Influence of <i>Eucommia ulmoides</i> Extract on the Growth, Glucose Metabolism, and Antioxidant Capacity of Largemouth Bass (<i>Micropterus salmoides</i>)

This study aimed to evaluate the impact of <i>Eucommia ulmoides</i> extract (EE) supplementation on the expression of genes related to glucose metabolism and antioxidant capacity of <i>M. salmoides</i> in response to different starch levels. In order to evaluate the effect of EE on fish metabolism and especially to enhance the metabolism of <i>M. salmoides</i> towards glucose metabolism, especially in high and low starch formulations, we designed six experimental feed groups: PC (high-starch control), NC (low-starch control), and four groups supplemented with EE on the basis of PC, with EE concentrations of 0.05%, 0.10%, 0.15%, and 0.20%, respectively. Each feed was administered to fish with an average weight of 36.98 ± 0.08 g, which were cultured for seven weeks, and the water temperature was 31–33 °C. The results demonstrated that increasing the EE concentration in the feed significantly influenced fish growth without affecting the body composition. Regarding the antioxidant activity, the highest CAT (catalase) enzyme activity in the intestine was recorded in the 0.15% EE group. Additionally, the mRNA expression of the antioxidant gene <i>keap1</i> (kelch-like ECH-associated protein1) increased with higher EE supplementation, and <i>sod</i> (superoxide dismutase) mRNA expression was significantly elevated in the 0.10% EE group compared to that in the PC group. A plasma biochemical analysis revealed a significant increase in the ALP (alkaline phosphatase) activity in the 0.05% EE group relative to the PC group, while the TG (triglycerides) levels progressively decreased as the EE levels increased. Furthermore, the GLU (glucose) levels were significantly reduced in both the EE-supplemented and NC groups compared to those in the PC group. Among the genes associated with glucose metabolism, both <i>gk</i> (glucokinase) and <i>pepck</i> (phosphoenol pyruvate carboxykinase) exhibited a pattern of initially decreasing, followed by an increase, as the EE levels rose, with the <i>pepck</i> (phosphoenol pyruvate carboxykinase) expression being lowest in the 0.10% EE group. In conclusion, appropriate EE supplementation in the diet may promote growth performance, enhance antioxidant capacity, and support the expression of genes related to glucose metabolism of <i>M.salmoides</i> in response to different starch levels.