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.

Zinc (Zn) is an essential trace element that plays important roles in growth, digestion, antioxidant defense, immunity, and inflammation regulation in fish. This study investigated the effects of graded dietary Zn levels on growth performance, serum biochemistry, digestive enzyme activity, zinc transporter expression, antioxidant capacity, immune responses, and inflammatory regulation in juvenile black carp (Mylopharyngodon piceus). Six isonitrogenous and isoenergetic diets were formulated to contain 27.95, 34.38, 44.90, 66.52, 116.14, and 199.56 mg/kg Zn by supplementing ZnSO₄·7H₂O. Juvenile fish with an initial weight of 2.88 ± 0.12 g were fed the experimental diets for 60 days in triplicate tanks. Growth performance increased with dietary Zn and then plateaued at 44.90–199.56 mg/kg; broken-line regression estimated the optimal dietary Zn requirement at 44.6 mg/kg. Adequate Zn supplementation also reduced whole-body lipid content, increased digestive enzyme activities, improved serum HDL-C and ALP levels, and decreased AST and ALT activities. In addition, adequate dietary Zn (44.90 mg/kg) significantly modulated the expression of zinc transporter genes in the liver and intestine. Adequate dietary Zn supplementation enhanced antioxidant capacity by activating the Nrf2/Keap1 signaling pathway, improved intestinal immunity, and strengthened barrier function by increasing the expression of tight junction proteins and mucins. Moreover, adequate dietary Zn could alleviate inflammatory responses by upregulating anti-inflammatory factors and downregulating pro-inflammatory cytokines via the MAPK14 signaling pathway. These findings suggest that dietary zinc at 44.60 mg/kg is sufficient to promote growth, antioxidant status, immune function, and intestinal health in juvenile black carp.