Metabolic responses of juvenile GIFT strain of Nile tilapia (Oreochromis niloticus) to dietary L-tryptophan supplementation

A 60-day indoor feeding trial was conducted to evaluate the effects of dietary tryptophan supplementation on growth performances, whole-body chemical composition, expression of muscle growth-related genes (MyoD,myogeninandmyostatin), and haematological and biochemical responses of juvenile genetically improved farmed tilapia (GIFT). Five corn-soy-based isonitrogenous and isoenergetic diets were formulated to contain graded levels of dietary tryptophan (2.6, 3.2, 3.7, 4.2 and 4.8 g/kg of diet). Each diet was randomly assigned to triplicate groups of 30 fish (5.3 +/- 0.1 g) per experimental unit, which were fed thrice a day (9:00, 13:00 and 17:00 hr). Maximum growth performances and feed utilization were observed in fish fed tryptophan at 3.7 g/kg of diet. There was no significant (p > .05) effect on whole-body composition and amino acid profile by dietary tryptophan supplementation. However, significant (p < .05) differences were observed in plasma metabolites and the mRNA expression ofMyoD, myogeninandmyostatin. Serum cortisol level was found significantly lowest in fish fed tryptophan at 3.7 g/kg of diet. Second-order polynomial regression analysis of weight gain and nitrogen gain against dietary tryptophan levels indicated that the optimum dietary tryptophan requirement for maximum growth and feed utilization of juvenile GIFT tilapia was 3.8 g/kg of diet.