Impact of immune system stimulation on metabolism and requirements for sulfur amino acids in growing pigs

Immune system stimulation (IS S) and stress conditions modify protein and amino acid (AA) metabolism in animals and humans. During ISS, AA are redirected from growth and reproduction towards mounting an immune response, which impacts AA requirements both qualitatively and quantitatively. Special attention has been given to sulfur amino acid (SAA; methionine plus cysteine) metabolism during ISS due to their role as precursors for the synthesis of proteins and metabolites (e.g. glutathione; GSH) that are involved in the immune response. Research objectives were to determine the impact of ISS on ileal digestibility of nutrients and various aspects of SAA metabolism including plasma cysteine (CYS) flux, GSH synthesis rate, irreversible loss of CYS, and dietary SAA requirements. In all of the studies, pigs were feed restricted to eliminate the confounding effect of ISS on feed intake, and diets were formulated to be first limiting in SAA and up to five levels of SAA were fed.;The first experiment was conducted to establish a pig model of ISS, representing moderate chronic disease. Impact of three disease causing agents, turpentine, Escherichia coli lipopolysaccharide (LPS) and mycotoxins (a mixture of deoxynivalenol, fusaric acid, zearalenone and acetyldeoxynivalenol) were evaluated. Among these agents, repeated injection with increasing amounts of LPS altered the measures of immune function without profoundly compromising pig performance (e.g. feed intake), and thus it was considered an appropriate model for studying nutrient metabolism during ISS in growing pigs.;Moderate ISS did not alter ileal digestibility of energy, crude protein and vital amino acids, indicating that the impact of ISS on nutrient utilization efficiency is attributed largely to post-absorptive metabolic processes.;In an isotope tracer study it was shown that ISS increases plasma CYS flux, as well as fractional (%/d) and absolute (mmol/kg tissue/d) synthesis rate of GSH in the liver and several other tissues. In this study, it was also demonstrated that ISS modifies irreversible loss of CYS, by reducing production of sulfate while increasing production of taurine (TAU).;Based on whole body nitrogen and sulfur mass balance, it was observed that ISS alters the partitioning of SAA in favor of non-protein nitrogenous compounds such as GSH and TAU. In addition, it was shown that ISS enhances urinary N excretion to a greater extent than S excretion. Based on the relationship between digestible SAA intake and whole-body protein deposition (PD), ISS does not influence partial efficiency of ileal digestible SAA utilization for whole-body protein deposition but it substantially increases the maintenance SAA requirements. It was also demonstrated that ISS reduces dietary SAA requirements for maximum protein deposition.