Dietary Malondialdehyde Impair Intestinal Health and Fillet Quality of Hybrid Grouper (Epinephelus fuscoguttatus♀ × E. lanceolatus♂)

Aquafeed kept at elevated temperatures and humidity can result in malondialdehyde (MDA) formation, adversely affecting aquafeed quality and triggering negative reactions in fish. To investigate the detrimental effects of dietary MDA on fish, six experimental diets with varying MDA levels (ranging from 0.03 to 17.72 mg/kg, on dietary crude lipid basis) were administered to three replicates of hybrid grouper for 8 weeks. Dietary inclusion of 4.43 mg/kg MDA significantly decreased serum complement 4 content and lysozyme activity, along with intestinal complement 3, complement 4, and immunoglobulin M contents. Furthermore, dietary inclusion of 8.86 mg/kg MDA significantly increased the activities of interleukin-1 receptor-associated kinase, ubiquitin-protein ligase, p38 mitogen-activated protein kinase, and tumor necrosis factor-α, downregulated the relative expression of <i>Occludin</i> but upregulated the relative expression of <i>HSP70</i> in the hindgut. Additionally, the highest inclusion of MDA (17.72 mg/kg) significantly upregulated the relative levels of pro-inflammatory cytokines (<i>IL-1β</i> and <i>TNF-α</i>), caused intestinal inflammation, and damaged the intestinal microbial structure and fish fillet texture. This study demonstrated a dose-dependent response of MDA on hybrid grouper. A low dietary dose of MDA (<2.21 mg/kg) exhibited minimal impact on immune response and fillet quality. However, higher inclusion levels (≥4.43 mg/kg) impaired the intestinal health and fillet quality. Consequently, the safety limit for MDA content in the diet for hybrid grouper has been established at 4.43 mg/kg based on dietary crude lipid basis.