Effects of different dietary carbohydrate-to-lipid ratios on growth, plasma biochemical indexes, digestive, and immune enzymes activities of sub-adult orange-spotted grouper Epinephelus coioides.

A 10-week feeding trial was conducted to investigate the effects of dietary carbohydrate-to-lipid (CHO:L) ratios on glycogen content, hematological indices, liver, and intestinal enzyme activity of sub-adult grouper Epinephelus coioides. Five iso-nitrogenous (496.0 g kg-1 protein) and iso-energetic (21.6 KJ g-1 gross energy) diets with varying CHO: L ratios of 0.65 (D1), 1.31 (D2), 2.33 (D3), 4.24 (D4), and 8.51 (D5), respectively, were fed to triplicate groups of 20 fish (average 275.1 ± 1.86 g). Results showed that the weight gain rate (WGR), specific growth rate (SGR), and protein efficiency ratio (PER) of sub-adult grouper increased and then stable when dietary CHO:L ratios reach D4 (CHO:L = 4.24). The trend of feed conversion ratio (FCR) was opposite to PER. Along with the dietary CHO:L ratios, the liver and muscle glycogen level increased gradually. Plasma triglycerides (TG) and glucose (GLU) were all maximized at D5 (CHO:L = 8.51) group, cholesterol (CHOL) at D4 (CHO:L = 4.24) group. Digestive enzyme activities were significantly affected by dietary CHO:L ratios. Liver hexokinase (HK), alkaline phosphatase (AKP), and glucose-6-phosphate dehydrogenase (G6PDH) activity increased significantly as CHO:L ratios increased. Liver lysozyme (LYZ) and superoxide dismutase (SOD) activity of sub-adult grouper fed the D4 diet was significantly higher than that of the D2 (CHO:L = 1.31) diet. The trend of acid phosphatase (ACP) is opposite to AKP. The regression model analysis showed that the most suitable dietary CHO:L ratio to reach the highest SGR is 6.06.