摘要Background Tibetan sheep grazing on the Qinghai-Tibet Plateau require dietary protein supplementation;however,they face economic constraints due to the high cost of feed transportation in this region.Given that the leucine metabolite β-hydroxy-β-methyl butyrate(HMB)enhances both protein synthesis and intestinal nutrient absorption,this study employed metagenomics and untargeted metabolomics to systematically evaluate HMB's effects on anti-oxidant capacity,immune response,microbiota,metabolites,and the health of the small intestine in Tibetan sheep.A total of 120 healthy weaned 60-day-old male Tibetan lambs were assigned to diets containing 0 mg/kg(control group,CON),430 mg/kg(low HMB,L-HMB),715 mg/kg(medium HMB,M-HMB),or 1,000 mg/kg(high HMB,H-HMB)for 90 d.At the end of the experiment,6 lambs from each group were slaughtered for intestinal tissue and content sampling.Results The M-HMB treatment significantly increased average daily gain of the lambs without affecting feed intake,thereby improving feed utilization efficiency.M-HMB promoted the development of small intestinal morphologi-cal and elevated villus height,while also enhancing the activities of digestive enzyme and disaccharidase activities.Furthermore,M-HMB enhanced the antioxidant capacity,immune response,and barrier function of the small intes-tine.Metagenomic analysis revealed that M-HMB supplementation improved the composition of the small intestinal microbiota in Tibetan sheep,specifically increasing the relative abundance of Ruminococcus bacterium P7 and R.bromii,and enhanced microbial carbohydrate degradation capacity.Metabolomic analysis demonstrated that M-HMB supplementation significantly altered the small intestinal metabolite profile,enhancing carbohydrate metabolic path-ways and increased the production of short-chain fatty acids(SCFAs).M-HMB upregulated PLCβ1 and ERK1/2 protein expression levels in small intestinal tissue and elevated the proportion of Ki67-positive cells at the basal crypt region of small intestinal crypts,suggesting enhanced proliferative activity of intestinal epithelial cells.Conclusions In summary,dietary supplementation with M-HMB(715 mg/kg)promoted small intestinal growth and development,enhanced digestive and absorptive functions,optimized the microbial composition,improved carbohydrate degradation,and increased the production of SCFAs,ultimately improving the growth performance of Tibetan sheep lambs.