The oral-gut microbiome axis in diabetes mellitus: a systematic review and emerging clinical perspectives

Emerging evidence suggests that diabetes mellitus (DM) is not only a metabolic disorder but also a mucosal disease shaped by microbial interactions across body niches. This review synthesizes current evidence on the oral-gut microbiome axis in DM, focusing on microbial transmission, functional overlap, and clinical relevance. A systematic search of six databases identified studies profiling paired oral and gut microbiomes in individuals with diabetes. Across included studies, consistent findings demonstrate concurrent dysbiosis in both niches. Notably, oral-associated taxa such as Streptococcus, Prevotella, Fusobacterium, and Porphyromonas were detected in the gut, suggesting ectopic colonization and inter-niche microbial transmission. Functional analyses revealed shared disruptions in key metabolic pathways, including short-chain fatty acid production and glycine betaine metabolism, with downstream effects on inflammation and insulin resistance. These microbial alterations correlated with established clinical markers such as HbA1c, fasting glucose, and inflammatory indices. Emerging machine-learning models integrating oral and gut microbiota demonstrated promising diagnostic performance (AUC > 0.83). Collectively, these findings support a potential bidirectional oral-gut axis associated with metabolic dysregulation in DM. Despite limitations including cross-sectional design and heterogeneity, this axis represents a novel target for biomarker development and therapeutic intervention. Future longitudinal and interventional studies are required to determine causal relationships and clinical utility.