Bile Acids as Functional Additives in Plant-Based Tilapia Diets: A Dose-Response Study on Growth, Lipid Metabolism, and Hepatoprotection

The replacement of fishmeal with plant-based ingredients in aquafeeds, while economically and environmentally advantageous, can impair lipid metabolism and liver function in fish due to the lack of specific bioactive compounds such as bile acids (BAs). BAs are amphipathic steroid molecules that facilitate lipid digestion and act as signaling hormones, yet their optimal inclusion levels in conventional, balanced diets for Nile tilapia (Oreochromis niloticus) remain undefined. This study evaluated the effects of dietary BA inclusion on growth performance, feed efficiency, body composition, and serum biochemical parameters of juvenile Nile tilapia (GIFT strain, initial weight 18.04 ± 3.67 g) and estimated the optimal inclusion level. Six isoproteic (320 g kg−1) and isoenergetic (3300 kcal kg−1) plant-based diets were formulated with increasing BAs levels (0, 50, 100, 200, 400, and 600 mg kg−1) and fed to quadruplicate groups for 45 days. Only the Linear Response Plateau (LRP) model converged for weight gain data, estimating the optimal BA level at 479.70 mg kg−1, with a plateau weight gain of 76.60 g. Inclusion of the 50–600 mg kg−1 BAs significantly improved specific growth rate (up to 4.53%), crude protein retention efficiency (up to 81.11%), and whole-body crude protein content (up to 50.52%) compared to the control (p < 0.05). Fish fed 200 mg kg−1 BAs exhibited the highest protein retention and lowest ether extract deposition, indicating a protein sparing effect. Serum lipase activity increased proportionally with BAs levels, while alanine aminotransferase (ALT) and aspartate aminotransferase (AST) activities were reduced at 400 mg kg−1 BAs, reflecting improved liver health. No mortality was recorded. In conclusion, dietary BAs inclusion enhances growth, protein utilization, and hepatic function in juvenile Nile tilapia fed plant-based diets. The recommended optimal level is 479.70 mg kg−1 (dry matter basis), although significant benefits already occur from 50 mg kg−1. These findings support the strategic use of BAs to improve the sustainability and efficiency of tilapia production.