Effects Of Low-protein Diets Supplemented With Crystalline Amino Acids Or Intact Protein On Growth Performance, Nitrogen Metabolism And Immune Function In Pigs

A moderate reduction of dietary CP level, with the supplementation of crystalline AA, has been recognized as an effective strategy to ease the increasing shortage of protein source, reduce N excretion, and improve the efficiency of N utilization, while maintaining normal growth performance as pigs fed high CP diet. However, a deep reduction of dietary CP level would affect pig growth performance, which may be due to a deficiency of DAA or intact protein and-related release of peptides. Meanwhile, there is little information whether there are alterations on immunological traits of pigs receiving extremely low-CP diet supplemented with AA. Thus, the objective of the current study were conducted to evaluate the effects of low-protein diets supplemented with crystalline amino acids or intact protein, to maintain appropriate amounts of IAA, DAA and intact protein, on growth performance, nitrogen metabolism and immune function in pigs.Experiment 1:Effects of low-protein diets supplemented with indispensable amino acids on growth performance, nitrogen balance and immunological parameters in 13-to 35-kg pigs. The objective of this study was to determine if a moderate or high reduction of dietary CP, supplemented with indispensable amino acids (IAA), would affect growth, nitrogen balance and immunological parameters of pigs. A total of 40 barrows (initial BW=13.50±0.50 kg,45±2 day of age) were used in a completely randomized block design, and allocated to 4 dietary treatments containing CP levels at 20.0,17.2,15.3 and 13.9%, respectively. Crystalline AA were added to meet the IAA requirements of pigs. On day 22, a N balance trial was carried out using 6 pigs from each group for 5 days. After 4-week feeding, blood and tissue samples were obtained from pigs. The results showed as follows:(1) Reducing dietary CP level from 20.0 to 13.9% resulted in a significant decrease in average daily gain (ADG), plasma urea nitrogen (PUN) concentration and relative organ weights of liver and pancreas (P< 0.01). and increased feed gain ration (F:G) (P<0.01). Pigs fed the 13.9% CP diet had significantly lower ADG and higher F:G than that of pigs fed higher CP at 20.0.17.2 or 15.3%.(2) Reducing dietary CP level in different degree. decreased fecal, urinary, and total N excretion (P< 0.01). villous height in duodenum (P< 0.01) and crypt depth in duodenum, jejunum and ileum (P< 0.01). The reduction in the dietary CP level increased plasma concentrations of methionine, alanine (P< 0.01) and lysine (P< 0.05). and decreased plasma concentration of arginine(P<0.05).(3) Reducing dietary CP level from 20.0 to 13.9% resulted in a significant decrease in plasma concentration of IgG (P< 0.05). percentage of CD3+T cells of the peripheral blood (P< 0.01). also down-regulated the mRNA abundance of innate immunity-related genes on toll-like receptor 4 (TLR4). myeloid differentiation factor 88 (MyD88) (P< 0.01) and nuclear factor kappa B (NF-κB) (P< 0.05) in the ileum. However, reducing dietary CP level from 20.0 to 15.3% only resulted in a significant decrease in percentage of CD3+T cells of the peripheral blood (P< 0.05).These results indicate that reducing dietary CP level from 20.0 to 15.3%. supplemented with IAA, had no significant effect on growth performance, reduced N excretion and had a limited effect on immunological parameters. However, a further reduction of dietary CP level up to 13.9% would lead to poor growth performance and associated with the modifications of immune function.Experiment 2:Effects of low-protein diets supplemented with intact protein on growth performance, nitrogen metabolism and immune status in 15-to 35-kg pigs.Based on Experiment 1. to further study whether the poor growth performance and altered immunological parameters in low-CP diets was due to deficiencies in dispensable AA (DAA) or intact protein. Thus, this study was conducted to evaluate the effects of low-protein diets supplemented with DAA or intact protein on growth performance, nitrogen metabolism and immunological parameters of pigs. A total of 80 barrows (PIC breeding, initial BW 15.57±0.13 kg.48±2 day of age) were used to receive 4 dietary treatments.5 pens per treatment and 4 pigs per pen. Treatments were:1) 16.7% CP:2) 14.7% CP:3) 13.2% CP; 4) 12.9% CP plus Casein. All diets were supplemented with IAA to provide suffieient IAA according to NRC (2012). diets 3 and 4 were fortified with dispensable amino acids (DAA) to provide a similar level of SID DAA in diet 2, and the casein in diet 4 was used as a source of protein bound AA to partly replace free AA in diet 3. After 4-week feeding, blood and tissue samples were obtained from pigs. The results showed as follows:(1) Pigs fed 13.2% CP and 12.9% CP plus Casein diet had lower (P< 0.01) ADG, ADFI and higher (P< 0.01) F:G,relative weights of kidney compared with those fed 16.7% CP diet. However, pigs fed 14.7% CP diet showed similar growth performance compared with those fed 16.7% CP diet.(2) Pigs fed 13.2% CP and 12.9% CP plus Casein diet had lower plasma concentrations of ALB, SUN, Val, Leu (P< 0.01), lower activitives of GPT and GOT in liver (P< 0.05). higher plasma concentrations of Lys and Thr (P< 0.05), compared with those fed 16.7% CP diet. Pigs fed 13.2% CP diet had higher plasma concentration of GPT (P< 0.05) and lower plasma concentrations of Ile, Arg (P< 0.05) compared with pigs in other groups.(3) Pigs fed 13.2% CP and 12.9% CP plus Casein diet had higher plasma concentrations of 1GF-1 (P< 0.01) compared with pigs fed 16.7% CP and 14.7% CP diet. The mRNA abundances of SLC7A7 and SLC15A1 were significantly decreased (P< 0.01) in the jejunum of pigs fed 13.2% CP diet compared with pigs fed 16.7% CP diet. The mRNA abundances of SLC7A7 and SLC6A19 (P< 0.05) were significantly decreased in the jejunum of pigs fed 12.9% CPCasein diet compared with pigs fed 16.7% CP diet. Pigs fed 12.9% CP plus Casein diet had a higher mRNA abundance of Akt (P< 0.05) compared with pigs fed 14.7% CP and 13.2% CP diet, and a higher mRNA abundances of mTOR and RPS6K(P< 0.05) compared with pigs in other 3 diets.(4) The percentage of CD3+T cells of the peripheral blood was decreased (P< 0.05) in pigs fed 13.2% CP diet compared with pigs fed 12.9% CP plus Casein diet. Pigs fed 13.2% CP diet had higher (P< 0.01) plasma concentrations of Il-1β and TNF-a compared with pigs fed the other 3 diets. The mRNA abundances of innate immunity-related genes on TLR4. NF-κB and Toll-interacting protein (TOLLIP) in the ileum were decreased (P< 0.05) in pigs fed 13.2% CP diet compared with those fed 16.7% CP diet.In conclusion, a moderate reduction of dietary CP to 14.7%, supplemented with IAA, had no significant effect on growth performance, and a limited effect on nitrogen metabolism and immunological parameters. A high reduction of dietary CP up to 13.2%, even though the low-CP diet had been supplemented with both IAA and DAA. would lead to poor growth performance, nitrogen metabolism and immunological parameters. However, using intact protein to partly replace free AA in extremely low-CP diet had shown better nitrogen metabolism and immune function. This observation provides evidence that a certain amount of intact protein is required to maintain health of pigs fed low-CP diet.