Re: Where does fat actually go when people lose weight ?

Fat, in the chemical sense, is composed of lipid molecules used to store energy. Lipids come in many forms, usually as glycerides, glycerol molecules with fatty acid chains attached. While mono- and di-glycerides (containing one or two fatty acid chains, respectively) are used for cell signalling and to build cell membranes, tri-glycerides (with three fatty acid chains) are used solely for storage. In the body, specialized fat cells (adipocytes) store triglycerides is huge vesicles that take up most of the cell's volume. As the adipocytes in an area fill up on triglycerides, they grow in size, leading to a visible increase in the size of that part of the body along with an increase in weight.

Weight is lost through the break down of triglycerides in the adipocytes into fatty acid, which are in turn broken down through a complex biochemical pathway, called Fatty Acid Oxidation, to produce acetyl CoA (a common fatty acid like oleic acid, 18 carbons long, could be used to acetylate 9 molecules of Coenzyme A). Acetyl CoA is shunted into the Krebs cycle (TCA cycle) in the mitochondria where it is used to generate energy, releasing carbon dioxide (two molecules of CO₂ per molecule of acetyl CoA). So the end product of fat catabolism is CO₂ , which is exhaled from the lungs during exercise.

(Ref. - Helena Curtis' introductory textbook, Biology provides an easy-to-read introduction, and further material concerning the biochemical processes mentioned above.)