The fat cells in your body are made up of triglycerides, which is the chemical form of 95% of thefat in your diet and in what is stored in your body. A triglyceride is a chemical with a three carbon backbone called glycerol to which three fat acids are attached. An enzyme in the fat cell called lipase breaks down the fat into glycerol and fatty acids. By measuring the glycerol released from fat cells, it is possible to determine the rate of fat breakdown from the fat cell. By capturing expired air and trapping fat breakdown products bearing a marker, you can determine how fast fat is being burned. It turns out that fat is being broken down three times faster than it is being burned for energy even at the lowest levels of exercise. So when you start walking on a treadmill or cycling, your body is already burning mostly fat in a carbohydrate flame. The stores of carbohydrate in the body are very limited with only about 300 grams or 1200 Calories stored in the liver and the muscle. While there aare about 160,000 Calories stored in fat. So the body uses fat for most endurance exercise including running and swimming while conserving carbohydrate.