Dietary Supplementation With Olive Leaf Extract Alleviated Lipid Accumulation and Improved Growth and Intestinal Flora in Trachinotus ovatus Fed High-Fat Diet.

With the application of high-fat diets (HFDs) leading to increased prevalence of fatty liver in aquaculture, plant extracts such as olive (Olea Europaea L.) leaf extract (OLE) have attracted much attention due to their potential bioactive effects. In the present study, five experimental groups were established with HFD (15.5% crude lipid) supplemented with 0.0%, 0.5%, 1.0%, 1.5%, and 2.0% OLE, respectively, and each was fed to three replicate cages containing 20 juvenile Trachinotus ovatus (initial body weight: 11.57 ± 0.19 g) per cage. Fish fed HFD supplemented with 1.0% OLE showed significantly increased growth and survival and reduced lipid content in whole fish and triglyceride (TG) and cholesterol contents in serum compared to fish fed HFD (p < 0.05). Analysis of liver gene expression showed that reduced lipid content might be associated with reduced expression of genes associated with lipid synthesis (sterol regulatory element binding protein 1 [srebp1], fatty acid synthase [fas], acetyl-CoA carboxylase [acc], and diacylgycerol acyltransferase 1 [dgat1]), and increased expression of genes associated with fatty acid oxidation (peroxisome proliferator-activated receptor α [pparα], hormone-sensitive lipase [hsl], and carnitine palmitoyl transferase-1 [cpt1]) and lipid transport (CD36 molecule [cd36] and fatty acid binding protein 1 [fabp1]). Inclusion of OLE not only significantly lowered serum aspartate aminotransferase (AST) and alanine aminotransferase (ALT) activities but also increased activities of serum alkaline phosphatase (AKP) and superoxide dismutase (SOD) and total antioxidant capacity (TAOC), while serum malondialdehyde (MDA) decreased in fish fed HFD with 0.5%-1.0% OLE compared to fish fed HFD (p < 0.05). Additionally, 0.5%-1.0% dietary OLE significantly enhanced the integrity of intestinal morphology, reduced intestinal lipase activity and expression of inflammatory factors (interleukin-1 [il-1] and il-8), and increased the diversity of intestinal microbiota (p < 0.05). The abundance of beneficial bacterial taxa (Bacteroides) was increased, while harmful bacterial taxa (Mycoplasma) decreased in T. ovatus fed diets with OLE compared to fish fed HFD. The data suggested that dietary OLE (particularly 1.0%) alleviated the negative effects of HFD in T. ovatus, which has highlighted the positive impacts of dietary OLE in fish and provided a new development opportunity for the olive industry through utilization of production waste.