The impacts of dietary carbohydrate levels on growth performance, feed utilization, glycogen accumulation and hepatic glucose metabolism in hybrid grouper (Epinephelus fuscoguttatus ♀ x E. lanceolatus ♂)

The present study was conducted to investigate the effects of dietary carbohydrate on growth performance, feed utilization, glycogen deposition and hepatic glucose metabolism in hybrid grouper. Triplicate groups of grouper (9.16 +/- 0.18 g) were fed diets with grade levels of carbohydrate (5.27, 8.95, 11.49, 14.37, 17.78, 20.82 and 23.65%) twice daily for 10 weeks. Results showed that specific growth rate were quadratically and linearly decreased with the increase of dietary carbohydrate, which was related to reduced feed intake and apparent digestibility coefficient of nutrients. Meanwhile, the increase of dietary carbohydrate resulted in both quadratic and linear increase of hepatic glycogen accumulation, which further resulted in the elevated viscerosomatic index. The specific activity of glycolysis and gluconeogenesis related enzymes were both quadratic and linear enhanced as dietary carbohydrate increased, mainly due to the deceased hepatic protein content. The highest expressions of glucokinase, liver-type 6-phosphofructokinase, pyruvate kinase were obtained in the 8.95, 8.95 and 11.49% carbohydrate group, respectively. However, any significant differences were not observed in the expressions of gluconeogenesis related genes as dietary carbohydrate increased. The expression of insulin receptor, insulin receptor substrate 1 and serine/threonine kinase 1 was quadratic and linear elevated in the grouper fed the diet with 8.95% carbohydrate in accordance with the variation of serum insulin content, indicating the activation of insulin pathway at least at the transcription level. These results suggested optimal dietary carbohydrate would benefit for improving growth performance and feed utilization, and activating the insulin pathway along with the promotion of glycolysis in this grouper.