Malic acid (MA) is one of the organic acids that has been less studied for its effects on fish growth performance and gut health. The present study aimed to assess the effects of dietary MA supplementation on growth performance, antioxidant and immunological parameters, and intestinal gene expressions in rainbow trout, Oncorhynchus mykiss. Fish with ∼25 g body weight were randomly distributed into 12 tanks at a density of 15 fish per tank and were fed with diets containing 0 (control), 0.25, 0.5, and 1% MA over 8 weeks. The results indicated that dietary MA persuaded no significant changes in growth performance, feed intake and survival rate. There were elevations in serum alanine aminotransferase (ALT; 0.5 and 1% MA), aspartate aminotransferase (AST; 0.5 and 1% MA), lysozyme (0.25, 0.5 and 1% MA), catalase (CAT; 0.5 and 1% MA), superoxide dismutase (SOD; 1% MA), glutathione peroxidase (GPx; 0.25 and 0.5% MA) activities, and total immunoglobulin (Ig) level (0.5% MA) in the fish MA-supplemented diets, compared to the control. Serum alkaline phosphatase (ALP) activity showed a decrease in 0.5% MA treatment compared to the control. Skin mucus lysozyme (0.25% MA), alternative complement activity (ACH50; 0.5% MA), and total Ig (0.25 and 0.5% MA) showed significant elevations, whereas skin mucus ALP (0.5% MA) exhibited a significant decrease in MA-treated fish, compared to the control fish. Intestinal il-1β (0.5 and 1% MA), hsp70 (0.5% MA), tnf-α (0.5% MA), tgf-β (1% MA), caspase 3 (1% MA), and caspase 9 (1% MA) expressions significantly increased in M-treated fish, compared to the control. In conclusion, this study showed that dietary inclusion of malic acid at the rate of 0.25% in O. mykiss diets improved the antioxidant status and immune responses of fish without any negative effect on growth performance.

苹果酸 (MA) 是一种有机酸，其对鱼类生长性能和肠道健康的影响研究较少。本研究旨在评估日粮 MA 补充剂对虹鳟鱼Oncorhynchus mykiss生长性能、抗氧化和免疫参数以及肠道基因表达的影响。. 体重约 25 克的鱼被随机分配到 12 个鱼缸中，每个鱼缸的密度为 15 条鱼，并在 8 周内饲喂含有 0（对照）、0.25、0.5 和 1% MA 的日粮。结果表明，日粮 MA 在生长性能、采食量和成活率方面没有显着变化。血清丙氨酸氨基转移酶（ALT；0.5 和 1% MA）、天冬氨酸氨基转移酶（AST；0.5 和 1% MA）、溶菌酶（0.25、0.5 和 1% MA）、过氧化氢酶（CAT；0.5 和 1% MA）升高，与对照相比，在鱼 MA 补充日粮中，超氧化物歧化酶 (SOD; 1% MA)、谷胱甘肽过氧化物酶 (GPx; 0.25 和 0.5% MA) 活性和总免疫球蛋白 (Ig) 水平 (0.5% MA)。与对照相比，血清碱性磷酸酶 (ALP) 活性显示 0.5% MA 处理降低。皮肤粘液溶菌酶（0.25% MA），与对照鱼相比，替代补体活性（ACH50；0.5% MA）和总 Ig（0.25 和 0.5% MA）显着升高，而皮肤粘液 ALP（0.5% MA）在 MA 处理的鱼中显着降低。肠il - 1 β (0.5 和 1% MA)、hsp70 (0.5% MA)、tnf-α (0.5% MA)、tgf-β (1% MA)、caspase 3 (1% MA) 和caspase 9 (1与对照相比，M 处理的鱼中 % MA) 的表达显着增加。总之，这项研究表明，在O. mykiss日粮中添加 0.25% 的苹果酸可改善鱼的抗氧化状态和免疫反应，而对生长性能没有任何负面影响。