Dietary fat powder replacement of soybean oil modulates muscle fatty acids, liver morphology and oxidation, and intestinal lipid metabolism-related gene expression in juvenile largemouth bass (Micropterus salmoides)

In aquaculture, maintaining dietary lipid stability is crucial for ensuring fish health and optimizing feed efficiency. Traditional methods of direct liquid oil incorporation into feeds often result in nutrient degradation by oxidation, adversely affecting the health and growth performance of aquatic species. Fat powder (FP) serves as a potential alternative to traditional liquid oils, enhancing feed oxidative stability and improving nutrient absorption. However, the effects of FP application in aquaculture feeds remain unclear, particularly for largemouth bass (Micropterus salmoides). An 8-week feeding trial evaluated soybean oil (SO) replacement with FP (0 %, 50 %, 75 % and 100 %) in 480 juvenile largemouth bass (initial weight: 10.26 ± 0.03 g), assessing growth, whole-body proximate composition, muscle fatty acid profile, hepatic and intestinal histology, hepatic and serum biochemical indices, and intestinal lipid metabolism-related gene expression. Results revealed no significant differences in final weight (FW), weight gain rate (WGR), specific growth rate (SGR) and protein efficiency ratio (PER) among all groups (P > 0.05). However, whole-body crude protein content was significantly increased in the 75FP and 100FP groups (55.32 ± 1.37 and 55.88 ± 0.32) compared to the SO group (53.04 ± 0.34, P < 0.05). Muscle docosahexaenoic acid (DHA) concentration was significantly higher in the 75FP and 100FP groups (9.43 ± 0.1131 and 9.14 ± 0.0557) than the SO group (8.86 ± 0.1526, P < 0.05). All FP substitution groups showed significantly elevated muscle n-3/n-6 polyunsaturated fatty acid (PUFA) ratios (0.85 ± 0.0060, 0.88 ± 0.0062 and 0.97 ± 0.0022) compared to the SO group (0.76 ± 0.0104, P < 0.05). Hepatic malondialdehyde (MDA) content increased significantly in the 50FP and 100FP groups (1.12 ± 0.12 and 1.3 ± 0.23) compared to the SO group (0.78 ± 0.04, P < 0.05). Similarly, hepatic sections showed nuclear displacement toward the cell membrane and mild hepatic lipid accumulation in the 100FP group (P < 0.05). Additionally, intestinal expression of lipolysis-related genes (cpt1 and atgl) was significantly upregulated in the 75FP group compared to the SO group (P < 0.05), while the expression of lipogenesis-related genes (fas, pparγ and srebp1) was significantly downregulated in the 50FP group compared to the SO group (P < 0.05). These findings suggest that FP’s substitution levels must be optimized to balance nutritional benefits against oxidative stress, providing a new insight for lipid source selection in aquaculture feed formulation strategies.