Glucose tolerance of grass carp Ctenopharyngodon idellus after a long-term adaptation to carbohydrate-to-lipid ratio diets

Unlike the case of mammals, teleost fish generally have poor ability to utilize carbohydrate, and the clearance of glucose from the plasma is sluggish. In this study, after a 8-week feeding trial, we conducted a glucose tolerance test to hypothesize that the capability of grass carp (Ctenopharyngodon idellus) to clear a glucose load is associated with its nutrient history of different carbohydrate-to-lipid ratio diets (CHO:L: 1.52%, 5.84% and 34.84%) through glycogen synthesis and glucose metabolic key enzymes. Our results showed that after a glucose load, the plasma glucose of fish fed with high CHO:L-ratio diet (34.84%) first declined to baseline conditions, and the peak value of liver and muscle glycogen content was significantly higher in high CHO:L-ratio but lower in low CHO:L group (1.52%). In addition, after glucose administration, hepatic GK (glucokinase) activity of fish fed with high CHO:L-ratio diet increased more significantly than that of the other groups, and PEPCK (phosphoenolpyruvate carboxykinase) activity obviously decreased in low CHO:L-ratio group. The results indicated that a long-term adaptation to the high CHO:L-ratio diet can improved the glucose tolerance of C. idellus through the promotion of glycogen synthesis and GK activity.