Interactive effects of dietary lipid and nutritional emulsifier supplementation on growth, chemical composition, immune response and lipid metabolism of juvenile Nile tilapia (Oreochromis niloticus)

The objective of this study was to investigate the interactive effects of supplementation with different levels of dietary lipids and a commercial nutritional emulsifier (NE) on the growth, feed utilization, immune response and lipid metabolism of juvenile Nile tilapia (Oreochromis niloticus). A 5 × 2 factorial design was formulated with five lipid supplement levels (0, 15, 30, 45 and 60 g/kg) and NE supplementation (with or without NE at 0.35 g/kg). Nile tilapia with an initial average body weight of 8.06 g were randomly placed in 30 cages and offered one of the ten diets for 8 weeks. The results show that dietary NE supplementation significantly increased the final weight, weight gain and specific growth rate compared with fish fed diets without NE (P<0.05). Interestingly, dietary NE supplementation significantly improved the feed conversion ratio (P<0.05). The supplementation of diets with NE significantly reduced the cholesterol levels (P<0.05). The activities of catalase, superoxide dismutase and glutathione peroxidase significantly decreased as dietary lipid supplement levels increased, but they were significantly improved by dietary NE supplementation (P<0.05). The results also show that the liver malondialdehyde (MDA) content significantly increased as lipid supplement levels increased, while NE supplementation resulted in significantly decreased MDA levels (P<0.05). As dietary lipid supplement levels increased, the crude lipid content of the fillet significantly increased in fish fed a diet supplemented with NE (P<0.05). Moreover, lipase activity was significantly increased when the diet was supplemented with NE (P<0.05). A total of 20 fatty acids were measured in the fillets of fish fed diets supplemented with different lipid levels and NE. Significantly higher levels of polyunsaturated fatty acids, particularly linoleic acid (18:2 n6c), γ-linolenic acid (C18:3 n6) and α-linolenic acid (C18:3 n3), were found in fish fed diets supplemented with NE. In summary, dietary NE supplementation has a positive effect on feed digestion and absorption, resulting in improved fish growth performance, immune response and nutritional value of the muscle due to increased essential fatty acid contents. NE supplementation is particularly effective when it is added to diets with low lipid levels.