Dietary Soy Isoflavones Ameliorate Muscle Quality in High-Fat Diet-Fed Rice Field Eels (Monopterus albus) by Modulating Myogenesis, Collagen Synthesis, and Antioxidant Capacity

High-fat diets are increasingly used to improve feed efficiency in aquaculture but may deteriorate fillet quality and health; soy isoflavones, plant-derived polyphenols, have emerged as promising modulators of muscle growth, antioxidant defense, and lipid metabolism in fish. This study investigated the effects of dietary soy isoflavone supplementation on myogenesis, collagen synthesis, fatty-acid composition, and antioxidant capacity in muscle of Monopterus albus fed a high-fat diet. Fish were assigned to four diets: control (CON, 6.16% crude fat), high-fat without soy isoflavones (HSIF0, 11.98% crude fat), and high-fat with 50 mg/kg (HSIF50) or 100 mg/kg (HSIF100) soy isoflavones. HSIF0 significantly elevated whole-body/muscle lipids, reduced ΣSFA/ΣMUFA/Σn-3/Σn-6 ratios (p < 0.05), increased Σn-6 (p < 0.05), impaired water-holding capacity/texture (higher losses, lower hardness/cohesiveness/gumminess/chewiness/resilience) (p < 0.05), induced loosely arranged myofibers with enlarged inter-fiber spaces, downregulated myogenesis (upregulated mstn; downregulated myod/tcap/mrf4/mrf5) and collagen genes (ets1/sp1/p4ha1) (p < 0.05), decreased collagen/hydroxyproline (p < 0.05), and weakened antioxidants (higher MDA/H2O2; lower T-AOC/GSH; downregulated nrf2/sod/cat/gpx1/gpx8) (p < 0.05). HSIF50 reversed these effects, enhancing ΣPUFA/Σn-3/EPA+DHA (p < 0.05), restoring structure/gene expression (p < 0.05), and boosting antioxidants (p < 0.05). In contrast, HSIF100 partially diminished benefits, indicating dose-dependency. Overall, 50 mg/kg soy isoflavones optimally mitigated high-fat-induced muscle quality decline via lipid remodeling, structural improvement, collagen promotion, and antioxidant enhancement.