Effects of different dietary carbohydrate levels and starvation on growth and glucose tolerance ability in grouper(Epinephelus coioides)

The experiment was conducted to determine the effect of dietary carbohydrate levels(CBH) and starvation on the growth performance, biochemical indices in plasma and liver/muscle glycogen content of the grouper.Total 300 fish were randomly distributed into 5 treatments with triplicate groups of 20 fish(initial weight:35±0.28 g) in each treatment.Three isonitrogenous and isoenergetic diets were formulated containing three carbohydrate levels(35% C_H, 21% C_M, 7% C_L), combined with starvation group(S) and refeeding group(R, starving 4W+refeeding 4W, 21% carbohydrate in the diet).Experiment lasted 8W.14 fish in each treatment were selected and injected with 300 mg glucose per 100 g body weight for glucose tolerance test(GTT) after the end of experiment and blood sampling at 0, 1, 3, 6, 12, 24, 48 h.The results showed that the weight gain and specific growth rate increased with dietary carbohydrate level increase but no significant difference(P>0.05), and the feed coefficient rate of C_L group was the highest(P<0.05).Plasma glucose levels significantly increased in 1-3 h and then decreased in C_H, C_M and C_L(P<0.05).Plasma glucose levels in C_M returned to the initial values within 6 h after being injected and initial level of plasma glucose content in C_H and C_L were obtained within 12 h.Plasma insulin content in C_H, C_M and C_L decreased at 1 h and then increased, but insulin level of C_L group significantly decreased at 3 h(P<0.05).Glycogen content in liver and plasma triglyceride levels were significantly increased in C_H, C_M and C_L within 1 h after injection(P<0.05).C_M, S and R groups presented persistent hyperglycemia after being injected and C_M restored initial level quickly within 6 h.Plasma insulin levels of C_M and R groups significantly decreased within 1 h(P<0.05), but that of R group continued to rise within 12 h.Plasma triglyceride levels of C_M were higher than those of S and R groups within 0 to 6 h(P<0.05).The results of GTT indicate that proper carbohydrate level in diet(21%) and continuous feeding can significantly improve glucose tolerance of the grouper.