Effects of adding quercetin to soybean meal-based feed on the growth, antioxidant capacity, and gut and liver health of juvenile turbot

Introduction The antioxidant and anti-inflammatory properties of quercetin and their regulatory mechanisms had been extensively studied in aquatic animals. However, limited information was available regarding the effects of quercetin in farmed fish fed diets with high soybean meal inclusion. The purpose of this study was to investigate the effects of adding quercetin to soybean meal-based feed on the growth, antioxidant capacity, and gut and liver health of juvenile turbot. Methods Five isonitrogenous and isolipidic experimental diets were formulated: a fish meal diet (FM), a diet in which 40% of the fish meal was replaced with soybean meal (SBM), and three diets based on the SBM diet, each supplemented with quercetin at graduated levels of 0.05%, 0.1%, and 0.2%, designated as Q0.05, Q0.1, and Q0.2, respectively. These diets were randomly assigned to 15 tanks (200 L), with 30 juvenile turbot (initial weight 8.41 ± 0.52 g) reared in each tank for a feeding trial of 70 days. Results The results showed that, compared with the FM group, the specific growth rates (SGR) in the SBM group decreased significantly ( P < 0.05). As dietary quercetin levels increased, SGR exhibited an increasing trend, although no significant differences were observed among groups. Among the antioxidant indicators, there was no significant difference in MDA levels between the FM and SBM groups. After the addition of quercetin, MDA levels in the quercetin-supplemented groups decreased significantly, and the Q0.1 group had the lowest MDA levels ( P < 0.05). Simultaneously, the Q0.1 group had the highest total antioxidant capacity (T-AOC) levels. Among the gut morphological indicators, the FM group and the quercetin-supplemented groups both showed significantly higher villus height, muscular layer thickness, and goblet cell count than the SBM group ( P < 0.05). Furthermore, the Q0.2 group had the highest villus height and goblet cell count ( P < 0.05), and the Q0.05 group had the highest muscular layer thickness ( P < 0.05). Among the indicators for gut inflammatory genes and tight junction protein genes, the results for both the FM group and the quercetin-supplemented groups were significantly better than those of the SBM group ( P < 0.05). claudin-7 and occludin mRNA expression in the Q0.05 group were significantly upregulated compared to the SBM group ( P < 0.05). Among the gut microbiota indicators, the FM group had significantly higher Shannon index and Simpson index than the SBM group ( P < 0.05), and the Q0.1 group had the highest Shannon index and Simpson index compared to the SBM group ( P < 0.05). At the same time, the gut microbiota composition in the Q0.1 group showed significant improvement. Serum level of lipopolysaccharide (LPS) in the FM and quercetin supplement groups were significantly lower than those in the SBM group ( P < 0.05), and the LPS in the Q0.1 group was significantly lower than those in the SBM group ( P < 0.05). Serum level of diamine oxidase (DAO) in the Q0.1 was significantly lower than those in the SBM group ( P < 0.05). Alanine aminotransferase (ALT) activity and aspartate aminotransferase (AST) activity in the FM group were significantly lower than those in the SBM group ( P < 0.05), the ALT activity in the Q0.2 group were significantly lower than those in the SBM group ( P < 0.05), and AST activity in the Q0.05 group were significantly lower than those in the SBM group ( P < 0.05). In the Q0.05 group, both γ-glutamyltransferase (γ-GT) activity and triglycerides (TG) content were significantly lower than those in the SBM group ( P < 0.05). Liver sections revealed that, compared with the FM group, the SBM group exhibited partial vacuolization of hepatocytes, swelling and rupture of the endoplasmic reticulum, widespread lipid deposition, and elevated levels of reactive oxygen species (ROS). After adding quercetin, the above conditions improved effectively. Furthermore, compared with the SBM group, lipid deposition was significantly reduced in the Q0.1 group, and ROS levels was significantly reduced in the Q0.05 group ( P < 0.05). Regarding the expression of inflammatory genes in the liver, expression levels in the SBM group were generally higher than those in the FM and the quercetin-supplemented group. The tnf-α and stat3 mRNA expression levels in the Q0.1 group were significantly lower than in the SBM group ( P < 0.05). Discussion In summary, dietary quercetin supplementation in diets with 40% fish meal replacement by soybean meal significantly enhanced antioxidant capacity and protected gut and liver health in turbot, with concomitant improvements in growth performance. The gut–liver axis was possibly involved in this regulatory process. The optimal dietary quercetin supplementation level was 0.2% in the present study.