Laminaria extract reshapes the gut microbiota and TCA cycle–centered metabolic profiles associated with immune modulation in Hu lambs

BACKGROUND: The global push to restrict antibiotic use in livestock has intensified the need for effective, safe feed additives, particularly for young lambs, which are highly susceptible to post-weaning diarrhea and immune dysfunction due to an underdeveloped intestinal barrier. Laminaria extract, rich in bioactive polysaccharides, has emerged as a promising candidate; however, its functional mechanisms in ruminants remain poorly understood. This study therefore investigated the effects of dietary Laminaria extract supplementation on growth performance, immune parameters, gut microbiota, and fecal metabolomic profiles in lambs. METHODS: Lambs were randomly assigned to five groups: a basal control (CON), three Laminaria extract doses (LL, 1 g/kg; LM, 3 g/kg; LH, 6 g/kg), and a diclazuril positive control (DIC). Growth performance was evaluated via body weight and average daily gain (ADG). Serum immune cytokines were measured by ELISA, gut microbiota via 16S rRNA sequencing, and fecal metabolomics by UPLC-MS, followed by integrative correlation and pathway analyses. RESULTS: Dietary Laminaria supplementation improved growth performance and immune markers in a dose-dependent manner, with the high-dose (6 g/kg) group exhibiting the most pronounced effects. Specifically, the high-dose group showed a 217.47% increase in ADG compared with the CON group (p < 0.05). Microbiome analysis revealed a selective enrichment of beneficial genera, particularly Prevotella and Muribaculum, in the supplemented groups, with the highest relative abundances observed at the 6 g/kg dosage. Furthermore, untargeted metabolomics revealed that the high-dose group exhibited the most substantial upregulation in the relative abundances of tricarboxylic acid (TCA) cycle intermediates, with citrate and α-ketoglutarate showing 2.93-fold and 2.95-fold increases in peak intensity, respectively (p < 0.05). Concurrently, elevated relative signal intensities of short-chain fatty acids (SCFAs) were detected in the high-dose group, further supporting the metabolic benefits of Laminaria supplementation. CONCLUSION: This study confirmed that 3-6 g/kg dietary Laminaria extract improves growth and immunity of Hu lambs by enriching Prevotella and Muribaculum, activating TCA cycle metabolism. Low dosage prioritized antioxidant improvement, while high dosage exerted superior growth-promoting effects, supporting Laminaria extract as a natural antibiotic alternative for lambs.