Synergistic effects of dietary carbohydrate and taurine on growth performance, digestive enzyme activities and glucose metabolism in juvenile turbot Scophthalmus maximus L.

Abstract

A 10-week feeding trial was conducted to evaluate the effects of taurine on growth performance, digestive enzyme activities and glucose metabolism of juvenile turbot Scophthalmus maximus (initial body weight, 3.66 ± 0.02 g) fed diets with different carbohydrate level. With a 2 × 3 two factoral experimental design, two levels of dietary carbohydrate (15% and 21%, respectively) with three levels of taurine (0.4%, 1.2%, and 2.0%, respectively) were used. In addition, the diet with 15% carbohydrate and without taurine supplementation was used as the control. The results showed that, compared with the control group, 1.2% of dietary taurine supplementation significantly increased the specific growth rate, the activities of amylase and lipase in intestine, glycogen content and the mRNA levels of glucokinase, phosphofructokinase, pyruvate kinase, glucose-6-phosphate dehydrogenase (G6PD), glycogen synthase (GS) and glucose transporter 2 in liver at 15% of dietary carbohydrate level. Meanwhile, it significantly decreased the blood glucose concentration and the mRNA levels of cytosolic phosphoenolpyruvate carboxykinase (cPEPCK) in liver. In addition, significant interaction between dietary carbohydrate and taurine were observed in blood glucose level. Compared to 15% dietary carbohydrate, 21% dietary carbohydrate significantly increased the blood glucose level at 1.2% and 2.0% dietary taurine level, whereas it had no significant effect at 0.4% dietary taurine supplementation level. At 21% of dietary carbohydrate level, although 1.2% and 2.0% of dietary taurine supplementation had no significant effects on blood glucose level, it significantly decreased the advanced glycationend products level in plasma and the fructose-1, 6-bisphosphatase mRNA level in liver. Moreover, compared to that fed diet with 15% carbohydrate, fish fed diet with 21% carbohydrate had lower activities of amylase and lipase in intestine and lower mRNA levels of G6PD, mitochondrial phosphoenolpyruvate carboxykinase and cPEPCK in liver. Furthermore, significant interactions between dietary carbohydrate and taurine were also observed in the expression of the genes involved in liver glucose metabolism above in addition to GS and cPEPCK. Overall, this study indicated that taurine had a beneficial effect on glucose homeostasis through enhancement of glycolysis, glucose transport, glycogen synthesis and the inhibition of gluconeogenesis in liver of turbot, thus decreased blood glucose level. However, the beneficial effect of taurine on glucose metabolism was affected by dietary carbohydrate level.