This is where people think fat goes when it's "lost":

Meerman/BMJ

But! Most people are wrong, according to physicist Ruben Meerman and biochemist Andrew Brown. Their calculations were published yesterday in the British medical journal BMJ (hence "faeces" in their survey results above), where the authors profess that despite soaring rates of obesity, there is "surprising ignorance and confusion about the metabolic process of weight loss."

The researchers use the calculation below to show that fat (plus oxygen) is metabolized mainly into carbon dioxide, and some water that goes into urine (uarine?) or gets used up in other metabolic processes.

Meerman explained in subsequent interviews that the calculation is not new to science, just misunderstood. He told ABC he was "flabbergasted when [he] first realized the extent of the ignorance about this really basic biochemical process."

Of course, the above equation does involve the release of energy. So I don't think saying colloquially that fat becomes energy really makes a person a fool. Meerman's point is just that, according to "science," matter is conserved, not easily converted into energy. The atomic bomb that the United States dropped on Hiroshima involved conversion of about one third the mass of a dime's worth of matter. If you were able to convert your fat stores directly into energy, you would explode in a glorious, catastrophic spectacle. Your treadmill would be destroyed, and so would your gym, and your city. But, the weight would be so off.