Over the last decades the nutritional status has changed where a fat-enriched diet progressively replaced dietary fibers. This is now considered a leading cause for the growing occurrence of metabolic diseases. An increased inflammatory tone is defined as a cornerstone of all metabolic diseases and is characterized by an augmented circulating and tissue cytokine concentration associated with an increased infiltration of cells from the innate immune system. However, the progressive onset of the disease in patients feeding on a fat-enriched diet leading to inflammation had no molecular origin. In the quest of a factor triggering inflammation we demonstrated that changes of intestinal metagenomic (i.e. bacterial genome) profiles occurred after a short period of high-fat feeding and were associated with an increased plasma concentration of lipopolysaccharides (LPS). Using genetic, antibiotic and pharmacological approaches in mice we causally demonstrated the role of intestinal microflora and LPS as triggering factors of adipose tissue, liver and muscle inflammation, leading to the onset of insulin resistance, overt hepatic glucose production, and fat pad enlargement. We next showed that LPS could directly target the adipose fat pads in mice with wild type or with CD14 inactivated adipose depots i.e. the LPS receptor. We also demonstrated that the origin of plasma LPS was linked to an increased leptin-regulated intestinal bacterial translocation process. Hence, a change in intestinal microflora induced by a fat-enriched diet was causally linked to plasma LPS concentration and metabolic inflammation at the onset of the development of metabolic diseases.