Comparative analysis of muscle metabolomics and gut microbiome provides new insights into the high fat intolerance mechanism of juvenile grass carp, Ctenopharyngodon idella

In aquaculture, increasing dietary fat levels can help to spare expensive protein resources and reduce water pollution. In some fish species, such as herbivorous fish, limited tolerance to dietary fat has emerged as a key factor affecting aquaculture sustainability, although the underlying mechanisms remain poorly understood. To address this, we investigated changes in juvenile grass carp Ctenopharyngodon idella on growth, muscle metabolomics, and gut microbiota in response to dietary lipid levels. A total of 270 fish (33.28 ± 0.05 g) were randomly assigned to three groups (triplicate) and fed isonitrogenous diets (30 % protein) with graded lipid levels (5 % control, 7 % and 9 % high-fat) for 8 weeks. Relative to the control, growth was enhanced at 7 % lipids but was significantly reduced at 9 % lipids (P < 0.05). The visceral and mesenteric fat ratios were elevated at 7 % and 9 % lipids and were highest at 7 % lipids (P < 0.05). Myocyte diameters were smallest in the control and largest at 7 % lipids (P < 0.05). Muscle metabolomics were significantly altered, at 7 %, the primary enriched metabolic pathways were lysine biosynthesis and glycine metabolism, while at 9 %, they were purine metabolism and taste transduction. Increasing lipid levels reduced the number of operational taxonomic units number and α-diversity of the gut microbiota. Bacteroidota abundance and phytuberin levels were significantly positively correlated at 7 % lipids; similarly Nakamurella abundance and guanosine levels showed a significant positive correlation at 9 % lipids (P < 0.05). Unclassified_f_Rhodobacteraceae abundance and acetic acid levels were negatively correlated at 9 % lipids (P < 0.05). Overall, Grass carp responded differently to varying dietary lipid levels. A moderate lipid increase (7 %) improved growth and affected amino acid metabolism, while excessive lipid levels (9 %) inhibited growth and altered muscle energy balance and nerve conduction. These effects were partly correlated with changes in the gut microbiota.