I read that a carb rich diet dis activates brown fat and being in cold activates it. The conclusion was that during summer, humans ate a more plant rich diet but in the winter hunted. I also read that the high carb year around modern diet  destroys most of it by adulthood. I can believe that because I'll be shivering in my car with my big goose down coat while kids are at the bus stop in slim jackets thinking "How the fuck can they stand that?"