Effect of Adding L-carnitine to High-Fat/Low-Protein Diets of Common Carp () and the Mechanism of Regulation of Fat and Protein Metabolism.

L-carnitine is a low molecular weight amino acid that plays an essential role in the oxidation of long-chain fatty acids. The regulatory effects and molecular mechanisms of L-carnitine on fat and protein metabolism in common carp () were investigated in this study. Common carp ( = 270) were randomly divided into three groups and fed either (1) common carp diet, (2) high-fat/low-protein diet, or (3) L-carnitine-high-fat/low-protein diet. Growth performance, plasma biochemistry, muscle composition, and ammonia excretion rate were all examined after 8 weeks. Additionally, each group's hepatopancreas was subjected to transcriptome analysis. The results showed that decreasing the feed protein/fat ratio resulted in a considerable increase in feed conversion ratio and a significant decrease in common carp-specific growth rate to 1.19 ± 0.02 ( < 0.05). Similarly, total plasma cholesterol sharply increased to 10.15 ± 2.07, while plasma urea nitrogen, muscle protein, and ammonia excretion levels dropped ( < 0.05). After adding L-carnitine to the high-fat/low-protein diet, it was found that the specific growth rate and protein content of the dorsal muscle increased significantly ( < 0.05). In contrast, the plasma total cholesterol and ammonia excretion rate decreased considerably at most time points after feeding ( < 0.05). The expression of genes in the hepatopancreas differed substantially between the different groups. Through GO analysis, it was demonstrated that L-carnitine increased the ability of fat decomposition by up-regulating the expression of in the hepatopancreas and decreased the expression of and to reduce the production and extension of lipids. Simultaneously, was more abundant in the hepatopancreas, implying that L-carnitine can increase protein synthesis. According to the findings, adding L-carnitine to high-fat/low-protein diets can stimulate growth by enhancing lipolysis and protein synthesis.