Brown fat is a fundamental tissue in mammals that, at the time, was mostly understood as something that rodents used to help heat themselves during hibernation. The spongy, fatty tissue looks a lot like ordinary white fat that most mammals use to store excess caloric energy. But where white fat can serve as an insulator, brown fat has an active role to burn white fat to generate body heat. It’s the only mammalian tissue whose sole purpose is to make heat, or, in scientific terms, thermogenesis. In humans, however, BAT was only considered important in newborns. The very first challenge that a
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