When you lose weight, where does it go? It turns out, most of it is exhaled. Our body stores the excess protein or carbs in a person's diet in the form of fat, specifically, as triglyceride molecules, which consist of just three kinds of atoms: carbon, hydrogen and oxygen. For people to lose weight, their triglycerides must break up into building blocks, which happens in a process known as oxidation.
When a triglyceride is oxidized (or "burned up"), the process consumes many molecules of oxygen while producing carbon dioxide (CO₂) and water (H₂O) as waste products. To burn 10 kilograms (22 pounds) of fat, a person needs to inhale 29 kg (64 lbs) of oxygen. The chemical process of burning that fat will produce 28 kg (62 lbs) of carbon dioxide and 11 kg (24 lbs) of water, the researchers calculated. That's a LOT of heavy breathing.