Where did it go…
As unlikely as it may seem, we exhale our weight. At least, we mostly do.
Fat is burned for the energy that drives our metabolism. However burning fat does not reduce weight. There is a fundamental principle in chemistry: after any chemical reaction, there is no change in the total number of atoms of any type. If we have the same number of atoms before and after a fat-burning reaction, then we haven’t lost any weight.
If we did lose weight by turning fat into energy, we would explode. Spectacularly. Turning matter into energy is what happens inside an atomic bomb or a nuclear reactor. It’s a nuclear reaction, not a chemical reaction. Very small amounts of matter equate to a vast amount of energy.
That’s what E=mc² is telling us. The letter ‘c’ stands for the speed of light, which is pretty quick. Then it is squared, giving us a massive number: 90,000,000,000,000,000 (in metric units). Multiplying this by the mass (m) gives the amount of energy (E) released. Thus, turning just 1g of fat into energy would release over 20 billion Calories. To put it the other way around, to get the total number of Calories an average person needs in a day would only require the energy from about one tenth of one millionth of one gram of fat.
Which means that whatever we are doing, we are certainly not losing weight by turning fat into energy.
Fats come in many forms (saturated, unsaturated, trans, omega etc) however they share one thing: they are all made up of atoms of carbon, hydrogen and oxygen. We ‘burn’ fats by combining them with oxygen. We get the oxygen by breathing it in. The process is known as cellular respiration, and is well understood but breathtakingly complex (pun intended, sorry).
In doing this we produce energy and molecular byproducts. The byproducts contain the same atoms that were in the fat and the oxygen, but now arranged as different molecules — carbon dioxide and water. The ‘weight’ (strictly, mass) of the carbon dioxide and water after the reaction = the weight of the fat and oxygen before it, because all we have done is re-arrange their atoms. We get chemical energy from this because the small carbon dioxide and water molecules occupy a lower energy state than the highly structured and complex fats that they came from, and so the reaction releases energy.
We then breathe out the carbon dioxide and some of the water as water vapour. The body can use some of the remaining water, and the water that is not needed is eliminated by the kidneys, bowels or sweating.
In ridding ourselves of the carbon dioxide and water we ‘lose’ weight. It turns out that most of the weight is exhaled.
So, that’s where our weight mainly goes — peacefully puffed into thin air.