The Human Gut Microbiota and Glucose Metabolism: a Scoping Review of Key Bacteria and the Potential Role of Short Chain Fatty Acids.

The gut microbiota plays a fundamental role in human nutrition and metabolism and may have direct implications for type 2 diabetes and associated pre-conditions. An improved understanding of relationships between human gut microbiota and glucose metabolism could lead to novel opportunities for type 2 diabetes prevention, but human observational studies reporting on such findings have not been extensively reviewed. Here, we review the literature on associations between gut microbiota and markers and stages of glucose dysregulation and insulin resistance in healthy adults and in adults with metabolic disease and risk factors. We present the current evidence for identified key bacteria and their potential roles in glucose metabolism independently of overweight, obesity and metabolic drugs. We provide support for short-chain fatty acids (SCFA) mediating such effects and discuss the role of diet and gut microbiota x diet derived metabolites. From 5983 initially identified PubMed records, 45 original studies were eligible and reviewed. Alpha diversity and 45 bacterial taxa were associated with selected outcomes. Six taxa were most frequently associated with glucose metabolism: Akkermansia muciniphila, Bifidobacterium longum, Clostridium leptum group, Faecalibacterium prausnitzii and Faecalibacterium (inversely associated) and Dorea (directly associated). For Dorea and A. muciniphila, associations were independent of metabolic drugs and body measures. For A. muciniphila and F. prausnitzii, limited evidence supported SCFA-mediation of potential effects on glucose metabolism. We conclude that observational studies applying metagenomics sequencing to identify species-level relationships are warranted, as are studies accounting for confounding factors and investigating SCFA and postprandial glucose metabolism. Such advances in the field will, together with mechanistic and prospective studies and investigations into diet-gut microbiota interactions, have the potential to bring critical insight into roles of gut microbiota and microbial metabolites in human glucose metabolism and to contribute towards the development of novel prevention strategies for type 2 diabetes, including precision nutrition.