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.