Investigation of the nutritional requirements of Australian snapper Pagrus auratus (Bloch & Schneider 1801):: digestibility of gelatinized wheat starch and clearance of an intra-peritoneal injection of D-glucose

Two experiments were performed to investigate the digestibility and utilization of carbohydrate sources by Australian snapper Pagrus auratus. In the first experiment, snapper of two different size classes (110 and 375 g) were fed a reference diet containing no starch (REF) or diets containing 150 (PN15), 250 (PN25), 350 (PN35) or 450 g kg(-1) (PN45) of 100% gelatinized wheat starch to investigate the interactive effects of fish size and starch inclusion level on apparent organic matter (OM) or gross energy (GE) digestibility (ADC), post-prandial plasma glucose concentration, hepatosomatic index (HSI) and liver or tissue glycogen content. A second experiment used a 72 h time course study to investigate the ability of larger snapper (300-481 g) to clear an intra-peritoneal injection of 1 g D-glucose kg(-1) body weight (BW). Organic matter and GE ADCs declined significantly in both fish sizes as the level of starch increased (PN45 < PN35 < PN25 < PN15). Only GE ADC was significantly affected by fish size (ADC(energy) small fish < ADC(energy) large fish). There was no interaction between fish size and inclusion level with respect to GE or OM ADCs. Gross energy ADC for both sized fish was described by the linear function GE ADC=104.97 (+/- 3.39)-0.109 (+/- 0.010) x inclusion level (R-2=0.86). Hepatosomatic index, liver and muscle glycogen concentrations were significantly elevated in both small and large snapper-fed diets containing gelatinized starch compared with snapper fed the REF diet. Three-hour post-prandial plasma glucose concentrations were not significantly affected by fish size, inclusion level or the interaction of these factors (REF=PN15=PN25=PN35=PN45), and ranged between 1.60 and 2.5 mM. The mean +/- SD resting level of plasma glucose (0 h) was 2.4 +/- 1.1 mM. Circulating levels of plasma glucose in snapper peaked at 18.9 mM approximately 3 h after intra-peritoneal injection and fish exhibited hyperglycaemia for at least 12-18 h. There were no significant differences between the plasma glucose concentrations of snapper sampled 0, 18, 24, 48 or 72 h after injection (0=18=24=48=72 < 12 < 1 < 3=6 h), indicating snapper required almost 18 h to regulate their circulating levels of glucose to near-basal concentrations. Australian snapper are capable of digesting moderate levels of gelatinized wheat starch; however, increasing the dietary content of starch resulted in a reduction in OM and GE digestibility. Smaller snapper appear to be less capable of digesting gelatinized starch than larger fish, and levels above 250 and 350 g kg(-1) of diet are not recommended for small and large fish respectively. Snapper subjected to an intra-peritoneal injection of D-glucose have prolonged hyperglycaemia; however, the post-prandial response to the uptake of glucose from normally digested gelatinized starch appears to be more regulated.