The Science of Fat Loss

By Christine Hronec  

Have you ever wondered how fat leaves your body? I’ve polled this question and got a wide range of guesses from urine, sweat, bowel movements, to cells shrinking, and even fat being converted into muscle. The reality is that fat doesn’t leave the body via ANY of these methods. So where on earth does it go once you burn it?

The moment you decide you want to start eating healthy and making consistent exercise a priority, does not mean that excess body fat will magically disappear. You can workout every single day performing an hour of cardio and still look pretty much the same without any notable difference in your body fat. This is because burning calories does not necessarily mean that your body is mobilizing stored energy from adipose tissue as your preferred fuel source. For this reason it is essential to understand the role of each macronutrient in the body, what foods to get them from, and how to determine how much you need of each macro to support your specific goals.

When you are performing any form of physical activity whether it is walking to your mailbox or going on a run, the first source of energy the body will utilize for fuel are carbohydrates in the form of glucose. Glucose found in your cells or blood stream is converted to usable energy very quickly. In order for fat to be used for energy to result in fat loss, an energy requirement must be present that is not being satisfied by readily available resources such as glucose. As described in chapters 5 and 6, think of carbs as cash in your checking account and fats as your funds in a savings account. Before you go through all the hassle of making a transfer or withdrawal from a savings or money market account, you will use your liquid funds first in the form of cash you may have around the house for a rainy day or from a checking account. So when it comes to fat loss, available energy in the form of carbs is preferentially used first before tapping into fat stores.

Lipolysis is the breakdown of triglycerides into glycerol and free fatty acids. When fat is used as a fuel source, free fatty acids are metabolized through a process called oxidation. In order for this where all the funds are stored. The money is available for use, but cannot be accessed unless the process to occur, the body requires oxygen to be present. Think of oxygen as the key to the safe where all the funds are stored.The money is available for use, but cannot be accessed unless the code to the safe unlocks the vault of this precious energy source. To us we may think of fat as an unpleasant thing we desperately want to get rid of, but to the body, it sees it as its most energy dense resource and it does NOT want to part with it unless it absolutely has to.

Nutritionally speaking there many differences between proteins, carbs, and fat.
However from a chemical structure perspective, the amount of energy needed to oxidize each macronutrient can be better understood from the perspective of oxygen. See this drawing below of a basic protein, carbohydrate, and fat molecule (see below).

It should be observed that in order to oxidize each macronutrient, the amount of energy needed to do so will depend on the amount of oxygen present in each macro-molecule. What is unique is that both proteins and carbs have the same exact ratio of carbon to oxygen atoms and both have the same exact caloric density of 4 kcal/g. In order to break down the fat molecule it can be seen that there are substantially more carbon atoms compared to oxygen atoms.

In order to oxidize a fat and release CO2, 2 oxygen atoms are needed for every 1carbon atom. This means that significantly more oxygen is needed to burn off fat as opposed to carbs and protein. This is why you tend to breath heavier during intense exercise as the body’s way of infusing the blood with oxygen (where it should be noted that the air we breathe is 80% nitrogen and only 20% oxygen). This is the reason that carbohydrates are able to generate more aerobic energy compared to fat. Carbs are able to make the limited oxygen supply in a contracted muscle go further, which actually prevents fatigue. This is one of the main reasons endurance athletes rely on carbs so heavily as the primary fuel source as opposed to fats. In simplified terms is that the oxidation of fat is a series of complex biochemical processes that result in this guiding overall chemical reaction:

Basic governing fat oxidation reaction*
Fat + O2  CO2 + H20 + Heat
*The  represents a series of biochemical reactions that we are omitting for simplicity
After balancing this reaction with the appropriate stoichiometry, this results in the following balanced reaction:
If one loses 10lbs of body fat, based on this governing equation, the math shows that 10lbs of fat is broken down into:
This means that fat is actually being broken down into 84% CO2 and 16% water however when fat is lost, it needs to be realized that over 84% of the fat oxidation process occurs through carbon dioxide or more simply put, through your breath. Where only 16% of fat loss is in the form of water via sweat and urine, but the majority of fat loss occurs when one exhales.