Optimal Dietary α-Starch Requirement and Its Effects on Growth and Metabolic Regulation in Chinese Hook Snout Carp (Opsariichthys bidens)

This study investigated the effects of dietary carbohydrate levels on growth performance, body composition, and hepatic expression of metabolic genes in Chinese hook snout carp (Opsariichthys bidens). Fish were fed five isonitrogenous diets with graded α-starch levels (8%, 14%, 20%, 26%, and 32%) for 56 days. The diet containing 14% α-starch significantly increased the weight gain rate (WGR) and specific growth rate (SGR) of O. bidens (p < 0.05). Both broken-line and polynomial regression analyses on WGR and SGR consistently indicated an optimal dietary α-starch level of approximately 14–17%. High carbohydrate diets significantly elevated plasma glucose, triglyceride, and cholesterol levels, as well as hepatosomatic and intraperitoneal fat indices. Gene expression analysis revealed that moderate carbohydrate intake upregulated lipoprotein lipase (lpl), hormone-sensitive lipase (hsl), and carnitine palmitoyltransferase 1 (cpt1) gene expressions, enhancing lipolysis and β-oxidation, whereas excessive carbohydrate intake (>26% α-starch) suppressed these pathways but strongly induced acc1 gene expressions, promoting lipogenesis. Additionally, glycogen metabolism genes (glycogen synthase (gys) and glycogen phosphorylase (pyg) and glycolysis-related phosphofructokinase (pfk) were responsive to carbohydrate supply, while oxidative metabolism gene cs was downregulated under excessive carbohydrate, implying reduced mitochondrial oxidative metabolism. Overall, O. bidens exhibited limited carbohydrate utilization, with optimal intake supporting growth and metabolic balance, whereas excessive intake redirected glucose toward glycogen and lipid accumulation, leading to metabolic imbalance.