• Isaac J. Wedig MS, CSCS

Where does fat go when you lose it?

In one of our many infinite parallel universes, there exists a barren wasteland with mountain after mountain, as far as the eye can see, of yellowish colored jello-like substance, resembling the fake rubber chunk of fat that your health teacher kept on her shelf. This distant universe is where all of the lost body fat of dieters past has gone and can only be reached via passage through a massive black hole that lies thirty light years past the rings of Saturn.

I know, that's a pretty cute little story that I threw together. It almost sounds the introduction to another, and far less entertaining, version of Star Wars. But all joking aside, where does fat go when we lose it? Surely it doesn't pass through a black hole and end up in an alternative universe.

If you lose 10lbs of fat, where does it go? Chances are this is one of those question that you haven't given serious thought to. However, it’s quite a thought provoking and mysterious question. Fat doesn’t just vanish into thin air. The typical thought-out answer to this question is that body fat is burned up and turned into energy. This isn’t a wrong answer; however, it isn’t necessarily the whole answer. If we consider the laws of chemistry and physics, they suggest that there is a bit more going on in this process than simple energy conversion.

It is important to realize that body fat not only contains energy but it contains matter. It has mass, or in other words, its weighs something, you can touch it. Fat is made up of carbon, hydrogen, and oxygen atoms, which are held together by chemical bonds. The chemical bonds that lie between these atoms are where the energy is stored. Energy itself doesn’t have mass. When we burn fat for energy, we are breaking the fat down, pulling it apart is atoms and releasing the energy that is held within its bonds. That energy can then be captured and used to power various cellular processes.

However, after the bonds are broken and the energy is released, we are still left with the atoms, carbon, hydrogen, and oxygen, to be specific. These are the actual components of fat that give it mass. Therefore, the fat hasn’t actually physically gone anywhere. The law of conservation of mass states that mass cannot be created or destroyed. Atoms don’t vanish. Our fat has to go somewhere after we break it down for energy and ‘burn’ it, so to speak, just like a burnt log leaves behind a pile of ash.

Below is the chemical equation for fat breakdown, or more properly called fat oxidation. If you’ve taken a basic chemistry class, this type of equation might look pretty familiar. Quite simply, this equation represents a chemical reaction of body fat combining with oxygen, forming the products carbon dioxide and water.

C55H104O6 + 78O2 -----> 55CO2 + 52H2O

(Fat) (Oxygen) (Carbon Dioxide) (Water)

Why am I haunting you with the long forgotten memories of chemistry 101? Because this equation is what holds the answer to our question. The average chemical composition of a fat molecule in the body is C55H104O6, represented on the left side of the equation above. Fat can vary in its number of carbon and hydrogen atoms but for the most part, this is what fat looks like chemically. Oxygen in this formula, O2, represents atmospheric oxygen that we breathe into our lungs from the air. When fat is broken down, it combines with the oxygen and its atoms reassemble to form two products; carbon dioxide and water. These products, which lie on the right side this equation, are essentially what fat turns into after its atoms are broken apart for energy. Carbon dioxide, CO2, is the primary component of what we exhale in our breathe. Water, H2O, is well, water. And yes, we can make it in our bodies.

So, where does our fat go when we lose it? Oddly enough, most of it goes into the air! Eighty-four percent of the fat that we burn is transformed into and expelled from the body as carbon dioxide. Take a look at your breathe really quick, some of that is fat. Pretty crazy, isn’t it? The remaining sixteen percent of the fat that you burn is made into metabolic water and later excreted as sweat, urine, or spit. If you lose 10lbs of fat, about 8.4lbs leaves as carbon dioxide and 1.6lbs leaves as water.

BOOM! Science. Turns out there is no alternative parallel universe where all the lost body fat goes.


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