INS and obesity due to melanocortin 4 receptor deficiency: SCFA-producing bacteria (Lachnoclostridium, Ruminococcus_1, Alloprevotella, and norank_f_Muribaculaceae) can not only cause lipolysis and fatty acid oxidation, improve insulin sensitivity, and alleviate host obesity [47,48], but also inhibit liver cholesterol synthesis [49].