When someone loses weight, where does the fat actually go? What
happens to it?
The fuel that your body runs on is called glucose. You get most of your
glucose from carbohydrates. Ideally, you will only take in enough to use.
If you take in more carbohydrates than you can break down you will store it
as fat. If you begin to use more carbohydrates than you need to fuel your
body, you have alternate pathways to break down fat which can be converted
into glucose. So the fat is converted back into carbohydrate and used as
fuel for your body.
Fat is the means through which your body stores energy in the form of large
complex molecules. When you lose weight your body tapes into its energy
stores (fat) and breaks down these molecules through metabolism. These
molecules end up in all sorts of places, sometimes as amino acids, sometimes
as other proteins, but most of the time they are broken apart and filtered
through your metabolic process to produce energy.
In the body, fat is combined with oxygen to make carbon dioxide and water.
Both of these products are excreted, mostly in the breath.
Richard Barrans Jr., Ph.D.
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Update: June 2012