The Metabolic System!

I believe that it is important that we fully understand how our body's function during exercise and the way our energy systems operate.

To fully understand how the body utilizes its energy stores to build muscle or burn fat, we must first understand how the body's metabolic system functions.

Our bodies use the energy stored in sugars and fats as the main fuel to produce muscular contractions. When these materials are taken into the body, they are stored within muscle, liver, blood, and fat tissues.

When carbohydrates are stored within the liver and muscle, they are referred to as "glycogen", while in the bloodstream it is referred to as "glucose". Fat is stored as fatty acids in the adipose tissues.

However, our muscles cannot directly burn sugars and fats for energy. Instead they use a material derived from them called "ATP". Otherwise known as Adenosine TriPhosphate, ATP is critical to the release of energy in order to produce muscular contractions.


ATP can be manufactured in a variety of ways. The body utilizes 3 main energy systems to produce it:

  • ATP/CP (Creatine Phosphate) System
  • Anaerobic Lactic Acid System
  • Aerobic ATP System

An understanding of these methods of ATP production is critical to understand how our body's function during exercise.

ATP/CP (Creatine Phosphate) System ///

Since the production of ATP from glucose and fat takes time to initiate, the body needs to have a source of immediate energy. This energy comes from a substance known as "creatine phosphate" which is stored within our muscles.

Creatine reacts immediately with Adenosine DiPhosphate (ADP) to produce creatine and ATP. Since our bodies have only a limited supply of creatine, this method of ATP production only lasts for up to 20 seconds.

This system is used for short-term activities such as sprinting, or in start/stop sports like hockey or rugby. Other means of producing ATP require the use of our energy reserves in sugars and fat. This takes time however, and this is where the ATP/CP system comes in handy. Think about the last time you had to run across the street quickly. The ATP/CP system allowed you to do this.

Anaerobic Lactic Acid System ///

This system produces ATP through a process called "anaerobic glycolysis," which is a breakdown of glucose to be converted to ATP without the use of oxygen.

This system is very inefficient and produces a lot of wastes. So inefficient, in fact, that only 5% of the energy contained in a glucose molecule is actually utilized. Lactic acid is the main waste product, changing the pH of the muscle cell and causing it become more acidic.

This acidity inhibits muscular contraction by interfering with energy production. The anaerobic lactic acid system is the main supplier of energy during weight training, sprints which last over 20 seconds, and racquet sports.

One molecule of glucose burned using the anaerobic lactic acid system produces water, carbon dioxide, lactic acid and only 2-3 ATP. Lactic acid causes muscles to become stiff and sore as it builds up.

The body deals with this lactic acid by removing it through the bloodstream, reconverting it to glucose, converting it to protein, or converting it to CO2 or H20. There is only a certain amount of lactic acid which the muscles can tolerate before fatigue and soreness sets in and the exercise must be stopped.

Aerobic ATP System ///

"Aerobic" means "in the presence of oxygen". This system comes into play when the amount of oxygen coming into the body is meeting the demand of the muscles. As I said earlier, in the first few minutes of an any strenuous activity, energy must be made anaerobically.

What Does Aerobic Mean?

"Involving or improving oxygen consumption by the body." Aerobic exercise, therefore, enhances respiratory and circulatory efficiency by improving oxygen consumption.

Eventually, after the heart rate and breathing rate have increased, the demand for oxygen will be met and we can create ATP aerobically. We will still be burning glucose/glycogen, but much more ATP can be produced (36-40 ATP rather than 2-3).

Now that oxygen is present in the muscle cells, fatty acid oxidation can begin to take place (creating up to 100 ATP). As the exercise carries on for longer and longer, eventually we will begin to burn more fat instead of glucose/glycogen.

This process is extended to use fat as the main fuel, since it has a much higher caloric density than glucose, allowing us to release far higher amounts of energy. Smaller and smaller amounts of glucose will be used as energy and the main reaction will come from fat. Don't think of fat on your body as fat, think of it as stored potential energy!


So to summarize: In the first few seconds of exercise, your body uses the ATP/CP system as a means of instant energy. After about 20 seconds, the body enters anaerobic glycolysis, where it breaks down carbohydrates without using oxygen. This system is very inefficient and produces a lot of wastes, such as lactic acid.

Eventually after the breathing rate and heart rates have increased, the demand for oxygen within the body can be met, and the body will produce energy aerobically by utilizing fatty acid oxidation.

What is the use of this, you ask? Not a whole lot, to be honest. It is more for overall knowledge, and I believe that it is important that we fully understand how our body's function during exercise and the way our energy systems operate. In addition, it's helpful to see why sustained cardiovascular exercise is prescribed for fat loss.