Exercising on an Empty Stomach
Muscles require energy for movement. This energy is found in the form of glucose (blood sugar), glycogen (sugar stored in the muscle), or body fat.
When exercising, the body first uses the glucose circulating in the blood for energy. If you have not eaten, your body will not have the available glucose and will drain the glycogen from the muscle cells. The result is a tired feeling and a lack of muscle endurance.
Fat is not immediately burned because the body conserves fat during times of deprivation. The metabolizing of fat is a last resort and usually does not occur for the first 24 to 48 hours of lowered glucose levels.
There are some experts, however, who say an activity such as running, or other aerobic type exercises, are better done on an empty stomach. The thinking is that you'll burn more fat because your body is forced to utilize its fat reserves during the aerobic activity.
But when it comes to muscles, they usually get their energy from carbohydrates. Therefore, if you haven't eaten before exercising your muscles, you body won't have many carbs in reserve. Scientists say that forces it to burn fat instead.
In summary, sound advice:
Aerobic exercise, do not eat before exercising.
Muscle/strength training, do eat 20 minutes or more before working out.