PYY and obesity due to melanocortin 4 receptor deficiency: In the large intestine, unconjugated primary BAs are converted into unconjugated secondary BAs by intestinal bacteria, mainly including Clostridium scintillans and Lactobacillus.81 These microbial transformation processes may have profound effects on the secretion and activity of PYY, providing new insights for the treatment of obesity and related metabolic diseases.