The potential role of intestinal microbiota in the etiology of various human diseases has attracted massive attention in the last decade. As such, the intestinal microbiota has been advanced as an important partaker in the development of obesity and obesity-related metabolic dysfunctions, amongst others. Experiments in animal models have produced evidence for a causal role of intestinal microbiota in the etiology of obesity and insulin resistance. However, with a few exceptions such causal relation is lacking for humans.