Abstract
The current study investigated the effects of dietary crude protein (CP) level on the liver lipid metabolism, gut microbiota, and bile acids (BA) profiles of growing pullets. Roman growing pullets (N = 180, 13-wk-old) were divided into 3 treatments groups with 6 replicates in each group and 10 hens in each replicate and provided 3 different dietary CP level diet treatments. The diet treatments included: a high-protein diet (15.5% CP, HP group), a medium-protein diet (14.5% CP, MP group), and a low-protein diet (13.5% CP, LP group). Compared with HP group, LP group significantly increased the lipid contents in the body (such as Breast intramuscular fat [BIMF], Leg intramuscular fat [LIMF], Percentage of abdominal fat [PAF], liver triglyceride [TG] and liver cholesterol [TC]), and the lipid metabolism-related parameters in serum (such as cholesterol (TC), high density lipoprotein cholesterol [HDL-C], low density lipoprotein cholesterol [LDL-C], very low density lipoprotein [VLDL]), and the mRNA expression of lipid metabolism-related genes (such as fatty acid synthase [FAS], CCAAT/enhancer binding protein β [C/EBPβ], and fatty acid translocase [FAT/CD6]) (P < 0.05). In addition, LP group significantly reduced the contents of lithocholic acid (LCA), isoLCA, and ursodesoxycholic acid (UDCA), and increased the deoxycholic acid (DCA) content compared with HP group (P < 0.05). The effects of LCA on lipid deposition were confirmed in chicken preadipocyte cell line (CPI), in which LCA supplementation significantly decreased the relative expression of PPARγ, FAS, acyl-CoA carboxylase (ACC) and SREBP-1c (P < 0.05). Correlation analysis further revealed a significant association between BA profiles and lipid metabolism-related parameters. Furthermore, 16S rRNA gene sequencing indicated that dietary protein level can significantly affect the richness, diversity, and composition of cecal microbiota in growing pullets. LP group significantly increased the abundance of Bacteroidetes and significantly decreased the abundance of Firmicutesa compared with the HP group. In summary, low protein diet in growing pullets influence the liver lipid metabolism through changing the gut microbiota and liver BA metabolism.