Effect of supplemental dietary leucine and immune system stimulation on whole-body nitrogen utilization in starter pigs.

The increase in circulating pro-inflammatory cytokines following an immune challenge (e.g., with bacterial lipopolysaccharide [LPS]) causes a disruption in normal AA metabolism and increases visceral protein synthesis at the expense of muscle protein synthesis. The objective of this study was to determine the effect of supplemental dietary Leu on the dynamics of whole body nitrogen (N) retention in starter pigs before and after immune system stimulation (ISS) induced by LPS. A total of 28 starter pigs (14.46 ± 0.73 kg BW) were assigned to isoenergetic and isonitrogenous diets formulated to supply essential AA 10% above estimated requirements for maximum whole-body protein deposition (PD) and to contain increasing amounts of Leu: CON (1.36% SID Leu); LEU-M (2.04% SID Leu); and LEU-H (2.72% SID Leu). Pigs were housed in metabolic crates, scale-fed every 4 h based on BW, and adjusted to dietary treatments for 5 d. The 108-h N-balance experiment was divided into two periods: prechallenge (before LPS challenge; six 12-h collections) and challenge (after LPS challenge; three 12-h collections) periods. In both periods, blood was collected to determine plasma AA and urea N concentrations. At the start of the challenge period, one-half of the pigs fed CON and all pigs fed LEU-M and LEU-H were challenged with LPS (ISS+; 30 µg/kg injected intramuscularly); the remaining pigs fed CON were injected with saline (ISS-). Whole-body N retention was determined during subsequent 12-h collections. Plasma free Leu concentration increased linearly with increasing Leu content in the diet before LPS was administered (CON, 124 µmol/L; LEU-M, 185 µmol/L; LEU-H, 227 µmol/L; < 0.01). During the prechallenge period, N retention was lower in pigs fed LEU-M ( < 0.01) and there was no difference between pigs fed CON and LEU-H (7.91, 7.18, and 7.71 g/12 h for CON, LEU-M, and LEU-H, respectively). During the challenge period, N retention in pigs fed CON was higher in ISS- than ISS+ (5.37 vs. 3.83 g/12 h; < 0.01) but was not affected by diet in ISS+ pigs (3.83, 3.21, and 3.45 g/12 h for CON, LEU-M, and LEU-H, respectively; > 0.10). In healthy pigs, feeding a high excess of dietary Leu induced an anabolic response to compensate for reduced N retention that occurred in pigs fed an intermediate excess of dietary Leu. There was no effect of supplemental Leu on N retention in pigs after an LPS challenge.