Simple SummaryIn contemporary dietary formulations for aquaculture, the amounts of fishmeal are being constantly reduced, and the inclusion of plant-protein sources is now a standard procedure. This approach may result in an unbalanced supply of selected nutrients, from which taurine was identified. Taurine is an amino acid that plays important physiological roles and is included in bile salts, which are essential for the emulsion, digestion, and absorption of dietary lipids and fat-soluble vitamins. In mammals, a hypolipidemic effect of taurine has been described. Senegalese sole (Solea senegalensis) is a marine fish species with increasing importance for aquaculture production in Southern European countries, with a high market value and low tolerance to dietary lipid levels. Thus, this study tested the effects of taurine supplementation to low-fishmeal diets on the physiological responses of Senegalese sole, with an emphasis on lipid metabolism. The results show that dietary inclusion of high levels of plant-protein sources to replace marine ingredients resulted in negative effects on lipid metabolism, due to the resultant low bile-acid concentration and/or the limited availability of taurine for bile-salt emulsification. Taurine supplementation mitigated part of the negative effects of plant-based diets, leading to better lipid utilisation.AbstractTaurine is a sulphur-containing amino acid with important physiological roles and a key compound for the synthesis of bile salts, which are essential for the emulsion and absorption of dietary lipids. This study aimed to evaluate the effects of taurine supplementation to low-fishmeal diets on the metabolism of taurine, bile acids, and lipids of Senegalese sole. A fishmeal (FM) and a plant-protein-based (PP0) diet were formulated, and the latter was supplemented with taurine at 0.5 and 1.5% (diets PP0.5 and PP1.5). Diets were assigned to triplicate tanks containing 35 fish (initial weight ~14 g) for 6 weeks. Fish from the PP0 treatment presented lower taurine and bile-acid concentrations compared with the FM treatment, and a downregulation ofcyp7a1andabcb11was observed. Triolein catabolism decreased in PP0-fed fish, resulting in increased hepatic fat content and plasma triglycerides, while no effects on plasma cholesterol were observed. Taurine supplementation to plant-based diets resulted in a higher taurine accumulation in fish tissues, increased bile-acid concentration, and upregulation ofcyp7a1andabcb11. Hepatic fat content and plasma triglycerides decreased with increasing dietary taurine supplementation. Taurine supplementation mitigated part of the negative effects of plant-based diets, leading to better lipid utilisation.