Oleanolic Acid Modulates the Gut–Liver Axis to Alleviate High-Fat Diet-Induced Hepatic Lipid Deposition in Nile Tilapia (Oreochromis niloticus)

This study examined the protective mechanisms of oleanolic acid (OA) against high-fat diet (HFD)-induced hepatic steatosis and intestinal dysbiosis in Nile tilapia. Fish were allocated to four groups: normal diet (ND), HFD, and OA-supplemented HFD (50 and 250 mg/kg). After 42 days, physiological, biochemical, and histological assessments demonstrated that OA markedly reduced hepatic lipid accumulation, mitochondrial injury, and intestinal shortening. Transcriptomic analysis revealed that OA alleviated lipid dysregulation by inhibiting de novo lipogenesis and promoting lipid trafficking and β-oxidation, effectively reversing HFD-induced changes in the PPAR, MAPK, mTOR, and autophagy-lysosome signaling pathways. 16S rRNA sequencing indicated that OA increased microbial alpha diversity, suppressing HFD-associated taxa (e.g., Nordella) while enriching beneficial genera such as Clavibacter, Bosea, and Bdellovibrio. Importantly, OA treatment restored HFD-induced depletion of intestinal butyric acid and suppressed hepatic pro-inflammatory cytokines (tnf-α, il-1β), while upregulating growth-related factors (igf1). Correlation analysis confirmed strong associations between microbial alterations (Nordella and Phreatobacter) and hepatic lipid metabolism and inflammatory gene expression. Overall, OA mitigates metabolic stress in Nile tilapia by reconfiguring the gut–liver axis, integrating microbial restoration with precise regulation of hepatic nutrient-sensing and inflammatory pathways, providing a potential therapeutic strategy for lipid metabolism disorders in aquaculture.