Effects of dietary lysine on muscle gene expression and fatty acid profiles and on selected carcass characteristics and plasma hormone concentrations in late-stage finishing pigs

Dietary inclusion of sufficient lysine is very critical for optimizing pig's growth performance. The objectives of this project were to study the effects of dietary lysine at different concentrations on (1) the growth performance and carcass characteristics, (2) the muscle gene expression profile and the possible alterations to the metabolic and signaling pathways, (3) the muscle fatty acid profile, and (4) the plasma concentrations of growth-related hormones of late-stage finishing pigs. Nine crossbred barrows were assigned to 3 dietary treatments (lysine-deficient, -adequate, and -excess diets) according to a completely randomized experimental design. During the 5-week feeding trial, pigs were allowed ad libitum access to experimental diets and water. All pigs and experimental diets were weighed individually each week during feeding trial to determine growth performance. After harvest, the carcass characteristics were determined and muscle samples were collected from longissimus dorsi for mRNA and fatty acid profiling, while the jugular vein blood was collected at the end of four weeks for analyses of three growth-related hormones. While the average daily gain showed a quadratic relationship, the dressing percentage and total lean cut weight both increased linearly with dietary lysine concentrations. Results of muscle gene expression data showed that dietary lysine deficiency may lead to decreased protein synthesis, increased protein degradation and lipid accumulation, while dietary lysine excess may lead to decreased protein degradation and increased lipid biosynthesis. Fatty acid (FA) composition data showed that different dietary lysine concentrations altered the intramuscular fat (IMF) content and FA composition, especially the unsaturated FAs. In particular, dietary lysine deficiency increased the IMF content and the proportion of mono-unsaturated FAs. Hormone analyses showed that the plasma concentrations of insulin and growth hormone were not affected by dietary lysine, whereas the concentration of insulin-like growth factor 1 was decreased by either dietary lysine deficiency or excess. Collectively, lysine may function as a signaling molecule to regulate the expression of genes related to protein turnover and lipid metabolism in the muscle of finishing pigs, causing differences in growth performance, carcass characteristics, and FA composition. IGF-1 may be a controlling growth factor that is sensitive to dietary lysine.