Fat Metabolism
Fat metabolism takes place during the three main Energy Transfer Process which are designed to break down this macronutrient’s (along with carbohydrates and proteins) chemical bonds to form ATP. Although most people have a negative connotation of fats, they are extremely important for good health and in achieving maximum performance. Fats play many roles in our bodies such as: regulation of hormones, transport vitamins and minerals through the body, are structurally important (along with triglycerides) in our plasma, and they are our largest source of energy in the body.
Our body is capable of synthesizing our own fatty acids, however; there are some that must come from the food we eat. Just like carbohydrates and proteins, these fats and broken down during the energy transfer process. For fat metabolism, there are six major pathways that this macronutrient is metabolized in its goal of creating ATP. They are: 1) The Triglyceride and Fatty Acid Pathway, 2) The Beta Oxidation Pathway, 3) The Ketone Pathway, 4) The Fatty Acid Synthesis Pathway, 5) The Triglyceride Synthesis Pathway, and 6) The Cholesterol Synthesis and Catabolism Pathway.
1) The Fatty Acid Pathway
Fatty acids are a very important source of energy and in ATP production. Fatty acids can be stores along with glucose and other nutrients very effectively in fat. Three molecules of stored fatty acids are joined together tightly with one molecule of glycerol to form triglycerides. This is important because triglycerides contain twice as much energy per mass as carbohydrates or proteins. The stored fatty acids come into play during our normal activities of daily life, during such activities as ultra trail running which is a long-duration, low-intensity exercise, and during rest periods in between exercises such a interval training. The fat that is in our body as well as the fat that comes from the foods we eat are bundled as triglycerides and must be broken down into fatty acids to receive the stored energy.
Triglycerides are stored in our liver, fat, and muscle tissues. When the body calls on this energy source, the triglyceride molecule is broken down through the lipolysis process to form three molecules of fatty acids and one molecule of glycerol. These fatty acids can be used for energy in the tissue that they were broken down in, or they can be used elsewhere in the body and delivered through the bloodstream. Think of it like this, muscles need energy to work, and their preferred source is stored fatty acids. The larger your muscles, the more stored fatty acids will be required to feed those larger muscles. This means that the best way to loose fat is to increase your lean body mass. And the best way to do this is through resistance exercises.
2) The Beta Oxidation Pathway
Fatty acids are metabolized through the oxidation pathway when they are need for energy transfer. In this process, the fatty acids are broken down into acetyl-CoA molecules. Acetyl-CoA molecules role in metabolism is either to enter the Krebs Cycle (Part of the Oxidative Energy Pathway that forms 32 molecules of ATP), or used for fatty acid synthesis. Fatty acids are capable of producing a lot of acetyl-CoA molecules. And when added into the Krebs Cycle this of course produces a lot of ATP. The end result of the break down of one triglyceride molecule is the production of 457 ATP molecules. To put this into perspective, one glucose molecule produces 36 molecules of ATP. When it comes to ATP production “fat is where it’s at”.
This Beta Oxidation Pathway process can only take place in the presence of oxygen, or in the aerobic pathway. Once you go into an intensity level where the amount of oxygen available is below the energy transfer requirements, the oxidation pathway will shut down. It is at this point that the body will use glucose that is processed through the glycolysis pathway which is anaerobic (without oxygen) pathway.
3) The Ketone Production Pathway
The liver produces ketones where they are used by mitochondria (the power-house of a cell) for ATP production. Ketones are produced in times of “famine” or when energy levels are very high and carbohydrate intake is low, which in turn causes glucose levels to be low. To put it into energy pathway terms, when carbohydrate demands exceed carbohydrate intake the beta oxidation pathway is the main fat metabolism pathway. Because of this, there are an excess of acetyl-CoA produced that can’t be transported to enter the Krebs Cycle to be turned into ATP. When this level is reached, the Krebs Cycles shuts down and the liver shifts into gear to start to produce ketones.
The ketones that are produced by the liver can be converted into acetyl-CoA and transported through the body through peripheral tissue. With this process, the brain, red blood cells, and muscles can receive the energy transfer of the nutrients they need when glucose is in short supply.
4) The Fatty Acid Synthesis Pathway
A large amount of the stored fat in our body comes from the food that we eat in the form of triglycerides. As we have discussed, these triglycerides are broken down into fatty acids. Once broken down they are regenerated back into triglycerides and stored in muscle, fat, and other cells ready to be used in energy transfer or just for storage. Not only are the dietary fats that we eat turned into and stored as fat, but other extra nutrients also have the potential be becoming fat through the fatty acid synthesis pathway.
These new fats are primarily produced in the liver but can be produced in the fat. The production of new fat that takes place in the fat tissue happens during times of dietary excess and low energy demands. So, you can see that a person that over eats and does not exercise is going to produce more fat in the fat that they already have. As you can probably guess, the fatty acid synthesis pathway is utilized when a person has a diet that is high in sugar and a sedentary lifestyle. With the constant introduction of sugar, carbohydrates, and glucose to the system when energy levels are low and energy transfer is not required, a build up of acetyl-CoA occurs. It is these excess acetyl-CoA that join together to form new fats.
5) The Triglyceride Synthesis Pathway
As discussed above, triglycerides are stored in our liver, fat, and muscle tissues. When the body calls on this energy source, the triglyceride molecule is broken down through the lipolysis process to form three molecules of fatty acids and one molecule of glycerol. Most triglycerides are transported through the body through the bloodstream. From a dietary perspective, triglycerides are introduced to the body in such foods as butter, margarine, and vegetable oils. Our body will convert any calories that it does not directly need for energy transfer into triglycerides as a way to store this untapped energy. The triglyceride synthesis pathway kicks into gear when our body reaches its capacity of glycogen. The excess glycogen is sent back to the liver and it is converted into triglycerides, which are then stored in the fat. A problem occurs when these excess triglycerides are not stored in the fat and travel through the bloodstream. This can cause the blood to start to thicken.
6) The Cholesterol Synthesis and Catabolism Pathway
Cholesterol is steroid of fat that is produced in the liver and intestines. PhD John Berardi states that, although the liver makes about 20% of the cholesterol there are many tissues within the body that produce the remaining 80%. It is an essential in the production of hormones, the absorption of dietary fat, and the production and functioning of cell membranes. Cholesterol synthesis is a four step process where acetyl-CoA is synthesized with the outcome being the production of ATP.
Cholesterol is not water soluble and needs to be transported in the bloodstream by lipoproteins. Lipoproteins are commonly known as LDL (Low Density Lipoproteins) and HDL (High Density Lipoproteins). LDL is considered the “bad cholesterol, while HDL is considered the good-cholesterol. LDL is considered the “bad-cholesterol” since it can break apart and deposit itself in the vascular system which can lead to plaque buildup. HDL to the rescue. HDL is considered the “good-cholesterol” because it has the ability to attached itself to LDL and transport it back to the liver for removal from the body.
