Protein nutrition on sub-adult triploid rainbow trout (1): Dietary requirement and effect on anti-oxidative capacity, protein digestion and absorption

Abstract

An 80-day feeding trial was conducted to assess the dietary protein requirement of sub-adult triploid rainbow trout (initial average weight: 233 g). Six isolipidic and isoenergetic diets were formulated to contain increasing levels of protein (31.6, 35.4, 40.7, 43.3, 45.8 and 52.4%). Each diet was fed to quadruplicate groups of fish (100 fish per group) in freshwater cages. Results showed that weight gain rate (WGR), protein gain, whole-body protein content, trypsin and chymotrypsin specific activities in intestine significantly (P < 0.05) increased with increasing dietary protein level and then plateaued (P > 0.05). Protein retention, protein efficiency ratio (PER), lipase in pyloric caeca and amylase specific activities significantly (P < 0.05) decreased with increasing dietary protein level. Fish fed the diet with 45.8% protein had higher feed efficiency (FE) and higher enteric peptide transporter 1 (Pep T1) gene expression level. Generally, fish fed diets with low dietary protein levels had lower feed intake (FI), higher hepatosomatic index (HSI), higher plasma alanine aminotransferase (ALT) and aspartate aminotransferase (AST) activities, higher hepatic glutamate dehydrogenase (GDH) activity, as well as higher malondialdehyde (MDA) contents in plasma, liver and intestine and lower total anti-oxidative capacity (T-AOC) contents in plasma and intestine (P < 0.05). Based on WGR, the optimum dietary protein requirement of sub-adult triploid rainbow trout was 45.8% dry matter with a diet containing 20% lipid and 22 kJ gross energy/g. This optimal dietary protein level improved the digestive and absorptive abilities of rainbow trout. Diets with lower protein levels (31.6–35.4%) led to increase lipid peroxidation level in whole fish body, while the diet with excess protein (52.4%) appeared to limit enteric Pep T1gene expression level.