There are basically three energy systems(or sources of energy available for muscular activity) found in the body: the aerobic system, and two anaerobic systems; the lactate system and the creatine-phosphate system. These three systems create energy by manufacturing something called ATP. ATP stands for adenosine triphosphate which is an energy source used in many metabolic reactions, especially those involving muscular activity. Each of these systems respond to very specific physical conditions.
The most efficient energy system in the body is the aerobic system. Aerobic, in this use, simply signifies that the energy is produced utilizing oxygen. The aerobic system burns both fat and sugar(glucose) to manufacture ATP. Each time a fat cell is broken apart, water and CO2 are produced. That is why you lose so much water weight when you begin to exercise aerobically. The fat cells are veritable storage batteries of energy. Each cell can generate up to 36 units of ATP, As the aerobic system improves through exercise, it becomes more efficient and burns a greater percentage of fat cells even at rest. Therefore the aerobic system can produce almost limitless amounts of ATP and is used for endurance exercise.
The lactate system is one of the anaerobic energy systems. Anaerobic signifies that the energy production takes place without the presence of oxygen. This system produces ATP by burning glucose, which is a simple sugar complex found in muscle tissue. Glucose can only generate approximately 4-9 units of ATP. The byproducts of anaerobic gylcosis include ATP and Lactic Acid. As lactic acid production increases the muscle soon reaches an overload wherein continued contraction becomes impossible. Lactic acid is thought to be the principal cause of immediate soreness(the “burn”) in an exercising muscle.
The creatine-phosphate system is the other anaerobic energy system.system. This system consists of a molecule that can be broken down very quickly into ATP. It is utilized for short bursts of energy. The major limitation of this system is that it is rapidly depleted. It takes no longer than ten seconds to exhaust the entire available supply of creatine-phosphate.
When your body is at rest it is actually relying on the aerobic energy system for production of ATP. That means that about 50% of the calories you are utilizing are coming from fat cells(for an athletic person that jumps all the way up to 70%). Unfortunately, when you are sitting on the couch, you are not burning that many calories(less than 200 calories per hour), so the fat burned is negligible.
For the first couple of seconds of exercise your body utilizes the creatine-phosphate energy system. Within the first three seconds your body has depleted the entire readily available store of creatine in the cell and begins to re-manufacture replacement molecules. Within ten seconds, the secondary supply is also depleted. No fat is consumed during these initial bursts of energy.
For exercise that lasts from ten seconds to seven minutes, the lactate system takes over. This exercise could include anything from the stop-and-go routine of weight lifting, to running a fast mile. This system burns only sugar(again no fat is consumed). Since oxygen is not needed for energy production, it is not breathlessness that makes you stop. It is the inability of the muscle to rid itself of the byproduct, lactate.
For exercises lasting longer than seven minutes, oxygen is a crucial component of energy production. That’s because the aerobic energy system gradually takes over as the main source of ATP. Aerobic energy production utilizes glucose and fatty acids. This means fat burning. As you increase the duration of exercise, the percentage of energy consumed from fat gradually increases to 80% . This percentage levels out after twenty minutes of continual exercise. For your body to take 100% of its’ energy requirements from fat, you would have to exercise for one and one half hours.
If you are interested in becoming a “fat burning machine”, duration of exercise seems to be the key. Gradually increase exercise duration to 50 to 60-plus minutes at least three to five days per week and raise the intensity as high as you safely can sustain it. If high intensity is not an option, try interval training to sustain the exercise session for as long as possible. Remember, total calorie expenditure during exercise ultimately determines the amount of fat used, and the longer you exercise, the more fat you will burn.