Effects of dietary carbohydrate kind and level on growth, body composition and glycemic response of juvenile sunshine bass (Morone chrysops x M-saxatilis )
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The ability of juvenile sunshine bass to use different levels (20 and 40% of diet) and kinds (glucose, maltose, or dextrin) of dietary carbohydrate was evaluated in terms of growth indices, glycemic response, and liver and muscle composition. Six semipurified diets containing 35% crude protein and each of the carbohydrates at either 20 or 40% of diet were maintained isocaloric (3.2 kcal/g) by adjusting lipid to 11 or 2% of diet, respectively. Two basal diets containing no soluble (digestible) carbohydrate were also formulated at each of the lipid levels. Diets were fed for 7 weeks to fish in triplicate tanks (15 fish/tank) connected to a brackish water (5-7) recirculating system. Fish fed the 20/11 (% carbohydrate/% lipid) diets had significantly (P < 0.05) better feed and protein efficiencies and weight gain compared to those fed the 40/2 diets regardless of carbohydrate kind. However, fish fed diets containing dextrin and maltose had significantly better feed and protein efficiencies than those fed diets with glucose regardless of carbohydrate inclusion level. Moreover, fish fed the 20/11 diets containing glucose, maltose, and dextrin had significantly higher intraperitoneal fat (IPF) ratio (IPF weight X 100/body weight) and lower muscle ratio (muscle weight X 100/body weight) values due to the higher dietary lipid level. There was no significant difference in hepatosomatic index (HSI; liver weight X 100/body weight) due to carbohydrate kind; however, those fish fed diets with soluble carbohydrate had significantly greater HSI values. The fish fed diets with soluble carbohydrates (regardless of kind) had more glycogen and less lipid, moisture, and protein in the liver than fish fed diets without soluble carbohydrates. After 7 weeks of feeding, fasting (15 h postprandial) plasma glucose concentrations were similar (5.0-6.0 mmol/l) among fish fed all diets. Postprandial plasma glucose peaked higher (18-22 mmol/l) and earlier (4 h) in fish fed diets with glucose; whereas, fish fed diets with dextrin exhibited lower (12-19 mmol/l) and significantly delayed (8 h) peak concentrations. Interaction between carbohydrate/lipid level and carbohydrate complexity was noted in the glycemic response data. In fish fed diets with glucose, hyperglycemia was more pronounced at the 40/2 inclusion level; whereas, fish fed diets with dextrin exhibited a higher transient at the 20/11 inclusion level. These data as well as liver composition changes indicate that both differences in substrate digestion/absorption rates and the glucose/fatty acid cycle noted in other species is operative in juvenile sunshine bass.
