Dietary Soy Isoflavone Alleviates Oxidized Fish Oil-Induced Growth Inhibition and Hepatic Injury in Rice Field Eel (Monopterus albus): Involvement of Antioxidant Capacity, Digestive Function, Endoplasmic Reticulum Stress, and Inflammation

An 8-week feeding trial was conducted to evaluate the potential of dietary soy isoflavones (SIF) to counteract growth inhibition, oxidative stress, endoplasmic reticulum stress, and inflammation induced by oxidized fish oil in Monopterus albus (initial body weight: 26.0 g). The fish were reared in outdoor pond-based net cages (2.0 m × 1.5 m × 1.5 m; 3 cages per treatment, 50 fish per cage) and fed five isonitrogenous and isolipidic experimental diets: a control diet (CON) containing 2.2% fresh fish oil, and oxidized fish oil diets (2.2% oxidized oil) supplemented with 0 (SIF0), 25 (SIF25), 50 (SIF50), or 100 (SIF100) mg/kg SIF. The fish were fed to satiation daily at 4:00 pm at a rate of 3–5% of initial body weight. Compared with the CON group, the SIF0 group showed significantly reduced growth performance, with a final weight gain rate of 84.13%, and decreased intestinal digestive enzyme activity. Hepatic superoxide dismutase (SOD) and catalase (CAT) activities declined to 1.78 U/mgprot and 4.13 U/mgprot, respectively, while serum glutamic-oxaloacetic transaminase (GOT) and glutamic-pyruvic transaminase (GPT) levels increased to 24.95 U/L and 37.56 U/L. The SIF0 diet also up-regulated the expression of hepatic endoplasmic reticulum stress-related genes (perk, ire1, atf6) and pro-inflammatory genes (tnfα, il-1β). Dietary SIF supplementation alleviated these adverse effects, with the most pronounced improvements observed at 50 mg/kg SIF based on overall performance. These results demonstrate that SIF can serve as a functional feed additive to mitigate oxidative stress and related metabolic disorders in M. albus.