There has been a significant paradigm shift surrounding the mechanism of action of metabolic surgery. Building upon initial presumptions of purely restrictive or malabsorptive effects of surgery, there followed a legion of research into the hormonal aspects. However more recently a nascent base of evidence has formed focused upon the role of the gut microbiota via bile acid changes in obesity pathophysiology.
We present the findings of a systematic review into the gut microbiota profiles after bariatric surgery in both murine and human patients. Databases of PubMed, CINAHL, Embase and Cochrane Central library were searched.
Animal studies suggest after SG (sleeve gastrectomy) and RYGB (Roux-en-Y Gastric Bypass) the percentage of Proteobacteria in the SG and sham groups remain markedly lower than the RYGB group. Sham surgery rats showed higher levels of Escherichia coli. Two cross-over studies trialled colonising germ-free mice with stools from postoperative mice. The surgically-altered faecal microbiota promoted reduced fat deposition and consequent weight loss in recipients.
By comparison, small studies of human faecal samples indicated specific bacterial genera positively correlated with weight loss (particularly Faecalbacterium and Enterococcus). RYGB was associated with a relative abundance of Firmicutes whilst SG resulted in increased Bacteroidetes phyla. There is a significant difference between sleeve gastrectomy and Roux-en-Y gastric bypass patients.
The area of the gut microbiome is a promising area of both diagnostic and therapeutic research. Unfortunately to-date the observational trials have been limited by a lack of standardisation in early studies, technical limits of earlier sequencing methods and heterogeneity of surgical procedures. A large-scale cohort trial observing the changes in faecal microbiota is necessary to adequately characterise the changes in the gut microbiota. Further determination of the optimal gut microbiota may provide guidance towards therapeutic measures to improve efficacy and durability of weight loss in reaching metabolic goals.