Dietary leucine level affects growth performance, whole body composition, plasma parameters and relative expression of TOR and TNF-ɑ in juvenile blunt snout bream, Megalobrama amblycephala

Abstract

Abstract A 9-week feeding trial was conducted to investigate the effect of dietary leucine levels on growth, body composition, plasma parameters and relative expression of target of rapamycin (TOR) and tumor necrosis factor-alpha (TNF-ɑ) mRNA in juvenile blunt snout bream. Six isonitrogenous and isoenergetic experimental diets were formulated to contain graded leucine levels ranging from 0.94% to 2.96% of dry diet. The results showed that survival rate was not significantly affected by dietary leucine level. Weight gain (WG), specific growth rate (SGR), feed efficiency ratio (FER) and protein efficiency ratio (PER) significantly increased with increasing dietary leucine level up to 1.74%, at 2.15% remained plateau and thereafter decreased with further increase of dietary leucine. 1.74% to 2.55% dietary leucine levels significantly improved protein contents of whole body and decreased lipid concentration. Plasma protein and cholesterol contents were not significantly affected by dietary leucine levels, while plasma triglyceride and glucose concentrations were significantly affected by dietary leucine levels. Plasma leucine concentration significantly increased with increasing dietary leucine level, whereas plasma isoleucine and valine concentrations showed reverse trends. Significantly higher relative expression levels of TOR mRNA was observed in fish fed with 1.74% leucine diet compared to those fed with 0.94% leucine diet. The highest relative expression level of TNF-ɑ mRNA was observed in fish fed with 2.96% leucine diet, while the lowest value was observed in fish fed with 1.74% leucine diet ( P Statement of relevance Fish cannot synthesize all amino acids, and 10 essential amino acids (EAAs) must be supplied by aqua-feed in an adequate level. EAAs play important roles in growth, nutritional physiology, immunity, behavior, larval metamorphosis, reproduction, and resistance to environmental stressors. Thus, optimizing the EAA supplementation in tune with the requirements is a generic imperative, and has been a major focus point in fish nutrition research. Leucine is the most effective amino acid regarding to the effects on protein synthesis and degradation in mammals, and plays important roles in insulin secretion stimulation, blood glucose homeostasis, body adipose deposition and so on. Leucine has been proven to be an EAA for fish, and a deficiency or an excess dietary leucine level reduced growth performance and feed utilization of several fish species. However, the knowledge on nutritional function of dietary leucine in fish is still limited compared to higher animals. Blunt snout bream, Megalobrama amblycephala , is a major cultured freshwater fish species in China, which has a long history of cultivation because of its excellent flesh quality, rapid growth performance and high larval survival rate. Its production has been rapidly increased and reached approximately 0.70 million tons in 2012. Economic fish production relies on low cost commercial diet development. However, there is no available data on dietary leucine requirement of blunt snout bream. Hence, a standard nutrient requirement study with 6 graded levels of leucine fed to 3 groups of fish for each diet was set up. The results showed that dietary leucine levels significantly influenced the growth performance, whole body composition, and relative expression of TOR and TNF-ɑ mRNA in juvenile blunt snout bream. Plasma leucine level significantly increased with increasing dietary leucine levels, while plasma isoleucine levels were enhanced in leucine deficient fish. The dietary leucine requirement of juvenile blunt snout bream was determined to be 1.44% of dry diet (4.24% of dietary protein) and 1.61% of dry diet (4.74% of dietary protein) on the basis of SGR and FER, respectively. It would be useful in developing EAA balanced commercial feeds for juvenile blunt snout bream.