Abstract
Background:
Bile acids (BAs) are made from cholesterol in the liver and are then coupled with taurine or glycine before being expelled by the hepatocyte. BAs are very important for emulsification of dietary fat for easy nutrients absorption processes.

Aim:
The aim of this study is to investigate the effects of dietary bile acid (BA) supplementation and dietary fat percent on the growth performance, morphology of intestine, immune-physiological responses, and transcriptomic responses of Nile tilapia (Oreochromis niloticus).

Methods:
Using diets containing three different inclusion levels of fat (5%, 7%, and 9%) with or without BA supplementation (0.4 g/kg) fish were fed for 90 days.

Results:
BA supplementation significantly (P < 0.05) improved growth performance and feed utilization, with fish-fed BA-supplemented diets exhibiting higher final weight, weight gain, and feed conversion ratio. Dietary fat levels also significantly affected growth performance, with higher fat levels leading to higher final weight, weight gain, and specific growth rate. BA supplementation also positively (P < 0.05) affected intestinal morphology, immune response, and antioxidant capacity. Fish-fed BA-supplemented diets had higher intestinal villus height, lysozyme activity, superoxide dismutase and catalase activities, and lower malonaldehyde concentration. Gene expression analysis revealed that BA supplementation upregulated (P < 0.05) the expression of antioxidant-related genes (SOD, GPX, and CAT), growth-related genes (GHr1 and IGF1), and intestinal mucin gene (MUC2) while downregulating (P < 0.05) the expression of fatty acid synthase (FAS) and pro-inflammatory genes (IL-1β and TNF-α).

Conclusion:
BA dietary supplementation accompanied with 7 % fat can be a valuable tool for improving Nile tilapia's growth performance, feed utilization, intestinal health, immune function, and antioxidant capacity.

Key words: Bile acid, Fat, Growth, Immune-oxidative response, Oreochromis niloticus