Soy Isoflavones Mitigate High-Fat Diet-Induced Oxidative Stress and Inflammation in the Gut of Monopterus albus via Gut Microbiota Remodeling

High-fat diets are commonly used in eel aquaculture to reduce protein costs, but excessive fat intake can impair intestine health. This study investigated whether soy isoflavones (SIFs), an emulsifying additive, could mitigate the negative effects of high-fat diets in eel. Six hundred eels (30.00 g) were randomly divided into four groups with three replicates: control (CK, 5.96% fat), high-fat diet (HFD, 11.96% fat), and HFD supplemented with 50 mg/kg (LSF) or 100 mg/kg SIF (HSF) for 8 weeks. Results show that SIF supplementation reversed adverse effects on eels fed high-fat diets, reducing intestine damage (better villi development and lower malondialdehyde levels) and improving antioxidant capacity (higher glutathione, <i>p</i> < 0.05). SIFs also increased key antioxidant-related enzyme activities (catalase, superoxide dismutase, <i>p</i> < 0.05). Gut microbiota analysis revealed that SIFs restored microbial balance by reducing Proteobacteria (<i>Pseudomonas</i>) and increasing Firmicutes (<i>Lactococcus</i>), while suppressing harmful metabolic pathways like lipopolysaccharide biosynthesis. These findings demonstrate that dietary SIFs (50 mg/kg) effectively counteract high-fat diet-induced intestine damage and gut dysbiosis in eels, offering a practical nutritional strategy for sustainable aquaculture.