Substituting Soybean Meal With Rapeseed Green Cake: A Holistic Assessment of Growth, Metabolic Response, and Intestinal Status in Juvenile Largemouth Bass ( Micropterus salmoides )

This study evaluated rapeseed green cake (RGC) as a partial replacement for soybean meal (SBM) in diets for juvenile largemouth bass ( Micropterus salmoides ; 15.63 ± 0.30 g). Five isonitrogenous (47.64%) and isolipidic (11.9%) diets replaced 0% (RGC0), 25% (RGC25), 50% (RGC50), 75% (RGC75), or 100% (RGC100) of SBM protein with RGC. Fish were fed for 8 weeks. The RGC75 group showed significantly lower WGR and SGR and higher CF, whereas SR, FCR, VSI, and HSI were not affected. However, no significant differences were observed in whole‐body composition. Blood analysis revealed reduced total cholesterol (TC) and high‐density lipoprotein (HDL) at RGC ≥ 50 and decreased total protein (TP) at RGC ≥ 75. Alanine aminotransferase (ALT), aspartate aminotransferase (AST), and lipoprotein lipase (LPL) decreased at RGC100, while glucose (GLU) peaked at RGC75/RGC100. Gene expression indicated suppressed protein synthesis ( mtor , rps6k , and igf1 ) at RGC75 and significantly suppressed lipid synthesis ( pparγ and fas at RGC75/RGC100; srebp1 at RGC100). Gluconeogenesis ( g6pase ) increased at RGC75/RGC100, while glycolysis ( gk ) decreased at RGC ≥ 50. Intestinal health markers showed elevated glutathione (GSH) and malondialdehyde (MDA) at RGC100 and glutathione peroxidase (GPx) activity at RGC ≥ 50. Gene expression analysis showed upregulation of nf-κb and downregulation of il-10 at RGC levels ≥ 50. Downregulation of nrf2 and gpx occurred at RGC levels ≥ 75. Il-8 expression increased specifically at the RGC100 level. Villus height was significantly reduced at RGC100, accompanied by downregulated expression of tight junction proteins ( occludin and claudin-1 ) at RGC ≥ 75; intestinal epithelial detachment occurred at RGC100, while goblet cell density increased at moderate RGC replacement (RGC50) but decreased at RGC100. Considering growth, metabolism, and gut health, RGC50 (50% SBM replacement) was optimal.