Nutritional control of lipid deposition in rainbow trout and European seabass: Effect of dietary protein energy ratio

The effects of dietary digestible protein (DP) to digestible energy (DE) ra tio on lipid deposition, activity of selected liver lipogenic enzymes and m uscle susceptibility to lipid peroxydation were studied in rainbow trout (O ncorhynchus mykiss) and European seabass (Dicentrarchus labrax). The experi mental diets were formulated to contain one of two crude protein levels (43 et 52% of dry matter (DM) for seabass and 38 et 45% DM for trout) and DP/D E ratios ranged from 17 to 26 mg/kJ. During the growth trials, each diet wa s hand fed to visual satiation, to triplicate groups of fish. Rainbow trout were grown in freshwater at a constant water temperature of 18 degrees C o ver 12 weeks. Seabass were grown in seawater (salinity: 35 parts per thousa nd; temperature: 18 degrees C) over an identical period. For rainbow trout the best results in terms of growth performance, feed efficiency and protei n utilisation were achieved at a DP/DE ratio 17-19 mg/kJ. For seabass, the optimal DP/DE ratio appears to be 19-20 mg/kJ. Whole body composition of ra inbow trout at the end of the experiment shows that an increase on dietary lipids led to an increase in fat content of fish. In seabass, the whole bod y composition was not much affected by experimental treatments. Trout fed h igh-energy diets showed highest lipid content in the muscle and particularl y in viscera. In seabass a similar tendance is observed but the liver seems to be the main site of lipid storage. Muscle homogenates from trout and se abass fed high-fat diets showed a higher susceptibility to lipid peroxidati on than fish fed low-fat diets. Activities of liver glucose-6-phosphate deh ydrogenase (G6PD), malic enzyme (ME) and fatty acid synthethase (FAS) were reduced with increasing fat intake. Activities of G6PD, ME and FAS were inc reased with increasing starch intake. G6PD was found to be the main NADPH-g enerating enzyme.