Effects of Schizochytrium sp. oil combined with exogenous EPA on fatty acid composition, FAD and ELO gene expression and lipid metabolism of juvenile Ctenopharygodon idella

The grass carp (Ctenopharyngodon idella) dietary Schizochytrium sp. stearin oil may show better growth performance than dietary fish oil by saving energy required for de novo synthesis of DHA, promoting lipolysis energy supply and inhibiting protein catabolism. The purpose of this study was to investigate whether DHA-rich Schizochytrium sp. stearin oil needs to be compatible with EPA and its possible mechanism. Equipped with five groups of iso-nitrogen and isoenergetic diets and fed grass carp (22.70 g±0.80 g) for 49 d. The 0.52% n-3 long-chain polyunsaturated fatty acid (LC PUFA) of the diets were provided with fish oil (F-O); Sc stearin oil (S-O); stearin oil (DHA)∶EPA =3∶2 (SE1-O); stearin oil (DHA)∶EPA = 1∶1(SE2-O); EPA (E-O), respectively. The results showed: 1) There was no difference in the weight gain rate (WG), specific growth rate (SGR) and feed conversion ratio (FCR) among all the groups; 2) The content of crude protein in muscle of S-O group was significantly higher than those of E-O group; 3) the content of DHA in muscle of S-O group was significantly higher than that of F-O group, SE1-O group and E-O group; 4) there was no significant difference in atherogenicity index among the groups, the thrombogenicity index of SE2-O group was significantly higher than that of F-O group, the hypocholesterolemic/ Hypercholesterolemic ratio in S-O group and SE2-O group was markedly lower than those in F-O group; 5) the adipose size of intraperitoneal fat in E-O group was notably higher than those in F-O group and SE1-O group, the adipose TAG lipase(ATGL)and carnitine palmitoyltransferase 1(CPT1) mRNA levels in E-O group were apparently down-regulated; in muscle, the fatty acid desaturase (FAD)mRNA level was markedly higher in E-O group than that in S-O group, the fatty acid elongase(ELO)mRNA level was obviously down-regulated in S-O group. Studies have shown that the use of Sc oil alone or in combination with EPA had no significant effect on growth, n-3LC PUFA content and lipid hydrolysis of adipose tissue of grass carp. When EPA was the sole source of n-3LC PUFA, the lipid hydrolysis of adipose tissue was reduced and the content of crude protein in muscle was reduced. High level of DHA in the diet would weaken the body's ability to synthesize LC PUFA. Compared to EPA, grass carp may need DHA more, and the Sc oil could be used alone in the feed of grass carp without combination with EPA.