Gut Microbiota in Type 2 Diabetes and Metabolic Disorders: Sources of Heterogeneity and Ways to Resolve Contradictions in Research

Metabolic diseases, including obesity and type 2 diabetes mellitus, represent a major global health burden and are closely linked to the composition and function of the gut microbiota. Advances in molecular methods have enabled detailed characterization of microbial communities and their interactions with diet, medications, and host physiology, positioning the microbiome as an active metabolic organ. However, the field faces persistent challenges in distinguishing causal relationships from associations, largely owing to substantial biological, exposure-related, and methodological heterogeneity. This review systematizes the principal lines of evidence connecting the gut microbiome to metabolic disorders and critically examines the sources of variability that limit reproducibility and cross-cohort transferability of these findings. We discuss the taxonomic, functional, and metabolite-based levels of microbiome analysis, evaluate the strengths and limitations of cross-sectional, case–control, cohort, and interventional study designs, and consider approaches for establishing causality, including fecal microbiota transplantation, Mendelian randomization, mediation analysis, causal diagrams, and triangulation of evidence. We conclude that only a comprehensive, standardized, and causally informed approach will allow reliable discrimination between true microbiota-driven effects and methodological artifacts, thereby advancing the integration of microbiome science into the management of metabolic diseases and diabetes.