Culture-independent methods to study microbial communities have advanced our knowledge of the human gut microbiota. The gut microbiota in humans includes various kinds of microorganism inhabiting the length and width of the gastrointestinal tract. The composition of the microbiota is host-specific, evolving throughout an individual’s lifetime and susceptible to both exogenous and endogenous modifications. It is estimated that the human gut microbiota contains as many as 10¹⁴ bacterial cells and 500 to 1,000 species. The process of initial colonization of gut microbiota begins at the time of delivery, when the fetus leaves the germ-free intra-uterine environment and enters the extra-uterine setting. It is now well accepted that the colonization of bacteria, including Bifidobacteria and Lactobacilli, is necessary for the normal development of intestinal innate and adaptive defenses. Most preterm infants are delivered by cesarean (C) section for various reasons, so the transfer of bacteria from mother to infant is completely absent during the delivery; thus, infants delivered by C section are colonized with anaerobic bacteria later than vaginally delivered infants, leading to an unbalanced composition of the intestinal microbiota, namely, dysbiosis. Diet is known to modulate the composition of the gut microbiota in humans over the long term. The alteration of gut microbiota composition, dysbiosis, may contribute to the risk and pathogenesis of both undernutrition and overnutrition through effects on nutrient metabolism and immune function.<br>The nexus between nutrient metabolism and the immune system occurs at many levels, ranging from endocrine signaling to direct sensing of nutrients by immune cells. The ability to use macronutrients is essential for the generation and maintenance of a protective effector immune response. Short-chain fatty acids (SCFAs) provide one of the clearest examples of how nutrient processing by the microbiota and host diet combine to shape immune responses. SCFAs are end products of the microbial fermentation of macronutrients, the most notable being plant polysaccharides that cannot be digested by humans.<br>Changes in lifestyle and an increase in the availability of energy-rich food are important contributors to the worldwide obesity epidemic. The microbial inhabitants of the gut can also have an influence on metabolic processes, such as energy extraction from food, and should be considered as an environmental factor that contributes to obesity and its comorbidities, such as insulin resistance, type 2 diabetes, cardiovascular disease and also cancer.<br>Accumulating evidence indicates that gut microbiota may be a target for preventing and treating obesity; this will require probiotics that are selected for specific clinical manifestations of metabolic syndrome.