The Chemistry of Weight Loss: Where Does the Fat Go When You Lose It?

Updated on May 11, 2018
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Chris enjoys writing about a variety of topics including science, health, and fitness.

Have you ever wondered what happens to your fat when you lose it? When asking some friends and family this question I got various answers ranging from "it burns up" to "it disappears" or that it is "excreted as solid waste" (they didn't say this exact phase but you get the picture). You've also probably heard that fat is "converted to muscle" as well. Unfortunately, none of these answers are correct. The truth may surprise you.

According to the law of conservation of mass, in any closed system matter cannot be created or destroyed, it can only be converted to other forms of matter. Therefore, if fat were to "burn up" or disappear, this would violate the law of conservation of mass. In addition to this, it is impossible for fat to be converted directly to muscle tissue since they are completely different from a biochemical and physical perspective.

Our bodies are made up of systems which designed to function using an innumerable amount of chemical reactions. Inputs such as food and water are ingested, nutrients are absorbed, and energy is created or stored as fat. When those processes are complete, anything that cannot be used by the body in combination with waste material is excreted. Any mass of material that not excreted by the body in various ways has to be retained and stored in the body as fat, muscle, and other tissue. Therefore, for weight loss to occur (or more specifically, fat loss) it must eventually be excreted from the body. To understand where the fat really goes, let's first take a look at what fat actually is and then what happens to it when it metabolizes.

What is Fat?

All fats are part of the larger family of soluble molecules known as lipids. Chemically speaking, fat cells in the human body are stored as molecules known as triglycerides. Triglycerides are made out of three fatty acid chains bonded together with a glycerol molecule. There are many different types of triglycerides, all of which are comprised of various combinations of three of fatty acids. The different combinations of fatty acids which make up the triglyceride molecule will determine if the fat exists either as saturated, unsaturated, monounsaturated, polyunsaturated or as a trans fat.

Some fatty acids are considered "Good" while others are considered "Bad." Various fat containing foods have different fatty acids which in turn will result in different health effects. The most common fatty acids are palmitic acid, oleic acid, and linolenic acid however there are several others as well.

When needed for energy, your body will take the triglycerides from your fat cells and break them down to form the glycerol and the fatty acid substrates. The glycerol can be converted directly into glucose which can then be used to generate energy. The fatty acids can also be used by your cells to produce adenosine tri-phosphate (ATP) which is the main energy carrier of body cells.

The chemical makeup of human body fat is such that the typical, or average, triglyceride can be best represented by the following chemical notation: C55H104O6 . Therefore, this "average" body fat molecule contains 55 Carbon atoms, 104 hydrogen atoms, and 6 oxygen atoms.

Metabolizing Fat

From a biochemical perspective, fat is metabolized through a process called oxidation. In a paper published in the British Medical Journal (BMJ) in 2014, researchers determined that when fat is metabolized, it is broken down and ultimately converted into to carbon dioxide and water. The chemical reaction for this process would be summarized as:

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

From the reaction, we can conclude that for every pound (0.454kg) of fat that is metabolized, 6.4 pounds (2.9kg) of oxygen are needed to make the conversion.

Furthermore, the analysis presented in the BMJ paper shows that 84% of metabolized body fat is converted to carbon dioxide and the remaining 16% of the fat is converted to water. This means that most of the fat that you burn leaves the body through exhalation via the lungs and the rest leaves through urine, sweat, and other bodily fluids.

How to Metabolize More Fat

With the knowledge of the above information it may seem that simply breathing more would help to promote weight loss. However, this tactic won't work for weight loss because breathing more doesn't actually consume any more oxygen than just breathing normally would. Instead, if you increase your breathing rate by exercising or moving more, then your body will require more oxygen to support the resulting increase in metabolic rate. With heavier breathing due to exercise and movement, more oxygen is used help break down fats resulting in a greater production of energy, carbon dioxide, and water. Thus, more fat is burned when more oxygen is consumed by the body.

Want to know how much CO2 you must exhale to lose 5lbs of fat? What about 10lbs? Use the table below to answer this question.

Fat Loss (lbs)
Carbon Dioxide Produced (lbs)
Water Produced (lbs)
1
0.84
0.16
2
1.68
0.32
5
4.2
0.8
10
8.4
1.6
15
12.6
2.4
20
16.8
3.2
25
21
4
30
25.2
4.8
40
33.6
6.4
50
42
8

A Brief Video Explanation

References

Meerman, Ruben. Brown, Andrew J. "When Somebody Loses Weight, Where Does the Fat Go?" British Medical Journal, Volume 349, no. Dec19 3, 2014. <https://www.bmj.com/content/bmj/349/bmj.g7257.full.pdf>

Trimarchi, Maria. "10 Myths About Body Fat" How Stuff Works. Accessed 2018. <https://health.howstuffworks.com/mental-health/human-nature/health-myths/10-body-fat-myths.htm>

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