The science of fat loss: Fat burn, fat oxidation – what does it actually mean?
In fact, both terms describe the same process. Fat burning means that our body uses stored fat to burn as fuel, which leads to fat loss. Fat oxidation is the process of fat burning – it requires oxygen.
Now, what happens in our body when we burn fat and how can we enhance this process with diet and exercise?
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Fat as fuel
First of all, our body uses carbohydrates as the main source of fuel for daily activities. Right after that comes fat. Protein can also be utilized as a source of fuel but it takes much longer to break down amino acids. In addition to that, the break down of amino acids burns quite a few extra calories as well (thermogenic effects of protein), which does not make protein the ideal source of energy.
Now, fat in itself comes in different variations. I have already written an article on “Full Fat to lose Fat”, explaining the healthy kinds of fats and which ones to avoid.
The type of fats we consume in our diet is very important when it comes to athletic performance and general health.
However, fat oxidation (burning fat with oxygen) is not linked to the amount of fat consumed through diet, neither does fat storage.
Digestion of Fat
Fats we consume through our diet are converted into free fatty acids in our intestines. These free fatty acids, also called triglycerides are stored in adipose tissue. Certain hormones in our body regulate the breakdown of fatty acids (called lipolysis). Insulin is a hormone that stops lipolysis and further enhances the storage of triglycerides in adipose tissue.
If lipolysis takes place, the triglycerides are reduced to glycerol (used by the liver for further use) and fatty acids which are then released into our bloodstream. From there they have access to every cells in our body: They are used to synthesize hormones in the liver, intracellular signalling and build up membranes of our cells.
A diet too low in fats will harm your whole body: Your metabolism will slow down because of low thyroid levels, osteoporosis begins because of low estrogen and/or testosterone, your immune system will get weaker and many more.
In our cells, fatty acids can be stored and used to provide energy. This is done in mitochondria. To transport fatty acids to the mitochondria, you need an enzyme called carnitine. This enzyme is found in red meets and poultry.
In our mitochondria, you can find stored DNA (deoxyribonucleic acid) and enzymes necessary for ongoing cellular construction. These enzymes also help in oxidation of the fatty acids to generate ATP, the energy equivalent in our body.
To sum this part up:
Fats are digested in our intestines and broken down to fatty acids through lipolysis. To generate energy from fats (to burn fat as fuel), we need to transport them into our mitochondria. This is done my carnitine. Once the fatty acids arrive in our mitochondria, they can be used to generate ATP through fat oxidation (which means you burn fat with the use of oxygen and as a result you get ATP).
I should add that long chain fatty acids cannot be utilized to generate ATP in our red blood cells and brain. Red blood cells don’t contain mitochondria and fats cannot pass the blood brain barrier.
What regulates fat burning?
As I mentioned above, there are hormones in our body regulating lipolysis and fat oxidation in our mitochondria.
Hormones increasing fat burning are:
These hormones are released by the pancreas and sympathetic nerves in adipose tissue. Blood levels increase when blood glucose levels are low, for example after meals. Increased glucagon and epinephrine also suppress the release of Insulin.
In contrast, Insulin is one of the main hormones decreasing fat burning processes. It also enhances the storage of triglycerides in our adipose tissue.
Fatty acids synthesis
To understand the regulation of fat burning, we need to know how fatty acids are synthesized in our body as well. Fatty acids can be formed from carbohydrates in the liver, adipose tissue and mammary glands during lactation, as well as cells of the central nervous system.
When glucose is broken down to provide energy, some of the products can be used to synthesize fatty acids. Among these is pyruvate. It is converted to Acetyl-CoA in the mitochondria. Acetyl-CoA is also the end-product of fat oxidation. Again, this process takes part in the mitochondria and Acetyl-CoA is used to synthesize fatty acids and cholesterol.
However, it is important to note that not all kinds of fatty acids can be synthesized. I’m sure you have heard of so-called “essential fats” we should consume in our diet. These healthy fats, mostly polyunsaturated fats, have very long chains of fatty acids. Our body cannot link so many fatty acids and only produce short and mid-chain fatty acids.
The fats produced are still essential for survival, as cholesterol is used to synthesize hormones.
However, if you do not supply the body with what it lacks, the long-chain fatty acids, it will start to produce too much of the short-chain fatty acids and cholesterol. High cholesterol levels are very common nowadays and are often a result of too low healthy fat consumption and too much unhealthy fats and carbohydrate consumption.
Regulation of fatty acid synthesis
The same rules as for fat oxidation apply to the synthesis of fatty acids. Insulin, aka high blood glucose levels, increases fat synthesis, while Glucagon and epinephrine, released when blood glucose is low, increase fat oxidation.
Disorders of fatty acid metabolism
Hypertrygliceridemia and Hyperlipidemia are disorders that may be genetic in most cases but can also be acquired. Oftentimes, certain enzymes needed to break down triglycerides are lacking due to genetic mutations.
However, your lifestyle contributes to high triglycerides and cholesterol levels even more than genetic mutations.
Risks of high cholesterol and triglyceride levels are metabolic syndrome (abdominal obesity, elevated blood pressure, elevated fasting plasma glucose, high serum triglycerides, and low high-density lipoprotein (HDL) levels), diabetes, heart disease, infections and many other diseases.
This concludes Part 1 of this series. I hope you enjoyed reading and learning about the physiological processes in your body. It is important that you fully understand this because we will use this knowledge to reach our fitness goals in the following articles!
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