The compositional and metabolic responses of gilthead seabream (Sparus aurata) to a gradient of dietary fish oil and associated n-3 long-chain polyunsaturated fatty acid content

The replacement of fish oil (FO) with vegetable oil (VO) in feed formulations reduces the availability of n-3 long-chain polyunsaturated fatty acids (LC-PUFA) to marine fish such as gilthead seabream. The aim of this study was to examine compositional and physiological responses to a dietary gradient of n-3 LC-PUFA. Six isoenergetic and isonitrogenous diets (D1-D6) were fed to seabream, with the added oil being a blend of FO and VO to achieve a dietary gradient of n-3 LC-PUFA. Fish were sampled after four months feeding, to determine biochemical composition, tissue fatty acid concentrations and lipid metabolic gene expression. The results indicated a disturbance to lipid metabolism, with fat in the liver increased and fat deposits in the viscera reduced. Tissue fatty acid profiles were altered towards the fatty acid compositions of the diets. There was evidence of endogenous modification of dietary PUFA in the liver which correlated with the expression of fatty acid desaturase 2 (fads2). Expression of sterol regulatory element-binding protein 1 (srebp1), fads2 and fatty acid synthase increased in the liver, while peroxisome proliferator-activated receptor alpha 1 pathways appeared to be supressed by dietary VO in a concentration-dependent manner. The effects in lipogenic genes appear to become measurable in D1-D3, which agrees with the weight gain data suggesting that disturbances to energy metabolism and lipogenesis may be related to performance differences. These findings suggested that suppression of beta-oxidation and stimulation of srebp1-mediated lipogenesis may play a role in contributing toward steatosis in fish fed n-3 LC-PUFA deficient diets