Effects of different dietary protein and starch levels on the growth and liver metabolism of grouper (Epinephelus coioides)

A 3×3 factorial experiment was designed to determine the interactive effects of dietary protein and starch levels on growth performance of grouper (Epinephelus coioides). Nine diets were formulated to contain three levels of crude protein (P) (38%, 45% and 52%) and three levels of starch (S) (10%, 20% and 30%). Fish were fed the diets twice daily over a 56-day study period. Fish fed 38% crude protein and 10% starch diet (38P/10S diet) had significantly lower weight gain rate than those fed other diets, and fish receiving 52P/10S diet achieved highest weight gain rate among the test diets, but comparable to fish receiving diets 45P/20S, 45P/30S and 52P/20S. Increasing dietary crude protein and starch levels led to increased feed efficiency, whole-body protein and lipid liver glycogen content and liver lipid contents, and reduced daily feeding rate and whole-body moisture content. Increasing dietary protein level decreased protein efficiency ratio, but increasing dietary starch level increased hepatosomatic index, protein efficiency ratio and viscerasomatic index. Dietary protein and starch levels did not affect whole-body ash content. Hepatic lipase, lipoprotein lipase, fatty acid synthetase, glutamic-oxalacetic and glutamic-pyruvic transaminase activities increased with the increase in dietary protein or starch level. Increasing dietary protein level resulted in lowered liver glucose-6-phosphatase activity, and elevated liver malic enzyme activity, but did not affect liver activities of glucokinase, pyruvate kinase, phosphofructokinase, phosphoenolpyruvate carboxykinase and glucose-6-phosphate dehydrogenase. Increasing dietary starch level led to increased liver activities of glucokinase, pyruvate kinase, phosphofructokinase, glucose-6-phosphate dehydrogenase and malic enzyme, but reduced activities of phosphoenolpyruvate carboxylase kinase and glucose-6-phosphatase. These results indicate that the growth and liver metabolism of grouper were affected by both dietary protein and starch levels, in which the enzyme activity related to glucose metabolism responds to dietary starch level more sensitively than dietary protein level. The diet containing 45% protein and 20% starch is suitable for the optimal growth of this fish species.