Effect of Dietary Carbohydrate/Protein Ratios and Feeding Frequency on Carbohydrate Metabolism of Common Carp

A 8-week 3×2 two-factorial experiment was conducted to investigate effects of dietary carbohydrate to protein ratio (C5%/P32%, C10%/P30%, C20%/P28%) and feeding frequencies (fourth daily and twice daily) on the hepatic carbohydrate metabolic enzyme activities, regulatory factor contents related to glycometabolism, the insulin receptor (IR), GLUT2, hexokinase (HK), GLUT2 and SGLTlmRNA expression levels of common carp with initial average weight of 55.37±3.55 g. Three replicates with 150 common carp were randomly allotted to one treatment with combination of dietary carbohydrate level and feeding frequency in a complete randomized design. The results indicated that: (1) Dietary C/P ratio and feeding frequency significantly influenced the amylase activities of midgut (P<0.05). Amylase activities of midgut of fish fed fourth daily were significantly lower than those fed twice daily (P<0.05). (2) The phosphofructokinase (PFK) and glucose 6 phosphate dehydrogenase (G6PD) activities were significantly affected only by dietary C/P ratio. In addition, the phosphoenolpyruvate carboxykinase (PEPCK) and Glycogen synthase (GS) activities were significantly affected by dietary C/P ratio and feeding frequency, and there was a significant positive correlation between those activities and dietary C/P ratio (P<0.05). (3) The IR, GLUT2 and HK mRNA expression levels of hepatopancreas and GLUT2 and SGLT1 mRNA expression levels of intestine were significantly affected by dietary C/P ratio and feeding frequency, which were significantly increased with increasing feeding frequency (P<0.05). Furthermore, the highest values of those expression levels were observed in the C10%/P30% group and these were significantly higher than those in other groups (P<0.05). Above all, fish fed C10%/P30% diet was suitable with feeding frequency at fourth daily in this study.