Energy balance is an equilibrium between the amount of energy extracted from the diet and the amount expended. Selective pressures throughout evolution have programmed animals to protect energy stores through the accumulation of adipose tissue; as diets have changed and energy-dense foods have become readily available, obesity rather than malnutrition has become the primary concern in developed nations. Nevertheless, factors other than the types of food and their availability appear to be important. Recent evidence suggests that the gut microbiota play a role in energy harvest, storage, and expenditure. The preponderance of the evidence demonstrates that germ-free mice are protected against obesity and that the transfer of gut microbes from conventionally raised animals results in dramatic increases in body fat content and insulin resistance. Moreover, the composition of the gut microbiota has been shown to differ in lean and obese humans and animals and to change rapidly in response to dietary factors. The gut microbiota may also influence the development of conditions characterized by low-level inflammation, such as obesity and type 2 diabetes, through systemic exposure to bacterial lipopolysaccharide derived from the intestinal microbiota. Together, these data suggest that modification of the gut microbiota may be a relevant therapeutic avenue for obesity and other metabolic disorders.

The prevalence of obesity and related disorders such as metabolic syndrome has vastly increased throughout the world. Recent insights have generated an entirely new perspective suggesting that our microbiota might be involved in the development of these disorders. Studies have demonstrated that obesity and metabolic syndrome may be associated with profound microbiotal changes, and the induction of a metabolic syndrome phenotype through fecal transplants corroborates the important role of the microbiota in this disease. Dietary composition and caloric intake appear to swiftly regulate intestinal microbial composition and function. As most findings in this field of research are based on mouse studies, the relevance to human biology requires further investigation.