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Where Does Fat Go When you Lose It?

When discussing Metabolic Efficiency Training (MET) the term "fat burning" is used frequently as the focus on MET is on nutrition. The term "fat burning" used in Metabolic Efficiency Training is in reference to the fat loss process, but the question, "where does the fat go when we lose it?," seems like a basic question we should all know the answer to. So, I asked a few folks and the answers went something like this:

- Hmmm....?

- Well......?

- I don't know

- Poo

- Sweat

- you burn it up as energy

- It doesn't go anywhere

Some people think fat is converted into energy or heat, while others think the fat is somehow excreted or goes nowhere. Essentially no one I asked knew the answer!

So what is the basic science behind fat loss and where the heck does it go when we lose it?

The chemical formula for human fat is C55H104O6. This is the average fat molecule formula in the human body. Some might have a few more carbon atoms and/or a few more hydrogen atoms or a few less. What always stays the same is the amount of oxygen atoms - they stay at 6.

The body needs a certain amount of energy to function, and it gets that energy from food. When you consume more energy than you expend, it gets stored in fat cells as triglycerides which are made of carbon, hydrogen and oxygen. Fat goes directly into the cells, while excess carbohydrate energy is converted to triglycerides. Protein resists conversion to fat because your body uses this nutrient for other functions.

When you consume less energy than you expend, your body is then able to tap into that stored fat. Those triglycerides then go into the bloodstream and break up into smaller particles of fatty acid, which tissues throughout your body can then use as fuel. These fatty acids get broken down yet again into smaller chemical components. The breaking of those chemical bonds produces energy, and then your body is left with a bit of water and a lot of CO2.

The general equation is Fat + Oxygen gives you Carbon Dioxide plus water:

C55H104O6(FAT) + 78O2(Oxygen) —> 55CO2(Carbon Dioxide) + 52H2O(Water)

So the answer to the question of where does the fat go when we lose it is: Into thin air!

Yes, the end product of fat burning is through your breath - breathed out as carbon dioxide. Our lungs are the primary excretory organ for actual fat loss.

Fat comes out of the body as carbon dioxide and water:

84% of fat is exhaled as CO2

16% of fat is excreted as H2O

10kg of Fat becomes 8.4kg of CO2 + 1.6kg of H2O

This 1.6kg of water exits our bodies in ways that are more familiar, as sweat, urine, tears and other bodily fluids.

Why bother with explaining where the fat goes when we lose it?

Aside from the fact that knowledge is power, and who doesn't want to be powerful? And that we currently have an obesity epidemic in our country which I believe is due in large part to a lack of education on nutrition basics. As I was surely in the "dark" when it came to these basics before I started on my own nutrition journey. But, I also come back around to Metabolic Efficiency testing for targeted weight loss and athlete fueling intake for training and racing.


Because the method used to gain qualitative data on an individuals unique metabolics is through metabolic testing. This is done via a gold-standard metabolic cart with a Certified Metabolic Efficiency Training Specialist. This process is done by measuring an athletes oxygen intake and carbon dioxide expenditure during exercise. This tells us how much fat you are actually losing and everyone is different based on many factors.

Now you know where the fat goes when you lose it! Now measure it and say good bye to it forever.

Into thin air!

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