Effects of Dietary Protein Levels on Growth, Serum Physiology, Protein and Lipid Metabolism, and Antioxidant Responses in Black Carp (Mylopharyngodon piceus)

Background: Dietary protein optimization is an important nutritional strategy for improving growth and physiological responses, and antioxidant homeostasis in fish. Methods: In this study, 540 black carp (initial body weight: 10.50 ± 1.00 g) were randomly assigned into recirculating tanks (500 L) fed with six dietary protein levels (30–44% crude protein) for an 8-week feeding trial with triplicates per treatment and 30 fish per replicate. After the trial, fish body, blood, hepatopancreas, and intestinal samples were collected for body composition, serum biochemical parameters, metabolism, and antioxidant indices’ analyses. Results: Results showed fish fed 38% protein (PT38) exhibited the highest weight gain (p < 0.05), with no further improvement at higher protein levels. Compared with PT30 group, PT38 group significantly promoted protein deposition by upregulating transcript levels of insulin-like growth factors (IGFs) via activating mechanistic target of rapamycin (mTOR) signaling pathway. PT38 could improve fatty acid oxidation by heightening levels of carnitine palmitoyl transferase 1α (CPT1α), peroxisome proliferator-activated receptor α (PPARα) and PPARδ. Meanwhile, PT38-PT41 significantly inhibit expression of fatty acid synthesis and lipid droplet deposition-related genes, including acetyl-CoA carboxylase (ACC), fatty acid synthase (FAS), and perilipin 2 (p < 0.05). PT38 significantly enhanced antioxidant homeostasis by increasing levels of superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx) via activating nuclear factor erythroid 2-related factor 2 (Nrf2) signaling pathway. Conclusions: Overall, Under the current experimental conditions, 38% dietary protein is suitable for promoting growth performance, improving protein and lipid metabolism, and enhancing antioxidant homeostasis in black carp.