Fig. 2

Involvement of gut microbiota in metabolic diseases. An "obesogenic" microbiota (higher Firmicutes/Bacteroidetes ratio), with a greater capacity to extract energy from the diet, may contribute to the state of obesity. Metabolic diseases are associated with chronic low-grade inflammation and the resulting imbalances in adipose tissue and pancreas. The microbiota can influence the inflammatory state via lipopolysaccharide (LPS), the gut barrier, and several of its metabolites (especially short-chain fatty acids (SCFAs)). LPS potentiates inflammation by inducing macrophage infiltration and pro-inflammatory cytokines in adipose tissue. The structure and permeability of the intestinal barrier (mucosa), which protects against inflammation by preventing bacterial translocation, is affected, positively or negatively, by the presence or absence of different types of bacteria. SCFAs improve the intestinal barrier by reinforcing tight junctions, reduce inflammation by increasing regulatory T cells (Treg cells) and anti-inflammatory cytokines and decreasing inflammatory cytokines, and improve glucose homeostasis and insulin sensitivity. SCFAs also intervene in the gut–brain axis by regulating the levels of hormones involved in the control of gastrointestinal motor function and food intake, such as leptin, ghrelin, peptide tyrosine tyrosine (PYY), cholecystokinin (CCK), and glucagon-like peptide-1 (GLP-1). MetS metabolic syndrome, T2D type 2 diabetes