Effects of DL- Versus L-Methionine on the Concentrations of Plasma Free Amino Acids, Muscle Gene Expression, and Growth Performance of Young Growing Pigs

Muscle growth requires a constant supply of various AAs from the blood and, therefore, the blood plasma AA profile is a critical parameter to be managed to maximize the lean gain of pigs. Methionine, the second or third limiting AA in typical grain-based swine diets, play important roles in promoting animal health and growth, especially the muscle growth. Currently available on the market are crystalline DL-Met and L-Met products. This research was conducted to study if DL-Met and L-Met would differently affect plasma AA concentrations, muscle gene expression, and growth performance of growing pig. Twenty crossbred young barrows (Yorkshire × Landrace; initial BW 21.2 ± 2.66 kg) were randomly assigned to 20 individual pens, and further assigned to 2 dietary treatments according to a completely randomized design with pigs serving as experiment units (n = 10). Both Diets 1 and 2 were corn-soybean meal based diets formulated to meet or exceed the recommended requirements for energy, AA, and other nutrients (NRC, 2012; AminoDat^® 5.0). Crystalline L-Met and DL-Met were supplemented (both at 0.13%, as-fed basis) to Diets 1 and 2, respectively. After 4 weeks of the ad libitum feeding trial, the BW and feed intake were measured to calculate ADG, ADFI, and G:F, the blood samples were collected from the jugular vein for analysis of plasma free AA concentrations using the established HPLC methods, and the longissimus dorsi muscle samples were collected with biopsy technique for myogenesis gene expression analysis using qPCR method. Data were statistically analyzed using Student’s T-test. There was no difference (P = 0.56 to 0.94) in ADG, ADFI, and G:F between the pigs fed the two different diets, and no differences (P = 0.20 to 0.99) were found either in all the plasma free AA concentrations between the two dietary treatments. There was no difference (P = 0.18 to 0.94) in the mRNA expression of 8 myogenesis related genes which were myogenic differentiation 1 (MYOD1), myogenic factors 4, 5, 6 (MYOG, MYF5, MYF6), and myocyte enhancer factors 2A, 2B, 2C, and 2D transcript variant X1 (MEF2A, MEF2B, MEF2C, MEF2D). From biochemistry standpoint, only L-Met can be used by pigs for protein synthesis and muscle growth. The findings from this study indicated that the enzymatic conversion of D-Met (in the DL-Met product) to L-Met by pigs is highly efficient and there is no difference in the nutritional efficacy between the DL-Met and L-Met products tested.