You've probably heard that if you don't exercise or work out, your muscle will turn to fat. If that was a concern, stop worrying about it; it's not possible. Nor can fat turn into muscle. Fat and muscle are different types of body tissues — fat is adipose tissue, and muscle is protein — and you can't change one type of tissue into another. While it may appear that you're turning fat into muscle or muscle into fat when you get slack or get serious about exercise, that appearance is only because fat is less dense than muscle, which means an ounce of fat takes up more space inside the body than an ounce of muscle does.
So if fat doesn't turn into muscle, where does it go when you lose weight? Does it just shrink? Does it melt into energy that you burn off? Researchers studying this biochemical process tracked fat molecules to figure out exactly where fat goes when you lose it, and they use a chemical formula to calculate the process: C55H104O6 + 78O2 → 55CO2 + 52H2O + energy. As it turns out, mostly, we just exhale it. If you were to lose 22 pounds (10 kilograms) of body fat, 20 pounds (9.4 kilograms) would be released as carbon dioxide (CO2) when you breathe; the remainder would become water that is excreted in our urine, sweat or tears [source: Meerman].