Probiotics,

Objective: To systematically evaluate the clinical efficacy of probiotics, prebiotics, synbiotics, and fecal microbiota transplantation (FMT) on glycemic control in adults with type 2 diabetes (T2D) and related dysglycemia, and to synthesize associated mechanistic changes in microbial metabolites and composition. Methods: A systematic review was conducted following PRISMA 2020 guidelines. PubMed/MEDLINE, Scopus, and Web of Science were searched from inception through August 2025 for randomized controlled trials (RCTs) in adults with T2D, prediabetes, or metabolic syndrome. Interventions included probiotics, prebiotics, synbiotics, or FMT compared to control. Outcomes were glycemic indices (e.g., HbA1c, HOMA-IR) and mechanistic biomarkers (e.g., SCFAs, bile acids). Risk of bias was assessed using the Cochrane RoB 2 tool. A narrative synthesis was performed. Results: Thirty studies were included. Multi-strain probiotics, prebiotics, and synbiotics yielded modest but significant improvements in HbA1c (≈-0.2 to -0.4 %), fasting glucose, and HOMA-IR, particularly with durations ≥12 weeks. These benefits were linked to mechanistic shifts, including increased circulating butyrate and ursodeoxycholate, enrichment of SCFA-producing taxa, and reduced endotoxemia. Efficacy was moderated by concomitant medications: metformin use was synergistic, while sulfonylureas attenuated effects. FMT consistently improved clamp-measured insulin sensitivity in insulin-resistant phenotypes, but its effects on HbA1c were less consistent and donor-dependent. Conclusion: Microbiome-targeted interventions, especially multi-strain probiotics and substrate-matched synbiotics, are effective adjuncts for improving glycemic control, with effects mediated through microbial metabolite production. FMT primarily modulates insulin sensitivity. Clinical outcomes are context-dependent, influenced by intervention design, duration, and pharmacomicrobiomic interactions.