Effects of High Dietary Carbohydrate Levels on Growth Performance, Enzyme Activities, Expression of Genes Related to Liver Glucose Metabolism, and the Intestinal Microbiota of Lateolabrax maculatus Juveniles

The present study was conducted to investigate the effects of high dietary carbohydrate levels on growth performance, enzyme activities, and gene expressions related to liver glucose metabolism and the intestinal microbiota of Lateolabrax maculatus juveniles. Two experimental diets with levels of carbohydrates (20% and 30%, named the NCD group and the HCD group, respectively) were designed to feed L. maculatus (initial weight 9.45 ± 0.03 g) for 56 days. The results showed that, compared with the NCD group, the condition factor (CF) was significantly elevated in the HCD group (p < 0.05). The plasma advanced glycosylation end products (AGEs), glycated serum protein (GSP), total cholesterol (TC), triglycerides (TG), high-density lipoprotein (HDL), low-density lipoprotein (LDL), and glutamate aminotransferase (AST) were significantly higher in the HCD group than those in the NCD group (p < 0.05). The intestinal lipase, chymotrypsin, and α-amylase in the HCD group were significantly higher than those in the NCD group (p < 0.05). The liver superoxide dismutase (SOD), total antioxidant capacity (T-AOC), and catalase (CAT) were significantly lower in the HCD group than in the NCD group (p < 0.05). The liver malondialdehyde (MDA) and hexokinase (HK) levels were significantly higher than those in the NCD group (p < 0.05). In the histopathological findings, liver cells in the HCD group appeared to have many vacuoles, and the number of lipid droplets increased. Compared with the NCD group, the relative expression of liver glucokinase (GK) and glycogen synthetase kinase-3 (GSK3β) genes in the HCD group was significantly increased (p < 0.05), while the relative expression of phosphoenolpyruvate carboxykinase (PEPCK) and glycogen phosphorylase (GP) genes in the HCD group was significantly reduced (p < 0.05). High-throughput 16S rRNA gene sequencing showed that high dietary carbohydrate intake changed the composition and structure of the intestinal microbiota. At the phylum level of the intestinal microbiota, high dietary carbohydrates decreased the relative abundance of Firmicutes and increased the relative abundance of Proteobacteria and Bacteroidetes. At the genus level of the intestinal microbiota, high carbohydrates decreased the relative abundance of Bacillus and increased the relative abundance of Photobacterium and Paraclostridium. From the results of this experiment on L. maculatus, high carbohydrates led to increased condition factor and liver glycogen, lipid deposition, decreased antioxidant capacity of the liver, increased relative abundance of harmful intestinal microorganisms, and disrupted glucose metabolism.