The effects of different concentrations of glutamine on host performance are important in the context of aquaculture. Here, four diets were fed to Xenocypris davidi Bleeker, with 0, 3, 6, and 9 g/kg of glutamine supplementation, for a period of 8 weeks. The results showed that supplementation with 6 and 9 g/kg of glutamine significantly increased the final body weight (FBW), weight gain rate (WGR), and specific growth rate (SGR) of the fish (P < 0.05). Conversely, there was a decreasing trend observed in the condition factor (CF), viscerosomatic index (VSI), hepatosomatic index (HSI), and feed conversion ratio (FCR) with increased glutamine supplementation. Dietary glutamine was found to improve the intestinal structure and function of X. davidi. This was achieved through an increase in the digestive and antioxidant enzyme activities in the intestine, as well as an increase in the muscle layer thickness (MT), villus height (VH), and villus width (VW) of the intestine. These effects were particularly evident in the 6 and 9 g/kg glutamine supplementation groups. This finding was further corroborated by oil red O staining and the relative areas of lipid droplets. A gradual increase in plasma total protein (TP) concentrations with increasing glutamine supplementation was observed, and was considerably higher in the 9 g/kg group than in the control group (P < 0.05). In contrast, plasma total cholesterol (TC) and triglyceride (TG) concentrations were decreased significantly after fish were fed with diets with appropriate glutamine supplementation levels. Notably, we further constructed the correlation analysis to find and verify the important factors that affect the growth performance of host after feeding glutamine. In the present study, we observed that PEP and AMS (belong to digestive enzymes), MT, VH and VW (belong to intestinal morphology), crude protein (belong to proximate composition) showed significant positive correlation with FBW, WGR, SGR (belong to growth performance, P < 0.05). Overall, these findings suggest that dietary glutamine can enhance host performance in X. davidi, particularly when supplemented at 9 g/kg. This may have important implications for the aquaculture industry, as it can improve the health and growth of farmed fish.
