Dietary Succinic Acid Enhances High-Starch Diet Utilization in Largemouth Bass (Micropterus salmoides) Through Gut-Liver Axis Modulation

A 10-week feeding trial was conducted to investigate the effects of succinic acid (SUA) supplementation in high-starch diets (HSD) on growth performance and enterohepatic health of largemouth bass (Micropterus salmoides; initial weight 11.96 ± 0.71 g). Six isocaloric and isolipidic diets were formulated, comprising: a standard diet containing 10% corn starch and 40% fish meal, an HSD with 15% corn starch and 36% fish meal, and HSDs supplemented with 0.5%, 1.0%, 1.5%, or 2.0% SUA. Compared to the standard diet, HSDs exhibited adverse effects including impaired feed utilization, growth retardation, diminished antioxidant capacity and immune response, along with metabolic dysregulation and gut microbiota disturbances. Significant linear and quadratic dose-responses (P < 0.05) were detected for multiple parameters: feed conversion ratio, hepatosomatic index, viscerosomatic index, glycogen content, amylase and protease activities, and hepatic antioxidant status. The 0.5% SUA supplementation group demonstrated: enhanced intestinal morphology; upregulated expression of tight junction proteins (ZO-1, Claudin-4) and anti-apoptotic Bcl2; increased abundance of beneficial microbiota; and suppressed endoplasmic reticulum stress markers (GRP78, PERK, IRE1, ATF6, eIF2α, Chopα). Additionally, dietary 0.5% SUA upregulated hepatic anti-inflammatory mediators (Nrf2, TGFβ1, IL10), glycolytic genes (PK, PFKL2, GK), β-oxidation-related CPT1, anti-apoptotic Bag, and glucose transporter GLUT2, while downregulating pro-inflammatory TNFα, gluconeogenic enzymes (PEPCK, G6Pase), lipogenic genes (ACC1, FASN), and pro-apoptotic Bad. Broken-line regression analysis identified 0.46–0.50% as the optimal SUA inclusion level based on growth parameters. This study demonstrates that appropriate SUA supplementation alleviates HSD-induced oxidative stress, enhances intestinal barrier function, modulates gut microbiota, and maintains metabolic homeostasis, thereby improving starch utilization and growth performance in largemouth bass.