Effects of Dietary Ursolic Acid on Growth Performance and Intestinal Health of Largemouth Bass ( Micropterus salmoides ).

This study aimed to investigate the effects of ursolic acid (UA) on the growth performance and intestinal health of largemouth bass ( Micropterus salmoides ). Four diets were formulated with UA supplementation at 0, 250, 500, and 1000 mg/kg, defined as the control (CON), UA250, UA500, and UA1000, respectively. After an 8-week feeding experiment, the results showed that, in the UA500 group, the final body weight (FBW), weight gain rate (WGR), and specific growth rate (SGR) increased, and the feed conversion ratio (FCR) and hepatosomatic index decreased. Total superoxide dismutase (T-SOD) activity exhibited a significant increase, and malondialdehyde (MDA) content decreased. An intestinal histological analysis revealed an improvement in the intestinal structural integrity of the UA500 group. The mRNA relative expression levels of physical barrier-related genes [ occludin , claudin-1 , and zonula occluden-1 ( zo-1 )] were upregulated. The mRNA relative expression of interlenkin 10 ( il-10 ) increased, and the mRNA relative expression of interlenkin 1β ( il-1β ) and tumor necrosis factor-α ( tnf-α ) significantly decreased. The abundance of Firmicutes and Proteobacteria decreased, and the abundance of Tenericutes increased. The abundance of Mycoplasma , Cyanobium , and Staphylococcus decreased, while the abundance of Clostridium increased. In conclusion, dietary supplementation of UA significantly enhanced the growth performance and antioxidant capacity of largemouth bass while improving intestinal barrier function through its influence on the abundance of intestinal flora, such as Tenericutes, Firmicutes, and Mycoplasma . Optimal dietary UA levels for largemouth bass were determined to be between 498 and 520 mg/kg based on quadratic regression analyses of WGR, SGR, and FCR or T-SOD and MDA content.