The individual and synergistic effects of dietary lipids and DHA on the growth performance, feed utilization, lipid metabolism, antioxidant capacity, and immune responses of grass carp (Ctenopharyngodon idellus)

Abstract

This study employed a 2 × 3 factorial design to formulate six diets (designated as NFD-DHA0, NFD-DHA0.5, NFD-DHA1.0, HFD-DHA0, HFD-DHA0.5, and HFD-DHA1.0), where NFD denotes a normal-fat diet (5.0 % lipid) and HFD denotes a high-fat diet (10.0 % lipid). These diets were administered to grass carp with an initial body weight of 229.71 ± 0.38 g for 8 weeks to assess the independent and synergistic effects of dietary lipids and DHA on growth performance, feed utilization, and liver health. The results reveal a significant synergistic interaction between dietary lipid levels and DHA content, which affected growth performance, feed utilization, lipid metabolism, and antioxidant capacity (P < 0.05). Supplementing both NFD and HFD diets with DHA significantly enhanced the growth performance of grass carp (P < 0.05), while markedly reducing their Feed conversion ratio (FCR), condition factor (CF), visceral index (VSI), and hepatosomatic index (HSI) (P < 0.05). At a 10 % lipid level, 1.0 % DHA not only decreased hepatic crude lipid (10.86 ± 0.98 %) and triglyceride levels (0.14 ± 0.01 μmol/g protein) by enhancing the expression of genes involved in lipolysis (hsl: 1.93 ± 0.14; atgl: 1.36 ± 0.03) and lipid utilization (pparα: 2.34 ± 0.25; cpt-1: 1.43 ± 0.09; echs1: 1.83 ± 0.06; and acad9: 1.40 ± 0.16) (P < 0.05), but also alleviated hepatic inflammation by increasing antioxidant enzyme activity (SOD: 115.18 ± 1.92 U/mg protein, CAT: 26.87 ± 0.85 U/mg protein) and the mRNA expression (sod: 1.59 ± 0.09, cat: 1.92 ± 0.03) of anti-inflammatory factors (P < 0.05). Notably, at a 5 % lipid level, the NFD-DHA0.5 group exhibited the optimal growth performance, lipid utilization, and antioxidant capacity. In conclusion, supplementing 0.5 % DHA in a 5 % lipid diet or 1 % DHA in a 10 % lipid diet can optimize both growth and liver health in grass carp, providing valuable insights for the formulation of fishmeal-free aquaculture feeds.