The role of gut microbiota in obesity, diabetes mellitus, and effect of metformin: new insights into old diseases

There is a recent growing evidence that abnormalities in the microbiota composition can have a major role in the development of obesity and diabetes and that some actions of metformin may be mediated by gut bacteria. Several mechanisms have been found. A reduced microbial diversity is associated to inflammation, insulin-resistance, and adiposity. In particular, a rise in the Firmicutes / Bacteroidetes ratio is related to a low-grade inflammation and to an increased capability of harvesting energy from food. Interestingly, high-fat-diet favors the growth of bacteria capable of extracting more energy from food. Changes in some metabolites, such as short-chain fatty acids (SCFAs), produced by gut microbiota, and decreased amounts of the Akkermansia muciniphila are associated with the presence of type 2 diabetes. Among the mechanisms by which metformin acts on glucose metabolism and on the cardiovascular risk, some of them are due to positive effects on gut microbiota. A shift toward positive SCFAs produced by bacteria, an increase in some bacterial strains, including A. muciniphila , and some actions on bile acids mediated by microbiota have been described. All these recent advances have been reported and discussed.