The Gut Microbiota–Metabolic Axis: Emerging Insights from Human and Experimental Studies on Type 2 Diabetes Mellitus—A Narrative Review

The rapidly advancing field of gut microbiota research has revealed its pivotal role in human health, with growing evidence implicating microbial dysbiosis in the development of metabolic diseases, particularly type 2 diabetes mellitus (T2DM). This narrative review synthesizes recent findings on the complex, bidirectional relationship between the gut microbiota–metabolic axis and T2DM, drawing upon data from human and experimental studies published in the past decade. Patients with T2DM consistently demonstrate marked gut dysbiosis, characterized by reduced microbial diversity and depletion of beneficial butyrate-producing taxa such as Faecalibacterium prausnitzii and Roseburia intestinalis. In contrast, increases in pro-inflammatory bacteria including Escherichia-Shigella and Lactobacillus are commonly observed. Such compositional changes are linked to metabolic dysfunction through altered microbial metabolites, including elevated trimethylamine N-oxide (TMAO), which has been associated with insulin resistance and increased diabetes risk. Moreover, gut microbiota imbalances correlate with systemic inflammation, as indicated by higher levels of cytokines such as IFN-γ and IL-6. These findings underscore the gut microbiota’s central role in energy metabolism and inflammation in T2DM. Understanding these mechanisms could inform novel therapeutic and preventive strategies—such as microbiota-targeted dietary, probiotic, or pharmacologic interventions—to improve metabolic outcomes and enhance clinical management of diabetes.