INS and type 2 diabetes mellitus: Finally, fecal microbiota transplantation (FMT) continues to provide proof-of-principle for causality; randomized and controlled metabolic-syndrome/T2DM studies report directionally favorable changes in insulin sensitivity and SCFA-producing guilds, albeit with heterogeneous durability—underscoring the need for better recipient stratification and standardized endpoints (70, 79, 80).