INS and obesity due to melanocortin 4 receptor deficiency: In a pilot study with 18 participants (n = 9 FMT recipients, n = 9 controls), and a subsequent larger follow-up study with 38 participants (n = 26 FMT recipients, n = 12 controls), one research group has observed that fecal microbiota transplantation (FMT) from lean donors can transiently improve peripheral insulin sensitivity in men with obesity and metabolic syndrome [23,24].