Wow! Two of the trendiest topics in metabolism, brown fat and the microbiome, meet up in this fascinating article. This report studies 2 groups of mice with no microbiome, that is, no bacteria in their guts. This occurred either because they were delivered by hysterectomy and raised in germ-free environments, or because their microbiome was wiped out by antibiotics. Both groups developed more brown fat than control mice, weighed less, and had higher insulin sensitivity. When their microbiome was reintroduced, these findings were reversed. Next steps in mice are to see if there are specific gut bacteria that have greater effects on the conversion of white, or beige fat, to brown fat. It’s remarkable that this type of research has only become possible during the last 5 years or so, as knowledge about the gut microbiome and about brown fat have increased.