So fun facts:  The technique scientists use to search for brown fat deposits in mammals (non-invasively, e.g. the tests that we run on living humans) is to feed them food with specially treated (mildly radioactive tracers) fats.  If the subject is rapidly metabolizing fat, the radioactive tracers will be concentrated in the tissue responsible and can be picked up with a scan.    Why didn't these studies find brown fat deposits in adult humans?  The answer is that they did find brown fat in a small sub-population of adult humans, who where lean and athletic.  That was a bit of a mystery for a bit, but this study is actually one of the keys that solves the puzzle. Why is it hard to find brown fat in adults? Two factors combined, they used Americans in the studies and as we have just learned in this article: being fat turns the metabolism of brown fat off.  The radioactive tracers used to search for brown fat will only be observable if the brown fat cells are active.  American adults are a rather over weight sample population and now we know that their brown fat cells are mostly deactivated as a result of being fat. Yeah Science!