DIETARY ISOLEUCINE LEVELS ON GROWTH, FEEDING, AND LIPID METABOLISM OF LEOPARD CORAL GROUPER (<italic>PLECTROPOMUS LEOPARDUS</italic>)

This study investigated the effects of dietary isoleucine (Ile) levels on growth, feeding, and lipid metabolism of leopard coral grouper (<italic>Plectropomus leopardus</italic>). Seven experimental diets, each with three replicates, were formulated with graded Ile levels of 1.96%, 2.11%, 2.29%, 2.40%, 2.53%, 2.68%, and 2.84% of dry matter, designated as Ile1.96, Ile2.11, Ile2.29, Ile2.40, Ile2.53, Ile2.68, and Ile2.84, respectively. Juvenile fish (initial body weight of 34.93±0.27 g) were stocked at 15 fish per replicate, with three replicates per diet, and fed for 9 weeks. Results showed that the Ile2.84 group exhibited significantly lower weight gain rate (WGR), daily feed intake (DFI), specific growth rate (SGR), and protein efficiency ratio (PER) compared with all other groups (<italic>P</italic>&lt;0.05), while its feed conversion rate (FCR) was significantly higher (<italic>P</italic>&lt;0.05). The analysis of two-slope broken-line model on WGR indicated that growth declined when dietary isoleucine levels exceeded 2.68%. The Ile2.84 group exhibited significantly lower hepatosomatic index (HSI), condition factor (CF), and crude lipid content in whole body, muscle, and liver compared to other groups (<italic>P</italic>&lt;0.05). Consistently, plasma levels of triglycerides (TG), total cholesterol (T-CHO), and low-density lipoprotein cholesterol (LDL-C) were significantly decreased (<italic>P</italic>&lt;0.05), while high-density lipoprotein cholesterol (HDL-C) levels exhibited the opposite trend (<italic>P</italic>&lt;0.05). Ile2.84 group showed significantly higher expression of brain orexigenic genes (<italic>npy</italic>) and intestinal anorexigenic genes (<italic>t1</italic><italic>r1</italic>, <italic>t1</italic><italic>r3</italic>, and <italic>cck</italic>) compared to other groups (<italic>P</italic>&lt;0.05). The Ile2.84 group exhibited significantly reduced expression levels of hepatic lipid synthesis-related genes (<italic>acc1</italic><italic>α</italic> and <italic>fas</italic>), but significantly increased expression levels of lipid catabolism-related genes (<italic>ampkα</italic>, <italic>pparα</italic>, <italic>cpt-1</italic>, and <italic>hsl</italic>) (<italic>P</italic>&lt;0.05). In conclusion, dietary isoleucine level ranging from 1.96% to 2.68% (3.80%—5.17% of dietary protein) are sufficient to support optimal growth in <italic>P. leopardus</italic>.