INS and obesity due to melanocortin 4 receptor deficiency: It is difficult to make such causal inferences in humans [102], but microbiomes are altered in individuals with obesity [103, 104], and proof‐of‐concept studies have shown that insulin sensitivity can be improved in individuals with metabolic syndrome using fecal microbial transplantation (FMT) from healthy lean donors [105, 106, 107].