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Environmental factors, such as social networks, have an influence on obesity pandemics. The gut microbial flora (microbiota) plays a role in converting nutrients into calories. Variations in microbiota composition are found in obese humans and mice. The microbiota from an obese mouse confers an obese phenotype when transferred to an axenic mouse. There is a large body of experimental evidence and empirical data in the food industry showing that both antibiotics and probiotics, which modify the gut microbiota, can act as growth promoters, increasing the size and weight of animals. The current obesity pandemic may be caused, in part, by antibiotic treatments or colonization by probiotic bacteria. Using metagenomics and microarray analysis, studies of microbiota modifications after antibiotic and probiotic intake may identify the modifications associated with increased size and weight. Epidemiological studies recording these factors in an obese population may be able to link obesity with the absorption of microbiota modifiers.