People throw that phrase around all the time, "We're burning fat." And I understand much of the physiology involved, but where does the fat go after it's metabolized?
Do we sweat it out?
Does it pass through the urine or feces?
This question becomes even more absurd if you consider the fact that I have a Bachelor's degree in Exercise Science and Master's degree in Sport Mgmt., lol.
The Wikipedia article on fatty acid metabolism contains the full answer to your question. To summarize in laymen's terms:
From Acton's comments below:
"Water is yielded in overall fatty acid oxidation. You left out the most important part, the electron transport chain, where the bulk of the energy is actually produced. Reduced protons from beta oxidation and the Krebs cycle donate their electons to oxygen via the electron transport chain to form water, producing ATP. "Burning" 100 grams of fatty acids produces roughly 100 grams of water."
If you're asking about the fat that we eat but don't store on a low-carb diet, some of the fatty acids get broken down into ketone bodies, and the ketone bodies can be expelled through breathing (keto breath) or expelled through urine. I believe both of these are rather minor amounts, however; most of the fat is converted to energy, making you feel "more energetic" and/or boosting your body temperature.
Dr Eades, in his newsletter email, references his response to a question by Ryan http://www.proteinpower.com/drmike/weight-loss/a-legitimate-use-for-orlistat/ from 2007 regarding "Where does the fat go." It is buried in the comment section.
Richard Nikoley references the same at http://freetheanimal.com/2010/03/isnt-it-time-for-anthony-colpo-to-get-a-life.html in his blog about Anthony Colpo and Dr Eades.
Edit: Dr Eades ties the whole thing up at http://www.proteinpower.com/drmike/metabolic-advantage/thermodynamics-and-the-metabolic-advantage/#more-4034
There is no functional difference, in terms of what eventuall happens, between the fat you've got stored in your fat cells and the fat that you ingest through your mouth.
In fact, most fatty acids you eat are first transported to your fat cells and then used.
The amount of fat cells stays almost the sama for all adult life. The fatty acids themselves disintegrate into CO2, water and energy (heat). The CO2 is breath out and I think you can guess how the water comes out. Energy is used by the body.