Mice with certain communities of fungi living in their gut gained more weight when eating processed food than mice whose gut microbiomes hosted different communities of fungi, according to our study published March 5 in the journal Communications Biology. Microbiomes are communities of microorganisms. In this study, we explored whether the fungal members of the gut microbiome—called the mycobiome—changed their host’s metabolic reaction to processed food. To do this, we obtained genetically identical mice from four different companies—each with different fungal microbiomes—and then fed the mice either standard mouse food or processed food resembling the typical American diet. After six weeks, we measured their body fat as well as genes and hormones involved in metabolism. We specifically looked at the relationship between the fungal microbiome and processed foods—foods that contain refined sugars, monounsaturated fats, and white flour, for example—because these foods are linked to unhealthy weight gain in humans. Eating processed food made …