When you exercise aerobically, you need energy to do so. But you also use energy to live throughout the entire day, not just when you exercise.
The term metabolism ("Glossary") describes the sum of all chemical processes necessary to maintain body functions. Your metabolic rate, or how much energy you will use during the day, will depend on how much physical activity you do. That is, the more physical activity you complete the more energy you will use.
The energy that you need to live, work, and exercise comes from the food that you eat which provides calories, or a measure of energy in food.
In short... Energy from Food = fuel for the body
There are four types of food products that provide a person with calories.
Carbohydrates, or carbs, provide a very quick source of fuel for your body.
Fats are more of a long term fuel and take a good period of time for your body to make energy out of them.
Protein is only used for fuel in rare cases. Protein is used more for building muscle and other tissues in your body.
And the last food source that provides you with energy, that most people don’t realize, is alcohol. Most freshman will blame the “freshman 15” on eating at the cafeteria. But what they don’t seem to comprehend is all of the calories that can come in through alcohol. When you drink alcohol, it is sent to your liver where you turn it into fat and store it in your body as fat. More on this later this semester.
When you take in a fat or a carb molecule (and sometimes protein) and want to turn it into energy, what you are actually trying to make is a molecule called adenosine triphosphate, or ATP. This is the most basic form of energy used by all cells in your body. There are three different ways that your body can make energy: an immediate system, an anaerobic system (or without the use of oxygen), and an aerobic or oxidative system.
ATP/PC system
When you first start to exercise or move for that matter, during the first 10 seconds or so, you will be using the immediate system to provide you with energy. This system is also called ATP/CP system. The CP stands for creatine phosphate, which possibly many of you have heard of. Creatine is sold as an ergogenic enhancer of performance, that is, it is supposed to help athletes compete better (short video does show how creatine works, but it will try to convince you that their supplement will improve your performance)...
Click Here for Video on Creatine
Research has shown that for creatine to really work, two conditions should be met. First, the person doesn’t eat a lot of meat. If you eat meat, you are getting enough creatine in your body to make energy using the immediate system. And second, if you only participate in activities that are considered power events, such as sprinting or weight lifting, and those events should last less than 10 seconds. Once you get past the 10 second point, you start using the anaerobic energy system. And at 2 minutes, you start to switch more to an aerobic energy system.
Anaerobic Energy system
This system creates ATP without the use of oxygen (An = without; aerobic = oxygen). To create ATP anaerobically, your body uses a process called gylcolysis. This process takes a Glucose ("Glossary") molecule (readily available in your blood as blood glucose or blood sugar) and puts it through a biological system to create ATP. Your body can also take a Glycogen ("Glossary") molecule from your muscle or liver and convert it to Glucose to use in the creation of ATP anaerobically.
Aerobic Energy system
This system creates ATP using oxygen. It is completed in two steps called the Krebs cycle and the electron transport chain inside of your Mitochondria ("Glossary").
A HUGE difference between this system and the previous two is that Fat or protein can be used to make the ATP wherein the other two (ATP/PC and anaerobic) could not use either fat or protein directly to make ATP.
If you are really interested in this process and want to learn more, here is a great video for you to watch (or perhaps clear up the process for you when you learned it in other classes... I am not going to hold you all of the details in video at all... it is there for those who want to learn more).