Effects of dietary leucine supplementation on whole-body nitrogen utilization and tissue protein synthesis in pigs

Abstract

Pigs raised under commercial conditions are routinely exposed to pathogens that cause immune system stimulation (ISS). During ISS, a diet can be provided to pigs that blunts the negative impact of ISS on whole-body amino acid (AA) metabolism and body protein deposition (PD). Dietary Leu supplementation may be beneficial due to its regulatory effects on skeletal muscle protein synthesis and degradation, but its anabolic properties may be tempered by ISS. The research objectives of this thesis were to determine the effects of increasing the SID Leu: Lys ratio and ISS on whole-body nitrogen (N) utilization, whole-body protein turnover, and tissue protein synthesis in starter pigs. Whole-body N utilization and protein turnover were evaluated with an N-balance and continuous infusion of 15N-Gly. Protein deposition in pigs before ISS was affected by Leu in a dose-dependent manner: increasing the standardized ileal digestible (SID) Leu: Lys ratio (% Lys) to 150 reduced PD, which appeared to be reversed by further increasing the SID Leu: Lys ratio to 200. Moreover, a linear decrease in whole-body protein synthesis and degradation was observed with increasing SID Leu: Lys before ISS. Although whole-body protein synthesis and PD were reduced during ISS, there was no effect of Leu on whole-body protein synthesis, degradation, or PD. Given the high energetic cost of protein synthesis, and the relationship between protein turnover and a minimum level of AA catabolism, it can be implied that dietary Leu supplementation improves the efficiency of using energy for PD in healthy pigs. Liver and skeletal muscle protein syntheses were determined with a flooding dose of L-[ring-2H5]-Phe. The ISS-induced increase in liver protein synthesis was reversed with increasing SID Leu: Lys, whereas skeletal muscle protein synthesis was increased with increasing SID Leu: Lys. The Leu-induced alterations of liver protein and skeletal muscle protein synthesis may facilitate AA utilization in skeletal muscle for PD. Since dietary Leu supplementation did not affect PD during ISS, the optimum SID Leu: Lys ratio to minimize visceral PD and maximize skeletal muscle PD should be considered in order to optimize AA nutrition for diseased pigs.