**Background:** Taurine is a sulphur-containing amino acid that is virtually absent in terrestrial plant ingredients but abundant in marine feedstuffs. It plays critical physiological roles, including bile-salt conjugation, which is essential for dietary lipid emulsification, digestion, and absorption. Marine fish have limited capacity for endogenous taurine synthesis, making it a conditionally essential nutrient when fishmeal is replaced by plant proteins. Senegalese sole (Solea senegalensis) is a marine flatfish of high commercial value in Southern Europe but has low tolerance to dietary lipids. This study aimed to evaluate whether taurine supplementation to low-fishmeal, plant-protein-based diets could improve taurine, bile-acid, and lipid metabolism in this species.
**Methods:** Four isonitrogenous (~55% crude protein) and isolipidic (8.6% crude fat) diets were formulated: a fishmeal-based diet (FM, 0.40% taurine), a plant-protein-based diet replacing 85% of marine ingredients (PP0, 0.08% taurine), and two taurine-supplemented versions of PP0 (PP0.5, 0.43% taurine; PP1.5, 1.40% taurine). Each diet was fed to triplicate tanks of 35 juvenile Senegalese sole (initial body weight 13.7 ± 5.0 g) for 6 weeks in a recirculated aquaculture system. At the end of the trial, fish were sampled for taurine content (liver and whole body), bile-acid concentration in gallbladder bile, plasma triglycerides and total cholesterol, hepatic gene expression (cyp7a1, taut, abcc2, abcb11, apoa1, apoa4, apob100, mtp, vldlr, plin2), and histological analysis of liver. A separate metabolic trial using 14C-triolein was conducted to quantify triolein evacuation, catabolism (14CO2 production), and retention in liver, viscera, and body over 24 h.
**Key Results:** Fish fed the PP0 diet had significantly lower taurine concentrations in liver and whole body compared with FM-fed fish (p < 0.05), and taurine supplementation produced a linear increase in tissue taurine (R² = 0.975 for liver, R² = 0.984 for body; p < 0.001). Bile-acid concentration in bile also increased linearly with dietary taurine (R² = 0.973; p < 0.001). Hepatic fat content was significantly higher in PP0-fed fish than in FM-fed fish (p < 0.001), but decreased with taurine supplementation such that PP1.5 was not significantly different from FM. Plasma triglycerides were significantly elevated in PP0-fed fish compared with FM (p = 0.004), and this effect was reversed by taurine supplementation, with PP1.5 not differing from FM. Plasma total cholesterol was unaffected by diet (p = 0.853). Gene expression analysis showed downregulation of cyp7a1 and abcb11 in PP0-fed fish relative to FM; taurine supplementation (PP0.5) significantly upregulated both genes, though expression remained intermediate. The vldlr gene was downregulated in PP0-fed fish (p = 0.001) and restored by taurine supplementation. PCA of gene expression data clearly separated FM and PP0 clusters, with taurine-supplemented groups positioned intermediately. In the metabolic trial, triolein catabolism was significantly reduced in PP0-fed fish compared with FM (p = 0.045), while taurine-supplemented groups showed intermediate values. Hepatic 14C-triolein retention was significantly higher in PP1.5 than in all other treatments (p = 0.011). Weight gain was significantly lower in PP0 and PP0.5 groups compared with FM (p = 0.002), but PP1.5 restored weight gain to levels not significantly different from FM. Hepatosomatic and viscerosomatic indexes were significantly higher in all plant-based diet groups regardless of taurine supplementation (p < 0.001).
**Clinical Implications:** This study demonstrates that high replacement of fishmeal with plant proteins in diets for Senegalese sole leads to taurine deficiency, reduced bile-acid synthesis, impaired lipid catabolism, hepatic fat accumulation, and elevated plasma triglycerides. Taurine supplementation at 1.5% of the diet (3.5 times the level in a fishmeal-based diet) effectively restored bile-acid concentrations, improved lipid utilisation, reduced hepatic fat and plasma triglycerides, and normalised growth performance. These findings support the use of taurine as a conditionally essential nutrient in plant-based aquafeeds for marine carnivorous fish. The linear relationship between dietary taurine and tissue taurine content also suggests potential human health benefits, as taurine-enriched fillets could provide an additional dietary source of this hypolipidemic compound. Further research is warranted to explore taurine supplementation in high-lipid diets for Senegalese sole and to evaluate long-term effects on liver health and market-size production.