Growth performance, lipid deposition and hepatic lipid metabolism related gene expression in juvenile turbot (Scophthalmus maximus L.) fed diets with various fish oil substitution levels by soybean oil

• Higher dietary SO level (> 66.7%) reduces growth and muscle quality of turbot.
• Increasing hepatic lipid deposition of turbot fed with high SO relates to FAS.
• Increasing hepatic lipid deposition of turbot fed with high SO relates to CPT I.

A 92-day feeding experiment was conducted to investigate the effects of dietary soybean oil (SO) on growth performance, fatty acid composition, lipid deposition and hepatic lipid metabolism related gene expression in juvenile turbot (Scophthalmus maximus L.) (mean initial body weight, 5.88 ± 0.02 g). Three experimental diets were formulated with substitution of 33.3%, 66.7% and 100% fish oil (FO) by SO. Each diet was randomly fed to triplicate tanks, and each tank was stocked with 35 fish. The results showed that specific growth rate of turbot fed diets with 100% SO was significantly lower than that in the 33.3% and 66.7% SO groups. Fatty acid (FA) composition of total lipid in the liver and muscle was closely correlated with those in diets. The lipid content in the liver and muscle significantly increased with increasing dietary SO level. The activity of LPL in the liver of turbot was not significantly affected by dietary SO level. Relative gene expression of LPL, PPARα, FAS and MTP significantly increased, while relative expression of LXR and CPT I significantly decreased with increasing dietary SO level. There was no significant difference in the expression of SREBP-1 among dietary treatments. These results suggested that the growth of turbot fed diets with 100% SO was significantly lower than the low SO group. The increase of lipid deposition in the liver of turbot fed diets with higher dietary SO level would be related to the up-regulation of fatty acid synthesis-related gene (FAS) and the down-regulation of fatty acids oxidation gene (CPT I) expression.