Nutritional Intervention of Zinc on Metabolic Disorders Induced by High-Starch Diets in Largemouth Bass (Micropterus salmoides): Insights From Glycogen Metabolism and Insulin Sensitivity

Metabolic disorders induced by high-starch diets are the primary cause limiting the use of carbohydrates in aquafeeds. This study aimed to investigate the intervention effects of zinc on metabolic disorders induced by high-starch diets in Micropterus salmoides. Five diets were formulated, including a control diet (115 g kg⁻¹ starch) and three high-starch diets (163 g kg⁻¹ starch) supplemented with 0 (HS-LZn), 20 (HS-MZn) and 150 (HS-HZn) mg kg⁻¹ Zn. After 8 weeks of feeding, intraperitoneal glucose (Glu) tolerance tests (IPGTTs) and intraperitoneal insulin tolerance tests (IPITTs) were conducted to evaluate insulin sensitivity. High-starch diets inhibited feed intake but improved feed utilization. Diet HS-HZn significantly increased Zn content in the hepatopancreas. Zn retention decreased significantly as the dietary Zn supplementation level increased from 0 to 150 mg/kg. High-starch diets significantly improved the retentions of protein and energy. High-starch diets exerted more pronounced effects on glycogen metabolism and glycolysis. Diets HS-LZn and HS-MZn induced severe hepatic glycogen accumulation, concomitant with significant decreases in activities of hexokinase (HK), phosphofructokinase (PFK), pyruvate kinase (PK), glycogen synthase (GS) kinase-3β (GSK-3β), and acid alpha-glucosidase (GAA). Dietary 150 mg/kg Zn supplementation significantly decreased hepatic glycogen and increased activities of HK, PFK, citrate synthase (CS), and GAA. High-starch diets did not induce severe lipid metabolic disturbances or obvious fat deposition. The insulin tolerance test revealed that the area under the curve (AUC) and incremental AUC (iAUC) for Glu in the HS-LZn group were significantly higher than the control and HS-HZn groups. Meanwhile, phosphorylated AKT (p-AKT) level and immunofluorescence intensity were significantly decreased in the HS-LZn group, while zinc improved AKT phosphorylation. Untargeted metabolomics analysis demonstrated that high-starch diets without zinc supplementation inhibited multiple amino acid metabolic pathways.