In “Gut Feelings: The Microbiome and Our Health,” Alessio Fasano, M.D., and Susie Flaherty point to a number of environmental factors that influence obesity, including “sleep deprivation, disruption to circadian rhythms, chronic stress, changes in food composition, labor-saving devices, drugs (including antibiotics), and endocrine disruptors” as well as socioeconomic factors. They also add another variable to the mix: the gut microbiome, the ecosystem of microorganisms in and on the human body.