For people who are trying to burn off excess weight and calories or perform better it is important to understand how the human body's energy system works in order to plan an effective exercise program. Here is some general information to give you an idea I have provided some key terms and description of the human energy system process.
The Krebs Cycle is a series of reactions that continues the oxidation of glucose, glycogen, or pyruvate to create ATP the universal molecule for energy.
Here are some basics about the body's energy system:
The Human Body stores energy in chemical bonds of ATP (Adenosine Triphosphate) which is the universal energy carrying molecule manufactured in all living cells as a means of capturing and storing energy.
Energy stored in the chemical bonds of ATP is used to power muscular activity.
The replenishment of ATP in human skeletal muscle is accomplished by three basic energy systems:
1. Phosphagen
2. Glycolytic
3. Oxidative
The Phosphagen System is the primary source of ATP (Adenosine Triphosphate) for "short term - high intensity" activities such as sprinting or jumping. It is active at the start or beginning of all types of exercises regardless of intensity. ATP is an immediate source of energy for almost all body processes, including muscle contraction. This high-energy compound, stored in muscles, rapidly releases energy when an electrical impulse arrives in the muscle. No matter what you do, scratch your ear or lift 200 pounds, ATP breakdown makes the movement possible. ATP must be present for the muscles to contract. The body has a limited supply of ATP and must replace it rapidly if muscular work is to continue. This energy system relies on the chemical reactions of ATP and creatine phosphate, both phosphagen systems. The food-sport supplement industry has capitalized on this known fact to sell phosphagen food supplements. A component of the Phosphagen System is the Lactic Acid System which cannot be used directly as a source of energy for muscular contraction, however it can be used to help replace ATP rapidly when necessary. If you are exercising at a high intensity level and need to replenish ATP rapidly, the next best source of energy besides the Phosphagen Creatine is muscle glycogen. To be used for energy, muscle glycogen must be broken down to glucose, which undergoes a series of reactions to eventually form ATP, a process called Glycolysis.
The second system Glycolytic System takes over when the body starts to breakdown carbohydrates, either glycogen stored in the muscle or glucose delivered in the blood (from food you ate), to produce ATP. The ATP provided by glycolysis supplements the phosphagen system initially and then becomes the primary source of ATP for high-intensity muscular activity that lasts up to about two minutes or more. Glycolysis is stimulated during intense muscular activity activities lasting over 2 minutes such as a 200 or 800 meter run. This is sometimes termed "Anaerobic Glycolysis. One of the important factors controlling the metabolic fate of muscle glycogen is the availability of oxygen in the muscle cell. In simpler terms, if oxygen is available, a large amount of ATP is formed. This is known as "Aerobic Glycolysis."
The third system is the Oxygen System or oxidative (Aerobic) system. The oxidative system is the primary source of ATP at rest and during aerobic activities and uses primarily carbohydrates and fats as substrates for energy. People who are walking on a track or a treadmill, doing water aerobics, or participating in a yoga workout are relying primarily on the oxidative system. Protein is normally not used or metabolized significantly, except during long-term starvation and long bouts (> 90 minutes) of exercise. Interestingly enough at rest approximately 70 % of the ATP produced is derived from fats and 30 % from carbohydrates. An example would be when you are sleeping as this is a non-intensity activity. Essentially the body is not producing muscle contractions with skeletal muscle. However, following the onset or beginning of physical activity and as the intensity of the exercise increases, there is a shift in substrate preference from fats to carbohydrates. During "high-intensity" aerobic exercise, almost 100 % of the energy is derived from carbohydrates, if an adequate supply is available. However during PROLONGED, submaximal, steady-state work or exercise there is a gradual shift from carbohydrates back to fats and protein as energy substrates (sources for energy).
The order in which substrates are used to replenish energy ATP is the Phosphagens first for immediate activity. Then as sustained activity continues the body will begin using carbohydrates or blood glucose usually available in the blood from food you ate during the day. When that runs out the glycogen stored in muscle tissue such as the liver and skeletal muscle will be used as a substrate. After prolonged periods of activity such as aerobic activity like walking or jogging for an hour or more the body with the aid of oxidative system will start using fat as a substrate to replenish your energy and this is where you really start burning off fat is during these prolonged aerobic activities. It is not recommended to push your body beyond it's limits to burn off fat. A reasonable timed workout should be in order along with a balanced diet will be key to losing excess body fat.
Quality complex and simple carbohydrates can provide you with sufficient energy for aerobic activities. Planning a balanced diet and eating sufficiently ahead of time will allow for your body to metabolize and make ready for the use of these necessary substrates.
It is important to remember that your body is burning calories all day long even while at rest. It's just a matter of which part of the human energy system is being emphasized more to produce energy which in turn burns calories. If you want to burn off those calories from the food you ate and excess body fat you need sustained exercise and physical activity with moderate or submaximal intensity to stimulate your body to burn off carbohydrates and fat by using it for fuel.
For more information on this topic you can buy the book "Nutrition for Health, Fitness, & Sport 8th edition by Melvin H. Williams. There are many other sources that can provide information as well in books from the exercise science field.
