The Effects of Different Dietary Carbohydrate-to-Lipid Ratios on the Growth and Hepatic Glucose and Lipid Metabolism of Scophthalmus maximus L.

In this study, we investigated the effects of different carbohydrate-to-lipid ratios in the dietary on the growth, glucose metabolism and lipid metabolism of juvenile turbot (Scophthalmus maximus L.), aiming to identify the optimal carbohydrate-lipid ratio in feed for turbot and to determine the relative utilization efficiency of carbohydrates and lipids. Five experimental diets with different carbohydrate-to-lipid ratios (2.01, 1.53, 1.25, 0.96, and 0.71) were used, and these diets were fed to five groups of juvenile turbot (D1, D2, D3, D4, D5). Turbots with an initial weight of (7.00±0.35) g were randomly allocated to 15 tanks, 40 individuals each. Feeding was conducted twice daily using satiation feeding strategy in a culture period of 56 days. The results showed that the weight gain rate, specific growth rate, protein efficiency ratio, and feed efficiency ratio of turbot in D4 group were significantly higher than those in other groups (P < 0.05). The activities of trypsin and lipase in the intestines were significantly higher in D4 group than in others while the amylase activity was significantly higher in D3 group than in others (P < 0.05). The content of plasma triglycerides in D3 and D4 groups was significantly higher than that in other groups (P < 0.05). In contrast, the levels of total cholesterol, HDL-C, and LDL-C in D4 group were significantly higher than those in other groups (P < 0.05). The expression of glucose kinase gene in liver was significantly higher in D3 group than in D1 group. The the expression of pyruvate kinase gene was significantly higher in D3 and D4 groups than in other groups. The expression of phosphofructokinase 1 gene was significantly higher in D4 group than in other groups. The expression levels of phosphoenolpyruvate carboxykinase, fructose-1, 6-bisphosphatase and glucose-6-phosphatase genes in liver were significantly higher in D5 group than in other groups (P < 0.05). The D4 group exhibited significantly higher expression of peroxisome proliferator-activated receptor-γ coactivator-1α and lipin1 genes in liver compared to other groups. The expression of peroxisome proliferator-activated receptor α was significantly higher in D5 group than in other groups. The expression of lipoprotein lipase gene was significantly higher in D4 and D5 groups than in other groups. The expression of liver X receptor was significantly higher in D3, D4 and D5 groups than in D1 and D2 groups (P < 0.05). In summary, the optimal physiological and growth performance of turbot was observed when the carbohydrate-to-lipid ratio in the diet varied between 0.96 and 1.25.