The adaptive mechanism of Ctenopharyngodon idellus to dietary lipid levels: Insights from microbiota-mediated bile acid enterohepatic circulation

Dietary lipid intake can dramatically alter the host microbiota and bile acids (BAs) metabolism. To explore the effect of endogenous BAs and intestinal microorganisms in the adaptive mechanism of grass carp ( Ctenopharyngodon idellus ) to different dietary lipid levels, triplicate groups of grass carp were fed three isonitrogenous diets with low (2 %), normal (6 %) and high (10 %) lipid levels and named group E2, E6 and E10, respectively. Results showed that a 6 % dietary lipid level was optimal for achieving the maximum growth performance and maintaining the liver health state of grass carp. The levels of glucose (GLU), high-density lipoprotein cholesterol (HDL-C), and malonaldehyde (MDA), along with the activities of acetyl-coA carboxylase (ACC), carnitine palmitoyl-transferase 1 (CPT1), fatty acid binding protein (FABP) and fatty acid transport protein (FATP) were significantly lower in fish that fed diet E2 than those that fed diet E6 ( P < 0.05). The activities of acid phosphatase (ACP), alkaline phosphatase (AKP), glutamic oxalacetic transaminase (GOT) and glutamic-pyruvic transaminase (GPT) were significantly higher in fish fed diet E10 than in those fed diet E6 ( P < 0.05). While there were no notable differences in the activities of lipid metabolism-related enzymes between fish fed diets E6 and E10 ( P > 0.05), fish fed diet E10 exhibited a greater presence of vacuoles and lipid buildup in the liver compared to fish on diet E6. In the gallbladder, the levels of multiple BAs in fish that were given diet E2 were considerably lower than those in fish that received diet E6. However, the intestinal contents of fish that were given diet E2 showed significantly higher levels of multiple BAs than those on diet E6, while fish on diet E10 exhibited the opposite pattern. Additionally, fish fed diet E6 showed a higher presence of Fusobacterium and Cetobacterium , while the ratio of Proteobacteria decreased. Furthermore, multiple BAs were positively correlated with lactic acid bacteria-related genera from Firmicutes and a significant negative correlation with dietary lipid levels. These findings indicated that microbial-mediated bile acid hepatoenteric circulation plays an important role in the adaptive mechanism of fish to different dietary lipid levels, and it is expected to be an important intervention target for lipid metabolism disorders of grass carp. • Lactic acid bacteria-related genera in the phylum Firmicutes exhibited a positively correlated with multiple BAs. • Cetobacterium may be the core flora of Fusobacteriota in responding to changes in dietary lipid levels. • Grass carp may adapt to elevated dietary lipids by boosting BAs synthesis and curbing microbial-derived BAs excretion.