The goal of the present study was to find whether higher soybean meal (SBM) levels might trigger soybean meal-induced enteritis (SBMIE) in turbot. If so, caution must be taken when mixing ingredients containing saponins and other antinutrients to avoid SBMIE like symptoms. In a 8 week feeding trial conduced on turbot, three isonitrogenous and isolipidic diets were formulated to include 26%, 40% and 54% SBM to progressively replace 30%, 45% and 60% fish meal (FM) in a FM based diet, respectively. The results showed that SBM caused dose-dependent decreases in growth performance and nutrient utilization. Enteritis developed in the distal intestine in the inclusion range of 26-54%. Dose-dependent increases in severity of the inflammation, with concomitant alterations in brush border membrane enzymes and inflammatory marker genes expression were seen. Our results confirm the hypothesis that high inclusion level of SBM may cause similar inflammatory changes as observed in several other fish species. Thus, caution must be taken when formulating turbot diets based on ingredients that may contain saponins and other antinutrients. Moreover, turbot is also a candidate species for the study of causes and mechanism of diet induced inflammation in the intestine of fish.

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Soybean meal-induced enteritis (SBMIE) is a well-described condition in the distal intestine (DI) of several cultured fish species, but the exact cause is still unclear. The work on Atlantic salmon and zebrafish suggested soya-saponins, as heat-stable anti-nutritional factors in soybean meal, are the major causal agents. However, this conclusion was not supported by the research on some other fish, such as gilthead sea bream and European sea bass. Our previous work proved that soybean could induce SBMIE on turbot and the present work aimed to investigate whether soya-saponins alone could cause SBMIE and the effects of soya-saponins on the intestinal barrier function in juvenile turbot. Turbots with initial weight 11.4 ± 0.02 g were fed one of four fishmeal-based diets containing graded levels of soya-saponins (0, 2.5, 7.5, 15 g kg) for 8 weeks. At the end of the trial, all fish were weighed and plasma was obtained for diamine oxidase (DAO) activity and d-lactate level analysis and DI was sampled for histological evaluation and quantification of antioxidant parameters and inflammatory marker genes. The activities of superoxide dismutase, catalase, glutathione peroxidase, glutathione reductase and intestinal glutathione level were selected to evaluated intestinal antioxidant system. The distal intestinal epithelial cell (IEC) proliferation and apoptosis were investigated by proliferating cell nuclear antigen (PCNA) labelling and TdT-mediated dUTP nick end labeling (TUNEL), respectively. The results showed that soya-saponins caused significantly dose-dependent decrease in the growth performance and nutrient utilization (p < 0.05). Enteritis developed in DI of the fish fed diet containing soya-saponins. Significantly dose-dependent increases in severity of the inflammation concomitant with up-regulated expression of il-1β, il-8, and tnf-α, increased IEC proliferation and apoptosis, and decreases in selected antioxidant parameters were detected (p < 0.05). The epithelial permeability (evaluated by the plasma DAO activity and d-lactate level) was significantly increased with the increasing of dietary level of soya-saponins (p < 0.05), which was concomitant with the destroyed the intracellular junctions. In conclusion, the present work proved that soya-saponins induced enteritis and compromised the intestinal barrier functions. Based on the present work, strategies focus on regulation of cell apoptosis, epithelial permeability, intracellular junctions and redox homeostasis worth further investigating to develop new and efficient ways for SBMIE alleviation.