Growth and metabolic responses of grouper juveniles (Epinephelus coioides) fed diets containing varying levels of leucine

We conducted an 8-week feeding trial to evaluate the effects of dietary leucine on growth performance, body and muscle composition, hepatic and plasma components, gut morphology, and relative expression levels of genes in the mTOR pathway. Six isonitrogenous and isolipidic diets with increasing levels of Leu (1.88%, 2.84%, 3.51%, 4.45%, 5.27%, and 6.06%, respectively) were formulated. Each diet was fed to 4-replicate groups of 20 groupers (average initial body weight of 35.0 g/fish) twice daily, to apparent satiety. Weight gain and specific growth rates; feed intake; body protein, lipid, and muscle protein content; and liver GOT and branched chain α-keto acid dehydrogenase kinase activities showed positive linear and/or quadratic responses to dietary Leu levels. In contrast, body moisture and ash content, liver triglyceride and total-cholesterol content, and plasma GOT and GTP activity had negative linear and/or quadratic responses to different dietary Leu levels. There were no differences in the feed conversion ratio and hepatosomatic index, condition factor, plasma GOT activity, and plasma triglyceride and total-cholesterol content among treatments. The mucosal fold number in three gut segments (proximal intestine (PI), middle intestine (MI), and distal intestine (DI)) and mucosal fold length and muscle layer thickness in the DI showed positive quadratic responses to dietary Leu levels. In addition, mucosal fold length and number, and muscle layer thickness in the PI were greater that those observed for the MI and DI. The mRNA level of SLC3A2, SLC7A5, RagC/D, Ragulator, Raptor, mTOR, mLST8, and Tel2 in the muscle and/or gut showed positive quadratic responses to dietary Leu levels, whereas negative quadratic responses were observed for mRNA levels of RagA/B and V-ATPase in the muscle and/or gut, and SLC7A5 in the liver. The optimal amount of Leu required for this fish was estimated to be 8.88% of the dietary protein content, based on the regression analysis of percent weight gains against dietary Leu levels. The results indicate that appropriate dietary Leu levels (range 3.51%–4.45%) could help to improve growth performance and nutrient deposition, maintain gut structural integrity and positively regulate genes in the mTOR signaling pathway in the fish.