Q: How Do We Convert Body-Fat Into Energy

A: We don’t!

By Steve Anthony

This one is a bit geeky, but I think it is cool.

Many people view the body as a machine that burns fuel. Burning the fuel, in turn, creates heat and that heat is what gets us to move and keeps us alive. The fuel can be the food we eat, stored glucose and/or body-fat. To the surprise of many, however, we don’t burn anything. In fact, we don’t even convert fat into energy!

It’s not surprising that many have this incorrect view of what’s going on, since we talk about “burning calories” all the time. But a calorie isn’t even a unit of stuff — it’s a measure of heat: A calorie is the amount of heat required to raise 1 gram of water by 1 degree Celsius. So “burning calories” is like saying “burning heat.”

Then what IS going on? If we don’t burn body-fat, for example, what happens to it? Where does it go when we “lose it?”

We can turn to Physics to help us — specifically, the Laws of Conservation of Matter and Energy. But stay with me, it’s not too complicated, and actually, it’s kind of cool.

The Law of Conservation of Matter states: Matter is neither created nor destroyed; it must be conserved. That means there is a fixed amount of each of the elements (on the Periodic Table) in the universe — you can combine, say, two hydrogen atoms with one oxygen atom to make a molecule of water (H2O) but if you break down that molecule, you will end up with the original 2 hydrogen atoms and the oxygen atom — the matter (the atoms) will be conserved — as will their mass or weight.

The Law of Conservation of Energy states: Energy is neither created nor destroyed, but it can be transformed. So, we will always have the same amount of energy in the universe, just in different forms (heat, motion, electricity, etc.).

The key here is the transformation. The transformation of the matter and the transformation of the energy are related. As a concept, these transformations are called metabolism!

For our discussion here, we just need to remember that the matter we are talking about is body-fat. To keep it simple, we will look at a single molecule of body-fat.

A molecule is a collection of atoms — so, in our case, the matter we will be starting with is the collection of atoms that make up one body-fat molecule. The energy is what’s holding that collection of atoms together in the form of the molecule of body-fat.

There are several different types of body-fat molecules. We will just work with one that is composed of 165 atoms: 55 carbon atoms, 104 hydrogen atoms and 6 oxygen atoms, or, C55 + H104 + O6. Keeping these atoms together is energy in the form of chemical bonds.

You might have heard people discussing “fat oxidation” when talking about using body-fat to power the body. To oxidize, you need to add oxygen molecules (O2). We add oxygen to our bodies when we breathe, so that’s easy. To oxidize 1 molecule of body-fat we need 78 additional molecules of oxygen 78O2 (or 156 oxygen atoms).

So, now we have C55 + H104 + O162 (O6 + O156 = O162). When the fat molecule reacts chemically with the additional oxygen molecules, the reaction produces 55 molecules of CO2 (carbon dioxide) plus 52 molecules of H2O (water) plus a release of energy when the chemical bonds holding the fat molecule together are broken.

The Law of Conservation of Matter says that we need to have with the same number of atoms at the end of this process as we did at the start. Does the math work out? Yes: We started with 55 carbon atoms and we ended with 55; we started with 104 hydrogen atoms and we ended with 52 X 2 = 104; we started with 162 oxygen atoms and ended with (55 X 2) + 52 = 162.

So the matter was conserved. Our fat leaves our body in the form of water and carbon dioxide! Calculating just how much water and how much carbon dioxide is more complicated, so I’ll cut to the chase: 16% leaves as water and 84% leaves as carbon dioxide. So when you lose a kilogram of body-fat, 160 grams leaves your body as water and 840 grams leaves as carbon dioxide.

Getting back to the title of the article, we see that the body doesn’t convert fat into energy. The body converts fat into carbon dioxide and water, and in doing so, releases the energy in the chemical bonds holding the original fat molecules together.

But if you add together the weight of the water and carbon dioxide created by the oxidation of the fat, it will weigh the same amount as the lost fat did.

Consider this to illustrate:

There is a sealed room that contains all the air and water you will need for the time it will take you to lose 1 kg of body-fat. It also has a TV, stereo, etc., books, exercise equipment and so on — including a sealable, self-contained toilet. But no food (just to keep it simple). The room weighs 1000 kg., empty, and is on a scale. You weigh 100 kg. So when you enter the room, the scale shows 1,100 kg. There is also a scale IN the room. When you stand on it, it shows 100 kg.

Time passes and you lose 1 kg. of body-fat. When you stand on the scale IN the room, it shows a little less than 99 kg, because you’ve lost some water-weight not related to the body-fat, as well. But the scale the room is on still shows 1,100 kg. Because all the weight you lost (the water and the carbon dioxide) is still in the room.

I hope you enjoyed this little trip down Geekydom Lane!