When you eat more calories than you need, your body has two ways to store this extra energy -- glycogen and triglycerides, or fat. When your body needs extra energy, it pulls it from these two sources, and what kind of exercise you do, how intensely and for how long affects where the energy to fuel it comes from.
Creation and Storage
When you eat carbohydrates, they are turned into glucose during digestion. The increase in glucose in your blood leads to the release of insulin, which in turn triggers the storage of glycogen in your muscle and liver cells. This glycogen gives your body an easily accessible source of energy for the future for your brain, nervous system and muscles during high-intensity exercise. Your body only stores about 1,200 to 1,600 calories worth of energy in the form of glycogen. Diets that contain a lot of carbohydrates are best for increasing glycogen stores.
If conditions aren't right for forming glycogen, you'll form intramuscular fatty acids. These are a type of triglyceride that your muscles can access relatively quickly and use during long periods of moderate exercise when there isn't enough glycogen. Between 2,000 and 3,000 calories worth of energy can be stored as intramuscular triglyceride. Once glycogen stores are full, your liver cells send any extra glucose to be turned into triglycerides and stored in your fat cells. Anywhere from 50,000 to 60,000 calories of energy are typically stored as triglycerides in fatty tissue.
Conversion to Energy
When your body needs energy for exercise, it comes from three sources -- glucose in your blood, glycogen in your muscles and liver, and triglycerides in your fat cells and in the form of free fatty acids. Blood sugar is the easiest for your body to access for energy, followed by glycogen. Fat takes longer to convert into the glucose your body needs for energy. Intramuscular triglycerides can be converted to glucose at about one-third the rate of glycogen.
Use Based on Exercise Duration
When you first start an exercise session, about half of the energy you use comes from glycogen, with the other half of your energy coming from an equal mix of muscle triglycerides, fatty acids made by the breakdown of body fat, and a small amount of blood glucose. As time goes on, the stored muscle glycogen and muscle triglycerides begin to get used up, so more of the energy comes from blood glucose and free fatty acids. After three hours, almost all of your energy will come from free fatty acids and blood glucose.
Use Based on Exercise Intensity
Exercise intensity also plays a role in determining whether you'll use triglycerides, glycogen or blood glucose for fuel. Low-intensity exercise relies mainly on free fatty acids along with a small amount each of muscle triglycerides and blood glucose. As exercise intensity increases, the amount of free fatty acids used decreases slightly, the amount of blood glucose used increases slightly and the amount of muscle triglycerides and glycogen used increases.