Effects of Fermented Mulberry Leaf Powder on Growth Performance, Meat Quality, Antioxidant Capacity, and Fat Metabolism in Grass Carp (Ctenopharyngodon idellus)

The use of plant-based fermentation products as substitutes for costly feed ingredients like fishmeal and soybean meal has been at the forefront of aquaculture research. This study investigated the effects of dietary fermented mulberry leaf powder (FMLP) on grass carp (0.86 ± 0.07 kg). Fish were fed a basal diet supplemented with 0%, 5%, 10%, or 15% FMLP for 70 days to assess impacts on growth, muscle quality, antioxidant status, and lipid metabolism. Results indicated that FMLP did not significantly affect growth performance (p > 0.05). However, it reduced the hepatosomatic index, viscerosomatic index, condition factor, and intraperitoneal fat ratio (p < 0.05). While muscle basic nutrition was unchanged, FMLP improved post-mortem muscle water-holding capacity, drip loss, cooking yield, and texture (p < 0.05). FMLP lowered malondialdehyde and elevated glutathione peroxidase (GSH-Px) and superoxide dismutase (SOD) activities in grass carp (p < 0.05). Concurrently, the gene expression of GSH-Px, Nrf2, and SOD was upregulated in the liver (p < 0.05). Regarding lipid metabolism, FMLP increased serum high-density lipoprotein cholesterol and decreased hepatic total cholesterol and triacylglycerol (p < 0.05). It promoted gene expression related to fatty acid oxidation (PPARα, CPT-1a, CPT-1b) while inhibiting the genes involved in lipogenesis (SREBP, FAS). Quadratic regression analysis determined that an inclusion level of approximately 7.55% to 11.40% (with the data suggesting an optimal range near 8–10%) improves muscle quality, enhances antioxidant defenses, and promotes healthier lipid metabolism in grass carp.