The science of weight loss is fascinating, but has been often misunderstood. In a paper published in the British Medical Journal in December 2014, the authors describe the metabolic process of weight loss. The researchers showed that during weight loss, 84% of the fat that is lost turns into carbon dioxide and leaves the body through the lungs, whereas the remaining 16% becomes water, which is excreted in urine, feces, sweat, breath, tears and other bodily fluids.
How does this happen? The body stores excess calories consumed in the form of triglycerides (which consist of carbon, hydrogen, and oxygen). Triglycerides are stored in fat cells. When you eat less and exercise more, a hormone breaks up the triglyceride molecules into these three building blocks in a process called oxidation. The process of oxidation (burning up triglyceride) requires a lot of oxygen while producing carbon dioxide and water as waste products.
Where the fat goes is all based on chemistry and science. Mass cannot be created or destroyed, so the carbon, hydrogen, and oxygen that made the triglycerides had to go somewhere else – breathed out into the atmosphere and excreted as bodily fluid.
British Medical Journal, 16 December 2014