BACKGROUND: People co-evolved with members of the microbiota and developed, used and adapted many complex immune mechanisms, which are used for monitoring and control of the microbiota. The gut microbiota in cooperation with humans became its essential part, so-called "hidden organ" with many important and indispensable functions. Quantitative and/or qualitative deficiency of the gut microbiota (dysbiosis) probably is a basis of many disorders, including obesity.AIM: To present an overview of the possible association between gut microbiota and obesity.METHODS: Meta-analysis of available scientific and published data including PubMed, Web of Science, Scopus and Cochrane Library.RESULTS: In the intestinal microbiota at obese people is detected a specific increase in the proportion between class Firmicutes and Bacteroidetes despite the non-obese people. Also, it is detected a decrease in this proportion if the person lost weight. These facts may be secondary to obesity. The colonisation of germ-free mice with microbiota from ordinarily feed or obese mice, without changes in the feed style leads to increase body fat to more than 50%.CONCLUSION: The human gut microbiota directly affects the food digestion, absorption and metabolism. The gut microbiota of obese people has a higher capacity for receiving energy from the food than the microbiota at slim people. The gut microbiota affects appetite control and energy balance. Lifestyle and food regimen affect the diversity of the gut microbiota and the presence of dysbiosis.