Where Does My Fat Go When I Lose Weight?

When you lose fat weight, where does it go?  How does it physically leave the body?  --Carrie S.

Dear Carrie,

I just love that question!  I'm sure it's something we've all wondered at one time or another.

Well, it's a matter of biochemistry. Each fat molecule is made up of atoms of Carbon, Hydrogen, and Oxygen. When your body needs energy (like when you are exercising, or if you haven't eaten enough to support your weight) the fat molecules break down to yield energy in the form of calories. The molecules go through a series of reactions which generate energy that has been stored up until now in neat packages of fat, in special cells for this storage purpose. When you burn a lot of energy (like when running) your body temperature actually goes up and you begin to sweat.

The final breakdown products (and these same atoms are present in protein and carbohydrate as well--which go through a variation of the biochemical reactions to yield calories) are Hydrogen Oxygen, and Carbon. The Hydrogen and Oxygen leave your body as water through the kidneys, the intestine, your breath, and a small amount through your skin (as in sweat). The Carbon atoms combine with the oxygen you breathed in, and are excreted as carbon dioxide.
 
Ha! Remember in 6th grade when you learned "you breathe in oxygen and you breath out carbon dioxide"? Now you know where the carbon comes from!