Effects of Dietary Lysine Restriction on the Concentrations of Free Amino Acids and Other Selected Metabolites in the Blood Plasma of Growing Pigs

The growth performance, especially muscle growth of pigs is dependent on the constant supply of balanced AA and other nutrients from the blood circulation. Lysine is typically the first limiting AA in grain-based swine diets. This project was conducted to study how dietary lysine restriction affects plasma concentrations of AA and other metabolites in growing pigs. Twelve individually penned crossbred barrows (Yorkshire × Landrace; initial BW 22.6 ± 2.04 kg) were assigned to two dietary treatments according to a completely randomized design (n = 6). Two corn and soybean meal based diets were formulated to contain 0.65% (lysine-deficient; Diet I) and 0.98% (lysine-adequate; Diet II) standardized ileal digestible lysine. During the 8 weeks feeding trial, pigs had ad libitum access to their respective diets and fresh water. Pigs’ BW and feed intake were measured biweekly, and ADG, ADFI, and G:F were calculated biweekly, as well as for the overall feeding period. Immediately after the feeding trial, jugular vein blood was collected for plasma sample preparation. The plasma AA contents were analyzed with the established HPLC method and plasma contents of selected metabolites were determined using Alfa Wassermann’s ACE Clinical Chemistry System. Data were statistically analyzed using Student’s T-test. Overall, the final BW and ADG in pigs fed Diet I were lower (P < 0.01) than in pigs fed Diet II. However, no difference in ADFI was found between the two groups. Thus, the G:F of Diet I pigs was lower than that of Diet II pigs (P = 0.04). Although the plasma concentration of β-alanine in Diet I pigs was higher (P < 0.01) than in Diet II pigs, the plasma concentrations of lysine, methionine, leucine, arginine, tyrosine, and total protein in Diet I pigs was lower (P < 0.05) than in Diet II pigs. The plasma concentrations of all other AA and metabolites (including urea nitrogen, albumin, glucose, and cholesterol) were not different (P> 0.10) between the two groups. Based on these results and nutrition knowledge, dietary lysine restriction is responsible for the reduced plasma lysine level and growth performance of Diet I pigs. Reduced levels of plasma methionine, leucine, arginine, tyrosine, and total protein indicate that pig’s consuming a lysine-deficient diet may utilize these metabolites for other metabolic functions. In conclusion, dietary lysine restriction can affect the plasma profile of free AA and other metabolites in addition to free lysine in growing pigs.