First feeding of grass carp (Ctenopharyngodon idellus) with a high-carbohydrate diet:the effect on glucose metabolism in juveniles

The effective utilization of carbohydrate feed sources is important to save protein feed resources and reduce formula feed costs in fish aquaculture. The present study investigated whether high-carbohydrate diet stimulus in the early larval stage of the grass carp (Ctenopharyngodon idellus) could affect glucose metabolism in subsequent life stages (i.e., the juvenile). A 73-day feeding trial was divided to five groups (four treatments and a control). The trial was conducted from the first feeding of larval fish. During the first 66 days, the control was continuously fed a commercial grass carp diet (CD), the FF-3 group was initially fed with 60% malty dextrin diet (MD) for 3 days and then with the CD for 63 days, the YE-7 group was initially fed with 60% MD after yolk-sac exhaustion for 7 days and with the CD for 56 days, the FF-10 group was initially fed with 60% MD for 10 days and then with CD for 56 days, and the FF-66 group was continuously fed to 60% MD. During the next 7 days, all the treatment groups and the control were fed with 35% MD as challenge test. The results showed that short-term stimulation with high-carbohydrate resulted in significantly increased body weights. After 73 days of feeding, the final body weights of the FF-3 and YE-7 groups were significantly higher than that of the control. Early high-carbohydrate manipulation changed the normal structure of the gut tissues of juvenile fish. After a short challenge test (7 days) performed with a 35% carbohydrate diet in juvenile grass carp, the results showed that high-carbohydrate dietary histories affected plasma metabolites and liver (muscle) glycogen storage in the juvenile fish. The mRNA levels of glucokinase and fatty acid synthetase genes increased significantly at the juvenile stage in the firstfeeding groups. The phosphoenolpyruvate carboxykinase gene expression in the FF-3 and FF-10 groups decreased by varying degrees. The glycogen synthase gene expression levels in all groups were not affected. Long-term intake of high-carbohydrate feed (FF-66 group) led to the nutritional deficiency and disorder of the glucose metabolism system. In conclusion, our results demonstrated that a high-carbohydrate diet stimulus at first-feeding stage can affect the growth performance, gut morphology, and the homeostasis of glucose metabolism in juvenile of grass carp.