Thymol-driven remodeling of rumen fibrolytic microbiota and muscle metabolism improves lamb meat quality.

High-concentrate (HC) diets in intensive sheep fattening frequently induce metabolic disorders and compromise the lipid nutritional quality of meat. Although thymol exhibits metabolic benefits, its role in regulating rumen fermentation and muscle lipid deposition remains unclear. In this study, lambs fed an HC diet were supplemented with different levels of thymol, and growth performance, rumen microbiota, volatile fatty acids (VFA), as well as metabolomic and fatty acid profiles of the longissimus lumborum were systematically analyzed. Thymol supplementation enhanced average daily gain (ADG), serum immunoglobulin A (IgA), and loin muscle area, while reducing the certain saturated fatty acids (SFA) in meat, indicating an improved lipid nutritional profile. Microbial analysis showed that thymol inhibited starch-degrading bacteria while enriching key fiber-degrading taxa, particularly Fibrobacter sp., which was associated with increased ruminal pH, elevated acetate, and reduced propionate concentrations. Muscle metabolomics revealed a pronounced shift in metabolic profiles, characterized by increased thymol sulfate, decreased succinate derivatives, and activation of unsaturated fatty acid (USFA) biosynthesis pathways. Integrated multi-omics analysis further indicated that the enrichment of fiber-degrading microbiota and the increase in acetate production jointly contributed to the accumulation of thymol sulfate in muscle tissue. Overall, thymol enhances the nutritional quality of lamb meat by modulating rumen microbial fermentation and host metabolic pathways, highlighting its potential as a dietary strategy for improving meat quality under HC feeding.