Effects of Dietary Steroid Saponins on Growth Performance, Serum and Liver Glucose, Lipid Metabolism and Immune Molecules of Hybrid Groupers (♀Epinephelus fuscoguttatus × ♂Epinephelus lanceolatu) Fed High-Lipid Diets

High-lipid diets are attributed to excessive lipid deposition and metabolic disturbances in fish. The aim of this experiment was to investigate the effects of steroidal saponins on growth performance, immune molecules and metabolism of glucose and lipids in hybrid groupers (initial weight 22.71 ± 0.12 g) fed high-lipid diets. steroidal saponins (0%, 0.1% and 0.2%) were added to the basal diet (crude lipid, 14%) to produce three experimental diets, designated S₀, S_0.1 and S_0.2, respectively. After an 8-week feeding trial, no significant differences were found between the S₀ and S_0.1 groups in percent weight gain, specific growth rate, feed conversion ratio, protein efficiency ratio and protein deposition rate (p > 0.05). All those in the S_0.2 group were significantly decreased (p < 0.05). Compared to the S₀ group, fish in the S_0.1 group had lower contents of serum triglyceride and low-density lipoprotein cholesterol and higher high-density lipoprotein cholesterol and glucose (p < 0.05). The activities of superoxide dismutase, catalase and glutathione peroxidase were significantly higher, and malondialdehyde contents were significantly lower in the S_0.1 group than in the S₀ group (p < 0.05). Hepatic triglyceride, total cholesterol and glycogen were significantly lower in the S_0.1 group than in the S₀ group (p < 0.05). Activities of lipoprotein lipase, total lipase, glucokinase and pyruvate kinase, and gene expression of lipoprotein lipase, triglyceride lipase and glucokinase, were significantly higher in the S_0.1 group than in the S₀ group. Interleukin-10 mRNA expression in the S_0.1 group was significantly higher than that in the S₀ group, while the expression of interleukin-6 and tumor necrosis factor-α genes were significantly lower than those in the S₀ group. In summary, adding 0.1% steroidal saponins to a high-lipid diet not only promoted lipolysis in fish livers, but also activated glycolysis pathways, thus enhancing the utilization of the dietary energy of the groupers, as well as supporting the fish’s nonspecial immune-defense mechanism.