Inflammatory responses in broilers are detrimental to the goals of the broiler industry as inflammation results in worsened growth performance by reducing feed intake and muscle protein accretion while increasing the rate of the metabolism, synthesis of immune related proteins, and liver mass (Klasing, 2007). This results in a shift of broiler energetics and metabolism from muscle growth to immune response development, potentially requiring muscle protein degradation to provide amino acids for the immune system. The limiting amino acids for immune system protein synthesis may be different than the limiting amino acids for muscle protein synthesis (Le Floc’h, 2004). Broilers diets are formulated to provide the optimal nutrients to promote muscle growth, and when under an immune challenge, these diets may not be optimally formulated for synthesizing immune proteins.
Also, during an immune response, appetite can be suppressed, leading to a reduction in the intake of amino acids from the feed, and again, these may not be the optimal amino acids for immune proteins. Thus, a catabolic state is initiated in an effort to supply the necessary amino acids for immune function. As skeletal muscle represents the largest pool of expendable amino acids, it can be degraded to provide a source of amino acids for the immune system (Sirimongkolkasem, 2007). Since the amino acid content of skeletal muscle and immune proteins may contain different relative amounts of specific amino acids, it may be necessary that more muscle is degraded to free amino acids that are less prevalent in skeletal muscle but needed in high amounts for the immune system. As such, excess mobilization of skeletal muscle amino acids may be needed to support the immune system at times of challenge. Although an immune response decreases broiler growth performance, proper function of the immune system is needed to combat pathogen invasions, which if allowed to persist could lead to decreased growth performance or even death. Therefore, it is important that any potential immune response is prevented or in the inevitable event of disease, the response must be monitored in order to minimize the length of the response and prevent excessive reductions in growth performance.
As AGP are removed from broiler production strategies, it will become increasingly important to better understand the broiler GI tract in terms of epithelial cell renewal and immune response. The GI tract is rapidly renewed by the intestinal stem cells, and an immune response can shift the metabolic and energetic processes of an animal. Without AGP, more stress is placed on the immune system within the broiler GI tract. GI mitotic activity and immune response represent two normal processes that can place a substantial drain on amino acid and energy partitioning in a rapidly growing broiler. Further research is necessary to better understand these functions, their importance, underlying mechanisms, relationship to growth performance in the absence of AGP, and appropriate AGP alternatives in order to maintain and improve the efficiency and sustainability of broiler production.