The effects of dietary leucine on the growth performances, body composition, metabolic abilities and innate immune responses in black carp Mylopharyngodon piceus

The present study was focused on the growth, body composition, metabolic abilities and innate immune responses in juvenile black carp Mylopharyngodon piceus fed with six levels of dietary leucine (7.3, 12.4, 16.2, 21.9, 28.3 and 34.5 g kg−1) for 9 weeks. Results showed that the highest weight gain (WG) and the lowest feed conversion ratio (FCR) was obtained at 23.5 and 23.9 g kg−1 dietary leucine using second-order polynomial model, respectively. Adequate dietary leucine content (21.9 and 28.3 g kg−1) could significantly up-regulate the expression levels of neuropeptide Y (NPY) and ghrelin (GRL) in the brain of black carp juveniles. The protein efficiency ratio (PER), feed efficiency ratio (FER) and protein deposition ratio (PDR) were also significantly increased by adequate dietary leucine content (21.9 and 28.3 g kg−1) (p < 0.05). Adequate dietary leucine content (21.9 and 28.3 g kg−1) could significantly up-regulate the activities of metabolic enzymes, such as α amylase, trypsin, chymotrypsin and elastase in the liver of Black carp (p < 0.05). However, the activities of alanine transaminase (ALT), aspartate aminotransferase (AST) and leucine aminopeptidase (LAP) were significantly reduced in the fish serum by adequate dietary leucine content (21.9 and 28.3 g kg−1) compared with leucine-deficient diet (7.3 and 12.4 g kg−1). In addition, 21.9 and 28.3 g kg−1 dietary leucine could significantly increase complement component 3 (C3) and C4 contents, lysozyme (LYZ) activities in the serum compared with the leucine-deficient diet (7.3 and 12.4 g kg−1) (p < 0.05). Furthermore, optimal dietary leucine could also significantly up-regulate the mRNA expression levels of LYZ, interferon α (IFN-α), hepcidin (HEPC), natural resistance-associated macrophage protein (NRAMP), C3 and C9 in the blood of juvenile black carp compared with the leucine-deficient diets (7.3 and 12.4 g kg−1) (p < 0.05). In conclusion, these results suggest that adequate dietary leucine (21.9 and 28.3 g kg−1) could increase growth performances, improve metabolic abilities and then enhance non-specific immunities in black carp juveniles.