Carbohydrate utilization and its protein-sparing effect in diets for grouper (Epinephelus malabaricus)

To investigate the utilization of different carbohydrate sources and the po ssible protein-sparing effects of carbohydrates for grouper (Epinephelus ma labaricus), six isoeneregtic purified diets were prepared. Three dietary pr otein levels (500, 460, 420 g/kg) were achieved by substitution with three levels (143, 195, 246 g/kg) and two sources (glucose and starch) of dietary carbohydrate. Each of the six diets was given to triplicate groups of grou per in a recirculating rearing system for 8 weeks. In the glucose group, we ight gain of fish decreased with increasing dietary glucose and decreasing protein content. In the starch group, weight gain of fish given the 500 g/k g protein diet was higher (P < 0.05) than fish given the 460 and 420 g/kg p rotein diets. At each of the dietary protein levels weight gain of the gluc ose-group fish and the starch-group fish were similar (P > 0.05). At the 50 0 g/kg dietary protein level body lipid content of the starch-group fish wa s higher than that of glucose-group fish. At 500 and 460 g/kg dietary prote in levels, fish given starch diets had higher hepatic hexokinase activity t han fish given the glucose diets. These results suggest that, at the levels studies, the utilization of starch and glucose by grouper is similar. Decr easing the dietary protein level from 460 g/kg to 420 g/kg by increasing th e starch content in the diet from 295 g/kg to 246 g/kg did not reduce (P > 0.05) weight gain and food efficiency, suggesting that starch could spare s ome protein when the dietary protein level is low.