine and cysteine are involved in the synthesis of glutathione, protecting immune cells from oxidative stress and enhancing their function.

Cellular Energy Supply:

• Glutamine and arginine provide energy for immune cells, supporting their proliferation, differentiation, and function during immune responses.

Barrier Function:

• Threonine is essential for the production of mucin in the gut, which acts as a first line of defence by preventing pathogen entry and promoting gut health.

Immune Signalling:

• Arginine’s role in nitric oxide production is vital for immune signalling and the direct killing of pathogens by immune cells.

Protein Synthesis:

• Amino acids are building blocks for proteins, including those involved in the immune system such as antibodies, cytokines, and complement proteins.

Amino acids and protein metabolism signalling

Amino acids are crucial regulators of protein metabolism and signalling pathways in broilers and layers, impacting growth, development, and productivity. Here’s a detailed look at how amino acids influence protein metabolism and the key signalling pathways involved:

Key Amino Acids and Their Roles in Protein Metabolism

Leucine:

• mTOR Pathway Activation: Leucine is a major activator of the mechanistic target of rapamycin (mTOR) pathway, which regulates protein synthesis and muscle growth.
• Protein Synthesis: Stimulates muscle protein synthesis by promoting ribosome assembly and translation initiation.

Arginine:

• Nitric Oxide Production: Enhances blood flow to tissues, improving nutrient delivery and waste removal, which is crucial for muscle growth and repair.
• Creatine Synthesis: Contributes to creatine production, which supports energy metabolism in muscle cells.

Glutamine:

• Nitrogen Donor: Acts as a nitrogen donor for the synthesis of other amino acids and nucleotides.
• Protein Turnover: Supports protein synthesis and turnover, maintaining muscle and tissue health.

Methionine:

• Methylation Reactions: Serves as a precursor for S-adenosylmethionine (SAM), which is involved in methylation reactions critical for gene expression and protein function.
• Antioxidant Production: Supports the synthesis of glutathione, protecting cells from oxidative stress.

Lysine:

• Collagen Synthesis: Essential for collagen production, important for tissue repair and growth.
• Growth Promotion: Plays a key role in overall protein synthesis, particularly in muscle development.

Key Signalling Pathways

mTOR Pathway:

• Leucine and mTOR Activation: Leucine directly activates the mTOR pathway, which is central to regulating cell growth, protein synthesis, and muscle hypertrophy.
• Protein Synthesis Regulation: mTOR signalling enhances ribosomal biogenesis and the translation of mRNA into proteins, driving muscle growth and repair.

AMPK Pathway:

• Energy Balance: The AMP-activated protein kinase (AMPK) pathway is involved in maintaining cellular energy balance. Amino acids influence AMPK activity, which in turn regulates protein metabolism.
• Catabolism and Anabolism: AMPK activation can inhibit mTOR signalling during energy stress, balancing catabolic and anabolic processes.

Insulin/IGF-1 Signalling:

• Nutrient Sensing: Insulin and insulin-like growth factor 1 (IGF-1) signalling pathways are influenced by amino acid availability, particularly leucine and arginine.
• Growth and Metabolism: These pathways regulate growth, protein synthesis, and overall metabolism, promoting muscle growth and development in response to nutrient intake.

Amino Acids and Meat Quality

Amino acids significantly influence the quality of meat in broilers and layers, affecting attributes such as tenderness, juiciness, flavor, and overall nutritional value. Here’s an overview of how amino acids impact meat quality:

Key Amino Acids and Their Roles

Lysine:

• Muscle Growth: Essential for muscle protein synthesis, leading to increased muscle mass and improved meat yield.
• Collagen Production: Contributes to collagen synthesis, affecting meat texture and tenderness.

Methionine:

• Antioxidant Properties: Precursor to glutathione, an antioxidant that reduces oxidative stress, improving meat quality and shelf life.
• Fat Metabolism: Involved in lipid metabolism, influencing fat deposition and meat flavor.

Arginine:

• Nitric Oxide Production: Enhances blood flow and nutrient delivery to muscles, supporting muscle growth and development.
• Creatine Synthesis: Aids in creatine production, which is important for muscle energy and overall meat quality.

Glutamine:

• Muscle Recovery: Supports muscle repair and recovery, contributing to better muscle development and meat quality.
• Immune Function: Enhances immune function, reducing the incidence of disease and improving overall meat quality.

Threonine:

• Protein Synthesis: Essential for the synthesis of structural proteins, influencing muscle structure and meat texture.
• Mucin Production: Important for gut health, indirectly supporting nutrient absorption and meat quality.

