Faecalibacterium-derived spermidine mediates the amelioration of fatty liver hemorrhagic syndrome by inulin in laying hens

Fatty liver hemorrhagic syndrome (FLHS) is a critical disease threatening the laying hen industry. Inulin, a widely used prebiotic, has shown promise in alleviating metabolic disorders, but its role in mitigating FLHS in laying hens is not fully understood. Here, we investigated the effects and underlying mechanisms of inulin-mediated alleviation of FLHS in a high-carbohydrate low-protein diet (HCD)-induced laying hen model. We found that inulin supplementation significantly ameliorated HCD-induced hyperlipidemia, hyperglycemia, hepatic steatosis, liver injury, and oxidative stress. These phenotypic improvements were accompanied by enhanced fatty acid oxidation and suppressed lipid synthesis and inflammation. Microbiota analysis revealed that inulin reshaped the HCD-perturbed cecal microbiota, with Faecalibacterium identified as the only dominant genus substantially depleted by HCD and restored by inulin. Targeted metabolomics showed that inulin elevated cecal spermidine levels, which strongly correlated with Faecalibacterium abundance and improved metabolic traits. Fecal microbiota transplantation (FMT) from inulin-treated donors replicated the protective effects, confirming the causal role of gut microbiota in mediating inulin's anti-FLHS activity. Further mechanistic investigation using the representative species Faecalibacterium prausnitzii demonstrated that inulin enhanced spermidine production through transcriptional activation of the spermidine biosynthetic pathway. Spermidine, in turn, upregulated hepatic ALDH1A2 expression, enhancing retinoic acid synthesis and activating the AMPK-SIRT1 axis, thereby reducing lipid accumulation in hepatocytes. Collectively, these findings establish a novel Faecalibacterium-spermidine-ALDH1A2-retinoic acid-AMPK-SIRT1 axis through which inulin alleviates FLHS, highlighting inulin as a dietary intervention targeting the gut-liver axis and offering novel therapeutic avenues for preventing this disorder in laying hens.