Effects of dietary protein and lipid levels on growth, body composition, enzymes activity, expression of IGF-1 and TOR of juvenile northern whiting, Sillago sihama

Abstract

The purpose of this study was to estimate the effect of dietary protein and lipid levels on growth performance, feed utilization, body composition, liver metabolic enzymes activity and expression of insulin-like growth factor-1 (IGF-1) and target of rapamycin (TOR) of the northern whiting, Sillago sihama. A 56-day feeding trial was a completely randomized 4 × 3 factorial design and twelve diets were formulated containing four different protein levels (400, 450, 500 and 550 g/kg) and three different lipid levels (80, 100 and 120 g/kg). 1080 healthy fish (average wt. 0.83 ±0.01 g) were equally distributed in twelve treatments in triplicates with 30 fish per tank. At the end of the experiment, fish fed higher-protein diets (50%) resulted in significantly higher weight gain rate (WG), specific growth rate (SGR), and protein efficiency ratio (PER) compared to those fed the lower-protein treatments (40%). However, when the protein level in diet was 55%, WG began to decrease. The WG of fish fed the 12% lipid diets was significantly higher than that of fish fed the 8% lipid diets (P < 0.05). Also, higher dietary protein levels significantly reduced body moisture content, improved morphometrical indices and increased body lipid (P < 0.05). High-energy diets led to a decrease in the daily feed intake (DFI) of S. sihama. Based on WG, the optimal dietary protein requirement for the quadratic regression models was estimated to be 48.42% of dry matter when the lipid level was 10% and 48.53% for a diet containing 12% lipid. The activities of glutamic-pyruvic transaminase (GPT) and glutamic-oxalacetic transaminase (GOT) in liver increased with increasing dietary protein levels, and the activity of glutamate dehydrogenase (GDH) was significantly higher in the high-protein diets (50% and 55%) than that in the low-protein diets (40% and 45%) (P < 0.05). When lipid levels of the diet were higher than 10%, lipoprotein lipase (LPL) activities were significantly higher than that of the low-lipid groups (8% groups) (P < 0.05). Fatty acid synthase (FAS) activities were inhibited by the increasing lipid levels in diet. Dietary lipid levels significantly improved the expression level of insulin-like growth factor 1 (IGF-1) gene (P < 0.05). The relative expression of the target of rapamycin (TOR) gene in liver increased with dietary protein level at 45%–50%, however, it decreased at 55% of dietary protein levels and was significantly lower than that of the 50% group (P < 0.05). In conclusion, results of this study showed that the dietary protein requirements based on the growth of S. sihama were 48.42% (lipid level: 10%) and 48.53% (lipid level: 12%).