More specifically, 84 percent of those fat molecules get exhaled as colorless, odorless carbon dioxide. 

A couple of years ago, Ruben Meerman took off 40 pounds. And that got him wondering: What exactly happened to all that fat?

Conventional wisdom was that he “burned” it off. Or sweated it off. Or excreted it. None of that satisfied Meerman, who has a physics degree and makes his living explaining science to schoolkids and for the Australian Broadcasting Corporation.  So Meerman tackled the problem and eventually came up with a surprising answer: Most of the lost fat disappears into thin air.