The addition of protein-bound amino acids in low-protein diets improves the metabolic and immunological characteristics in fifteen- to thirty-five-kg pigs

This study was to evaluate the effects of supplementation of AA form (crystalline vs. protein bound) in low-protein diets on growth, metabolic, and immunological characteristics of pigs. A total of 80 barrows (PIC 327 x 1050; 15.57 +/- 0.13 kg BW and 48 +/- 2 d of age), housed in 4 pigs per pen with 5 pens per treatment, were assigned to 4 dietary treatments of 17, 15, and 13% CP and 13% CP plus casein for 28 d. The crystalline AA were supplemented to meet the requirement of indispensable AA in pigs. Results showed that pigs fed the 13% CP diet or the 13% CP plus casein diet had lower (P < 0.01) ADG and ADFI and a greater (P < 0.01) feed: gain ratio than pigs fed the 17% CP or 15% CP diets over the 4-wk study period. Compared with other diets, pigs fed the 13% CP diet had decreased concentrations of plasma urea nitrogen, albumin (P < 0.01), and mRNA expressions of Toll-like receptor 4 (TLR4), nuclear factor kappa B (NF-kappa B; P < 0.05), and Toll-interacting protein (TOLLIP; P < 0.01) in the ileum and also increased activity of plasma glutamate-pyruvate transaminase (P < 0.05) and concentrations of IL-1 beta (P < 0.05) and tumor necrosis factor-alpha (P < 0.01); however, these characteristics were partly normalized by feeding the 13% CP plus casein diet. Furthermore, the plasma concentration of insulin-like growth factor 1 (IGF-1; P < 0.01) and mRNA expressions of protein kinase B (Akt), mammalian target of rapamycin (mTOR), and ribosomal protein S6 kinase (RPS6K) in longissimus muscle were increased (P < 0.05) in pigs fed the 13% CP plus casein diet relative to pigs fed the 17% CP or 15% CP diets. In summary, reducing dietary CP level from 17% to 15% had no effect on growth, metabolic, and immunological characteristics of 15- to 35-kg pigs. A further reduction of dietary CP level up to 13% would lead to poor growth performance, but metabolic and immunological characteristics were partly normalized using protein-bound AA to replace synthesized AA in the 13% CP diet.