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Using Energy, Burning Fat – Pt. 3, Aerobic Energy System

The aerobic system is what the body uses to produce energy throughout the day and even during the night while you are sleeping. Because the process uses carbs, fat and protein as its energy source, it can continue to produce energy for the resynthesis of ATP for long periods of time. On the other hand, because it is a complex process and requires oxygen, it does not necessarily create the energy quickly. Therefore, it works for daily activities and low intensity exercise. The aerobic system consists of three steps.

Step One – Metabolism of carbs, fat or protein
The first step is the conversion of carbohydrates, fat or protein. When carbs are metabolized, they are converted into pyruvic acid just like with the Lactic Acid System previously discussed. The difference is the presence of oxygen. In the aerobic system, the presence of oxygen converts the pyruvic acid into acetyl CoA.

Fats are stored as triglycerides in adipose tissue and skeletal muscle. The metabolism of fat produces more energy than carbs or protein. If I understand the information correctly, fat stored in muscle tissue is readily available whereas fat stored in adipose tissue has to be converted into free fatty acids (FFA). Using a process called beta oxidation, the fat is converted into acetyl CoA.

Protein yields the least amount of energy for the resynthesis of ATP. The main source of protein energy in the body is muscle. Under severe glycogen depletion, protein is converted to glucose by gluconeogenesis. As the protein is metabolized, it produces small amounts of acetyl CoA.

Step Two – The Krebs Cycle
The second step is the Krebs Cycle. During this cycle, the acetyl is combined with other compounds and is converted to citric acid. The citric acid goes through a series of reactions where hydrogen ions and electrons are removed. The end result is carbon which combines with oxygen which, in turn, is carried to the lungs and exhaled.

Step Three – The Electron Transport System
The third and final step is the electron transport system. The electrons “flow” through the system which supplies the energy to make a tremendous amount of ATP. The remaining hydrogen ions combine with oxygen and produce water.

To Summarize
The energy the body needs comes from the breakdown of ATP which produces ADP. The ADP needs to be resynthesized into ATP. The energy for the resynthesis comes from three energy systems: aerobic, ATP-PC, and Lactic Acid System.

Most of the time (day and night), the body gets its energy from the aerobic system. The aerobic system requires oxygen. Carbohydrates, fat or protein are metabolized into acetyl CoA. The acetyl is converted to citric acid during the Krebs Cycle and hydrogen ions and electrons are removed. The electrons move through the electron transport system which produces the energy for the resynthesis of ATP.

For very short, high intensity exercise, the body uses the ATP-PC system in which phosphocreatine (CP) is broken down to produce the energy to resynthesize the ATP. When the CP is used up after about 15-20 seconds, the body turns to the Lactic Acid system in which glucose and glycogen are broken down into pyruvic acid which is transformed into lactic acid when enough oxygen is not present. The breakdown of the glycogen and glucose produce the energy for the resynthesis of the ATP.

Using Energy, Burning Fat
Part 1: The Aerobic and Anaerobic States
Part 2: Anaerobic Energy Systems
Part 3: Aerobic Energy System

I'm not a doctor or trainer. I'm just another person like you who is trying to learn about the body and how it works in relation to health and exercise. The information from this blog entry comes from Personal Fitness Training: Theory & Practice. The information here is my understanding of what I read. As with anything related to health, be sure that you check with your physician or certified trainer to make sure it is accurate and appropriate for you.


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