Dietary Fiber Levels Modulate Intestinal Mucosal Architecture and the Microbiome–Metabolome Axis to Support Immune Homeostasis in Brooding Wanxi White Geese

Dietary fiber is a critical determinant of intestinal health, yet its optimal inclusion level for WWG during the critical brooding period remains undefined. This study aimed to evaluate the effects of varying dietary CF levels (approximately 3%, 5%, and 9%) on the intestinal morphology, immune function, and microbiome-metabolome axis of brooding WWG. A total of 120 one-day-old goslings were randomly assigned to the three dietary treatments for a 28-day trial. Histological analysis revealed that the 9% CF diet significantly improved gut morphology, yielding superior villus-to-crypt ratios in the jejunum and ileum. Molecular assays indicated that higher fiber levels (5–9%) upregulated the expression of nutrient transporters (SGLT1 and GLUT2). Concurrently, the 9% CF diet effectively suppressed the potent pro-inflammatory cytokine TNF-α in the jejunum while appropriately upregulating IL-6 and NF-κB, indicating enhanced mucosal immune vigilance and structural maturation. Multi-omics integration (shotgun metagenomics and LC-MS metabolomics) demonstrated that specific fiber levels significantly shifted microbial abundances, specifically enriching Bacteroidetes and Actinobacteria. These microbial shifts were strongly correlated with enriched metabolic pathways, notably lysine biosynthesis and purine metabolism, which synergistically support mucosal homeostasis. Collectively, these findings demonstrate that a 9% dietary CF inclusion is an effective nutritional strategy to optimize intestinal architecture and microbial-metabolic profiles in brooding WWG.