Effects of faecal microbiota transplantation on glycaemic outcomes in adults with metabolic disorders: a systematic review and meta-analysis of randomised controlled trials.

BACKGROUND: Faecal microbiota transplantation (FMT) may improve glucose metabolism, but findings from randomised trials and previous meta-analyses are inconsistent. We evaluated the effects of FMT on glycaemic outcomes in adults with type 2 diabetes, prediabetes, insulin resistance or related metabolic disorders. METHODS: We searched PubMed and Scopus for English-language randomised controlled trials published from January 2000 to May 2026. The primary outcome was change from baseline in HOMA-IR. Secondary outcomes included HbA1c, fasting glucose and clamp-derived peripheral insulin sensitivity. Random-effects meta-analyses used Paule-Mandel estimation with Hartung-Knapp-Sidik-Jonkman confidence intervals. Sensitivity analyses evaluated comparator type, data reconstruction, the assumed baseline-follow-up correlation and individual-study influence. Risk of bias was assessed using RoB 2. RESULTS: The searches identified 887 records, and 18 studies met the eligibility criteria. Six trials involving 120 participants included to the primary analysis. FMT did not significantly reduce HOMA-IR compared with control (MD, -1.29; 95% CI [-3.04, 0.47]; I2 = 72%). The sensitivity analysis combining HOMA-IR and HOMA2-IR also produced a non-significant result (MD, -1.11; 95% CI [-2.50, 0.29]). HbA1c data were sparse and were not pooled across comparator types. Clamp-derived findings suggested a possible donor-phenotype effect but remained inconclusive. The certainty of evidence was very low for HOMA-IR and HbA1c and low for peripheral insulin sensitivity. CONCLUSIONS: FMT did not demonstrate a clear benefit for glycaemic outcomes. The estimates were imprecise, heterogeneous and influenced by comparator design. Larger trials with standardised outcomes and predefined donor phenotypes are required.