Abstract

The intestinal microbiome is a dense, metabolically active ecosystem that shapes host energy balance, immunity, 
and endocrine signaling. In obesity and type 2 diabetes (T2D), characteristic alterations reduced microbial 
diversity, depletion of fiber-fermenting taxa, expansion of mucus-degraders and pathobionts, and shifts in the 
virome/mycobiome associated with insulin resistance (IR) and glycemic dysregulation. Mechanistically, 
dysbiosis modifies production of short-chain fatty acids (SCFAs), bile-acid (BA) pools, and tryptophan-derived 
indoles; increases branched-chain amino acids and imidazole propionate; perturbs intestinal barrier integrity; 
and primes innate immunity via endotoxin (LPS) and other microbe-associated molecular patterns. These 
changes alter gut–brain and gut–liver axes, decrease GLP-1/peptide YY signaling, promote hepatic steatosis, 
and impair skeletal-muscle insulin signaling. Interventions that restore eubiosis, dietary fiber and polyphenols, 
Mediterranean/plant-forward patterns, time-restricted feeding, physical activity, and sleep regularity 
consistently improve metabolic endpoints. Clinical trials of pre-, pro-, and synbiotics show modest but 
reproducible benefits on IR and inflammatory markers; fecal microbiota transplantation (FMT) yields transient 
improvements in insulin sensitivity in selected recipients but lacks durable efficacy without diet/lifestyle 
co-intervention. Emerging strategies include defined microbial consortia, engineered commensals, 
bacteriophages, and targeted postbiotics (e.g., SCFA donors, secondary BA modulators). Precision microbiome 
medicine combining metagenomics, metatranscriptomics, metabolomics, and host genomics/immune profiling 
may identify responder endotypes and guide individualized therapy. This review synthesizes current evidence 
linking the gut ecosystem to obesity/T2D pathophysiology, maps metabolite and immune pathways to clinical 
phenotypes, and evaluates modulation strategies across lifestyle, nutritional, and microbial therapeutics, 
emphasizing pragmatic translation and safety.