Effects of dietary betaine supplementation on antioxidant capacity and glycolipid metabolism in juvenile largemouth bass ( Micropterus salmoides ) fed high-carbohydrate diets

The purpose of this study was to investigate the effects of betaine supplementation in high-carbohydrate diets on the antioxidant capacity and glycolipid metabolism of juvenile largemouth bass ( Micropterus salmoides ). Six experimental diets were designed, including a low-carbohydrate diet (8% starch, LC), a high-carbohydrate diet (20% starch, HC), and four gradient betaine supplementation groups (designated as HCB1, HCB2, HCB3, and HCB4 in turn) with 0.5%, 1.0%, 1.5%, and 2.0% betaine supplemented to the high-carbohydrate diet, respectively. A total of 270 juvenile largemouth bass with an initial body weight of 4.34±0.11 g were used for an 8-week feeding experiment. The results showed that, compared with the LC group, the HC group exhibited significant oxidative stress in the liver and disorders of glucose and lipid metabolism in juvenile largemouth bass. However, betaine maintained the glycolipid metabolic homeostasis of the organism by significantly down-regulating the transcriptional expression of key lipid synthesis genes (ACC1 and FAS), inhibiting excessive hepatic lipid synthesis, reducing the serum levels of glucose (GLU), triglycerides (TG), total cholesterol (TC), and low-density lipoprotein cholesterol (LDL-C), and decreasing abnormal lipid accumulation. In addition, the improvement in hepatic antioxidant capacity induced by betaine was mainly associated with the Nrf2-Keap1 pathway, which significantly upregulated the transcription of key antioxidant genes (SOD and GPX), elevated superoxide dismutase (SOD) activity, and increased reduced glutathione (GSH) content. In conclusion, dietary supplementation with 0.5% betaine not only enhances the growth performance but also improves hepatic antioxidant capacity and maintains hepatic metabolic homeostasis of juvenile largemouth bass under high-carbohydrate feeding conditions, providing more comprehensive theoretical support for the research on betaine in carbohydrate diets for carnivorous fish.