Role of the Gut Microbiome and Bile Acids

In this extensive review article (1) I miss any mention of the gut microbiome, whose influence is increasingly the focus of research in the development of obesity as well as weight reduction and the metabolic effects after bariatric surgery.

The Firmicutes phylum is reduced after bariatric surgery in favor of proteobacteria and fusobacteria. Proteobacteria are considered of major importance especially regarding glucose homeostasis and weight loss (2). In my opinion, the results of a recently published study are very interesting, according to which the composition of the microbiome preoperatively can have a predictive value for the success of bariatric surgery on weight (3). In this context it should be explained in somewhat greater detail the importance of bile acids, which in the review article are mentioned only as bile salts. Their postoperative rise in plasma is associated with the impairment of their enterohepatic circulation. A mutual relationship between these and the change in the composition of the microbiome is assumed. They function, among others, as important signaling molecules via the farnesoid-X receptor (FXR) and the transmembrane G-protein coupled receptor TGR5 with crucial effects, among others, on the secretion of the incretins glucagon-like peptide 1 (GLP-1) und glucose-dependent insulinotropic polypeptide (GIP) and the appetite regulating gut peptides and the peptide hormone YY (PYY) and cholecystokinin. Their effect on the postoperative drop in lipid concentrations as well as on the rise in energy consumption as a result of the conversion of thyroxine into triiodothyronine are also under discussion (4).