Comprehensive Investigation on the Influence of Dietary Soybean Oil Levels in a Soybean Oil-Based Diet on Gibel Carp (Carassius gibelio): Insights Regarding Growth, Nutrition, and Physiological, Metabolic, and Transcriptomic Analyses

Five isonitrogenous (35%) diets with different lipid contents were prepared by adding 0%, 3%, 6%, 9%, and 12% soybean oil (SO) as the single lipid source (with groups designated SO0, SO3, SO6, SO9, and SO12, respectively), followed by a 12-week feeding experiment in juvenile gibel carp (Carassius gibelio) (initial weight: 11.76 ± 0.09 g). The final weight (FW), weight gain rate (WGR), and specific growth rate (SGR) were significantly higher in gibel carp fed diets containing 6%–12% SO than those of others (p  < 0.05), while the feed conversion ratio (FCR) was significantly reduced in the SO6 group (p  < 0.05). Muscle crude fat and polyunsaturated fatty acid (PUFA) levels significantly increased as the dietary SO content increased (p < 0.05). Meanwhile, muscle crude protein was present in notably higher amounts in fish receiving 6%–9% SO (p < 0.05). Serum low-density lipoprotein (LDL), triglyceride (TG), and total cholesterol (T-CHO) continuously increased with increasing SO levels (p < 0.05). Histopathological abnormalities were observed in the local regions of the hepatopancreas after supplementation with 9%–12% SO. Most genes involved in lipid metabolism were upregulated to varying degrees (p < 0.05) as dietary SO content increased, especially lipogenic genes. About 4481 differentially expressed genes (DEGs) were screened in the hepatopancreas between the SO12 and SO0 group, primarily linking to metabolism-related pathways, including steroid biosynthesis. Conclusively, the available content of SO as the sole fat source in diets for juvenile gibel carp was found to be 6%–9%, within which the optimal SO inclusion was 6.37% according to FCR–based regression analysis. Excessive supplementation of dietary SO (12% or beyond) could expedite hepatic lipid metabolism, particularly lipogenesis, by stimulating steroidogenesis, thereby evoking fat over-accumulation, lipid metabolism disorders, and other adverse effects in gibel carp. These results provide synthetic data for better understanding the advantages, disadvantages, and relevant mechanisms of SO–based feed for gibel carp and for optimizing SO–based aquafeeds in farmed fish.