Effect of Dietary Protein and Lipid Level on Growth, Antioxidant, and Gene Expression of Juvenile <i>Parabramis pekinensis</i>

Unreasonable ratio of protein to lipid in feeds could affect growth, antioxidant, and related pathway genes expression. This study aimed to investigate the suitable proportion of protein to lipid in feed with <i>Parabramis pekinensis</i>. The ratio protein-lipid (<i>P</i>/<i>L)</i> indicated by G1 (2.52), G2 (3.16), G3 (4.03), G4 (5.33), G5 (7.49), and G6 (11.67), which were fed to <i>P. pekinensis</i> (80 ± 10.52 g) for 56 days. The present results showed that diets with a protein-to-lipid ratio of approximately 3.5:1 (35% protein and 10% lipid, or less) were optimal for enhancing growth parameters, including body weight, WGR, PER, VSI, HSI, SGR, and feed conversion ratio (FCR). The quadratic regression analysis of FCR and protein efficiency ratio (PER) in <i>P. pekinensis</i> showed that <i>P/L</i> ratio performed best around 5.33. As the <i>P/L</i> ratio in feeds turned down, the best growth performance appeared at about 5.33 (<i>p</i> < 0.05), which was due to the unbalanced feed protein and fat levels. Meanwhile, <i>P/L</i> in 5.33 group exerted a protective function against oxidative damage in <i>P. pekinensis</i>. In addition, the increased antioxidant capacity contributed to the growth performance of the fish in 5.33 group, which showed the connection obviously. Thus, the connection existed in target of rapamycin (TOR) and Nrf2 signaling pathway, which was downregulated when the <i>P/L</i> ratio was around 2.52 and 11.67. On the contrary, the <i>P/L</i> ratio around 5.33 could enhance the expression of <i>tor</i> and <i>s6k1</i> to improve the growth of <i>P. pekinensis</i>. In the Nrf2 signaling pathway, the expression of <i>keap1</i>, <i>sod1</i>, and <i>gpx</i> affected antioxidant ability and the <i>P/L</i> ratio from 4.03 to 7.49 could be able to balance the antioxidant capacity, maintaining in normal level of <i>P. pekinensis</i>.