Improved hygiene that leads to a reduced exposure to microorganisms has been implicated as one possible cause for the "epidemic" of chronic inflammatory and metabolic diseases in industrialized countries during the past 3-4 decades. In addition to a genetic predisposition and exposure to environmental triggers, a third key element, increased intestinal permeability, which may be influenced by the composition of the gut microbiota, has been proposed in the pathogenesis of these diseases (1). An imbalance between caloric intake and energy expenditure, both of which coexist with unhealthy lifestyles, is the most frequent cause of obesity. However, intestinal factors that influence gut mucosal biological and immunologic functions, including modification of the microbiota composition and barrier function, seem to play a crucial role as well in causing obesity and its associated metabolic complications. The symbiotic lifestyle of enteric bacteria colonizing the gastrointestinal tract contributes to the human host metabolic needs with the elaboration of enzymes not produced by humans to the catabolism of dietary fibers. A specific microbiota composition highly influenced by a Western diet and characterized by a decreased ratio of Bacteroidetes to Firmicutes or an increase in specific microbiota species has been described (2). It is becoming increasingly evident from animal studies and metagenomic and metabolomic studies that, in obesity, a specific gut microbiota may be responsible for a more efficient intestinal absorption of calories and increased lipid deposition by the digestion and absorption of plant polysaccharides that are usually poorly digestible (3), which influences body weight and interferes with metabolic changes. Studies in children have shown that the success of specific dietary interventions to lose weight are dependent on the type of initial host microbiota, and that the microbiota in childhood is strongly influenced by the type of delivery, the neonatal feeding regimen, and the exposure to antibiotics (4).Another key intestinal biological function influenced by diet and microbiota composition is the gut permeability by macromolecules. In mice, a high-fat diet reduced Bifidobacteria and increased plasma concentrations of LPS and liver fat, along with the expression of a number of proinflammatory cytokines. High-fat meal-related hypertriglyceridemia and a concurrent acute increase in plasma endotoxin concentrations seem to be due to an increase in intestinal permeability secondary to reduced expression of 2 epithelial tight junction (TJ) proteins, occludin and zonula occludens-1 (5). Microbiota dysbiosis may trigger the release of zonulin, a modulation of intercellular TJs (6), leading to increased trafficking of endotoxins and other nonself antigens. The access of LPS and other endotoxins to the intestinal lamina propria initiates a mucosal immune response that causes chronic low-grade inflammation and possible liver insult, triggering metabolic disorders characterized by nonalcoholic fatty liver dise...