Fats provide twice as much energy per unit mass as compared to carbohydrates & proteins. Fat consumption offers multiple benefits like acting as a fuel source to provide us energy for daily activities, exercising muscles & acting as a reservoir for energy storage which can be used in the times of need.
Through Beta-Oxidation, our cells can generate energy by metabolizing fatty acids. During the period of fasting & exercise, our body breaks down fat stores to release fatty acids that can be used as fuel by our muscles. However, the effectiveness of your muscles to use fatty acids as fuel depend upon your Beta-Oxidation or fat metabolism trait which varies from individual to individual based upon genetic code. This becomes all the more important when you are looking to shed body fat &/or improve your overall fitness level & body composition.
When we talk about generating energy from fat, we refer to a specific fat molecule- triglycerides. It has two types of molecules-fatty acids & glycerol. Our cells can use fatty acids as a source of fuel to produce ATP. Therefore the first step in getting energy from fat is to break down triglycerides to fatty acids. Whenever we eat a meal containing triglycerides (fats), our pancreas releases enzymes called lipases into the small intestine. These enzymes break down triglycerides in our food into fatty acids & glycerol. Triglycerides are then packed with cholesterol to form chylomicrons.
Chylomicrons transport triglycerides into the bloodstream from intestine to liver. Blood vessels supplying our skeletal muscles & adipose(fat) tissue produces an enzyme called lipoprotein lipase. This enzyme breaks down triglycerides releasing fatty acids. These are called free fatty acids which can be used by muscle cells for energy or can be stored as fat in adipose tissue. The liver also produces its own triglycerides. These are packaged with cholesterol into VLDL,very low density lipoprotein, which allows triglycerides to be transported into the bloodstream. Triglycerides in VLDL are broken down by lipoprotein lipase enzymes and supplied to our skeletal muscle & adipose tissue. This process releases fatty acids which can be used by muscle cells to generate energy. Besides, we have a lot of triglycerides stored as fat in our adipose tissue. Through lipolysis, fat cells can break down triglycerides and release fatty acids into the bloodstream. These fatty acids are then used by our muscle to fuel muscle contraction. This is stimulated by our hormone adrenaline (the fight or flight hormone released during exercise or stress). Adrenaline, for example, released during HIIT or MRT enhances the breakdown of fat stores & is an effective way of reducing fat %.
Role of Mitochondria in transporting fatty acids
Mitochondria is the powerhouse of cells that is responsible for generating energy in the form of ATP from different fuel sources. Before fatty acids can be used as energy, it must enter mitochondria. For this fatty acids are first converted into fatty acyl-CoA. One class of enzyme responsible for this reaction is called ACSL (long chain fatty acyl-CoA ligase/synthetase). The activity of ACSL enzyme is very important for allowing your mitochondria to use fat as an energy source. This enzyme is encoded by the ACSL5 gene. Once the fatty acids are converted into fatty acyl-CoA, they can be transported across mitochondrial membranes to the inner part of mitochondria called mitochondrial mix. This process is facilitated by various transport proteins. Some of these transport proteins use amino acids like carnitine which is important for your cells to transport & use fats for energy. Our bodies are capable of producing enough carnitine to meet our fat metabolism needs.
Once the fatty acids are converted into fatty acyl-CoA & transported to mitochondria, they can be used to produce ATP. This generation of ATP from fatty acids is called beta-oxidation. Through a series fatty acyl-CoA is converted into molecule Acetyl-CoA. This would sound familiar since this is produced when mitochondria uses glucose as fuel. After the first stage of Aerobic respiration, Acetyl-CoA is formed from breakdown of glucose. This molecule then enters final two stages of respiration-Krebs cycle & electron transport chain which generate ATP which is used to power various cell processes including muscle contraction.
As both beta oxidation of fatty acids & aerobic respiration of glucose provides ATP, fatty acids are a perfect energy source when glucose is low. Therefore for those looking to reduce fat % should look at replacing your glucose with fatty acids.
As mentioned above, ACSL enzyme is used to convert fatty acids into Acyl-CoA enabling mitochondria to produce energy from breakdown of fatty acids. Variants of the ACSL5 gene can help you evaluate the effectiveness of this gene.
ACSL5 gene variants can alter the degree of fat oxidation in response to diet and exercise. A SNP of the ACSL5 gene creates two different gene variants: the ‘T’ allele and the ‘C’ allele.
People carrying the ‘T’ allele are shown to produce higher amounts of the ACSL5 enzyme in their muscles and lose more fat in response to a calorie restricted diet.
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