Effects on Meat Quality

Tenderness:

• Adequate amino acid intake promotes muscle protein synthesis and proper muscle fibre development, resulting in more tender meat.
• Lysine and threonine are particularly important for the synthesis of collagen and elastin, proteins that influence meat texture.

Juiciness:

• Amino acids like arginine and methionine influence fat metabolism and deposition, which can affect meat juiciness.
• Proper hydration and muscle cell integrity, supported by amino acids, contribute to meat juiciness.

Flavors:

• Methionine and cysteine are sulfur-containing amino acids that can influence the flavor profile of meat through their role in fat metabolism and antioxidant properties.
• Arginine’s role in nitric oxide production and blood flow can enhance the delivery of nutrients that affect meat flavor.

Colour and Appearance:

• Amino acids impact muscle pH and oxidative stability, affecting meat color and appearance.
• Methionine’s antioxidant properties help maintain meat color by reducing oxidative stress and preventing discoloration.

Nutritional Value:

• Adequate amino acid nutrition ensures a higher protein content and better amino acid profile in the meat, enhancing its nutritional value.
• Balanced amino acid intake supports overall health and growth, leading to better meat quality.

Amino acids as anti-oxidants

Amino acids play a crucial role as antioxidants in broilers and layers, helping to protect cells from oxidative damage, improving overall health, and enhancing productivity. Here’s how specific amino acids function as antioxidants and their benefits for poultry:

Key Antioxidant Amino Acids and Their Roles

Methionine:

• Glutathione Precursor: Methionine is a sulfur-containing amino acid and a precursor for cysteine, which is essential for the synthesis of glutathione, a major intracellular antioxidant.
• Lipid Metabolism: Methionine supports the proper metabolism of fats, reducing lipid peroxidation and oxidative stress.

Cysteine:

• Glutathione Synthesis: Cysteine is a direct precursor of glutathione, a critical antioxidant that protects cells from reactive oxygen species (ROS).
• Detoxification: Helps in detoxifying harmful substances, reducing oxidative damage.

Taurine:

• Antioxidant Activity: Taurine, derived from cysteine, has direct antioxidant properties and helps stabilize cell membranes, protecting them from oxidative damage.
• Immune Support: Enhances the immune response, which can indirectly reduce oxidative stress by improving overall health.

Glutamine:

• Cellular Protection: Glutamine supports the synthesis of glutathione and helps in maintaining cellular redox balance.
• Immune Function: Enhances immune cell function, reducing the oxidative burden during infections and stress.

Arginine:

Nitric Oxide Production: While primarily known for its role in nitric oxide synthesis, arginine also contributes to antioxidant defences by supporting immune function and reducing oxidative stress.

Mechanisms of Antioxidant Action

Glutathione Production:

• Reduction of ROS: Glutathione neutralizes ROS, preventing cellular damage and maintaining the integrity of proteins, lipids, and DNA.
• Redox Balance: Helps maintain the cellular redox state, crucial for normal cell function and survival.

Supporting Enzymatic Antioxidants:

• Superoxide Dismutase (SOD): Amino acids support the activity of SOD, which converts superoxide radicals into less harmful molecules.
• Catalase and Peroxidases: Enhance the activity of catalase and peroxidases that break down hydrogen peroxide, reducing oxidative stress.

Detoxification:

• Conjugation Reactions: Amino acids like cysteine participate in conjugation reactions that detoxify harmful substances, reducing oxidative damage.

Conclusions

• Critical Role in Protein Synthesis: Amino acids, particularly the essential ones like lysine, methionine, and threonine, are indispensable for the synthesis of structural and functional proteins in poultry. Their availability directly influences the rate of muscle accretion and overall growth performance.
• Precision Formulation for Optimal Growth: The precise formulation of poultry diets to meet specific amino acid requirements, based on ideal protein profiles, maximizes growth rates, improves feed conversion ratios, and minimizes excess amino acid wastage, enhancing production efficiency.
• Regulation of Metabolic Pathways: Amino acids serve not only as substrates for protein synthesis but also as regulators of key metabolic pathways. For example, branched-chain amino acids (BCAAs) such as leucine are involved in the activation of the mTOR pathway, which governs cellular growth and protein synthesis.
• Influence on Immune Function and Disease Resistance: Adequate levels of specific amino acids, such as arginine and glutamine, are essential for maintaining a robust immune system in poultry. These amino acids support the production of immune cells and antibodies, enhancing the bird’s resistance to pathogens and reducing the incidence of diseases.
• Minimization of Nitrogen Excretion: Formulating poultry diets based on precise amino acid requirements reduces the excess intake of crude protein, thereby minimizing nitrogen excretion. This not only improves nitrogen utilization efficiency but also mitigates the environmental impact of poultry operations by reducing ammonia emissions and soil contamination.