Walnut Green Husk Extract Enhances Antioxidant, Anti-Inflammatory, and Immune Functions by Regulating Gut Microbiota and Metabolites in Fattening Pigs

This study investigates the effect of walnut green husk extract (WE) on gut microbiota, metabolites, and immune-antioxidant changes in fattening pigs through gut microbiota-metabolite interactions. A total of 60 healthy fattening pigs (Duroc × Landrace × Yorkshire) with an initial body weight of 65.2 ± 3.1 kg were randomly assigned to two groups (n = 30 per group): the control group (NC), which was fed a basal diet, and the WE group, which was fed the basal diet supplemented with 0.1% walnut green husk extract (WE). Dietary supplementation with 0.1% WE significantly increased the relative abundances of beneficial bacteria (e.g., <i>Firmicutes</i>, <i>Lactobacillus</i>) and reduced pathogenic bacteria (e.g., <i>Proteobacteria</i>, <i>Shigella</i>). Untargeted metabolomics identified 170 differentially accumulated metabolites, among which propionic acid-a key short-chain fatty acid with immunomodulatory effects-was significantly upregulated by 1.09-fold (<i>p</i> = 0.03) and showed a positive correlation with beneficial microbial abundances. These metabolites were enriched in glycerophospholipid and α-linolenic acid metabolism pathways, where eicosadienoic acid inhibited the nuclear factor kappa-B (NF-κB) pathway for anti-inflammatory effects, and methyl cinnamate synergistically regulated mitogen-activated protein kinase (MAPK) signaling with <i>Lactobacillus</i>. Serum analyses showed that WE significantly enhanced IgA, IgM, and IgG levels by 3.97-fold, 4.67-fold, and 4.43-fold (<i>p</i> < 0.01), reduced malondialdehyde (MDA) concentration by 82.8% (<i>p</i> < 0.01), and trended to improve antioxidant capacity via glutamine. Mechanistically, WE promoted short-chain fatty acid production by beneficial bacteria, forming a "microbiota-metabolite-immunity" cascade to enhance lipid metabolism and alleviate intestinal inflammation. These findings highlight that WE provides multi-omics evidence for its application as a functional feed additive.