Fecal microbiota transplantation (FMT) has emerged as a potential therapeutic strategy for modulating gut dysbiosis in diabetes mellitus. This review critically evaluates preclinical and clinical evidence on FMT in type 1 (T1D) and type 2 diabetes (T2D). Studies suggest that FMT can restore microbial diversity, improve glycemic control, and modulate immune responses, with varying effects across diabetes subtypes. In T1D, preclinical models demonstrate that FMT influences regulatory T-cell expansion and β-cell preservation, though clinical translation remains limited. In T2D, FMT has shown transient improvements in insulin sensitivity, with sustained effects observed only in patients with specific microbiome signatures. However, heterogeneity in patient responses, donor variability, and methodological limitations complicate its clinical application. This review highlights the interplay between FMT, immune modulation, and microbial metabolism, advocating for phenotype-stratified trials and multi-omics integration to enhance therapeutic precision.
