Recent advances in gut microbiota-mediated regulation of fat deposition and metabolic disorders.

The gut microbiota critically regulates lipid metabolism through microbial metabolites and host signaling pathways. Short-chain fatty acids (SCFAs), derived from dietary fiber fermentation, suppress hepatic lipogenesis via inhibition of SREBP-1c and enhance mitochondrial β-oxidation through GPR41/43 activation. Microbial enzymes convert primary bile acids into secondary bile acids, which activate FXR to inhibit lipogenesis and TGR5 to promote adipose thermogenesis. Lipopolysaccharide (LPS) from dysbiotic microbiota triggers TLR4-NF-κB signaling, exacerbating insulin resistance and adipose inflammation. Branched-chain amino acids (BCAAs), metabolized by gut microbes, drive adipogenesis via mTORC1-PPARγ signaling, with elevated circulating BCAAs linked to obesity. In livestock, microbiota modulation optimizes fat deposition: probiotics in pigs enhance intramuscular fat via Lactobacillus-enriched communities, while dietary succinate or coated sodium propionate reduces abdominal fat in broilers by reshaping cecal microbiota. Fecal microbiota transplantation confirms microbial causality in transferring fat phenotypes. Dysbiosis-associated mechanisms are conserved across species, where SCFAs and bile acids ameliorate metabolic inflammation, whereas LPS and BCAA imbalances worsen lipid dysregulation. Metabolic disorders, including obesity, type 2 diabetes (T2D), and non-alcoholic fatty liver disease (NAFLD), are tightly linked to gut microbiota perturbations. Dysbiosis drives LPS translocation and barrier impairment. These changes, along with altered metabolites, promote inflammation and fat deposition. Future strategies should integrate multi-omics and precision engineering of microbial consortia to advance therapies for both livestock and human metabolic health.