A commonly used technique by bodybuilders seeking to lose excess body fat prior to a contest involves doing aerobic exercise while in a fasted state. The usual practice is to do the aerobics first thing in the morning, prior to eating or drinking anything. The rationale behind this is that when you first awaken, both your stored glycogen and insulin levels are low due to a lack of food intake over an 8-hour (or however long you slept) time period. Glycogen is basically stored complex carbohydrates and is the primary fuel for anaerobic exercises, such as typical weight-training sessions. Glycogen, however, is also involved in the initial stages of aerobics. In fact, during the first 30 minutes of aerobics, the usual fuel mixture being utilized is a 50/50 ratio of stored glycogen in muscle, along with circulating blood glucose. Fat doesn't even kick in until around the 30-minute mark, and the use of fat as a primary fuel increases with the duration of the exercise. By the 90-minute mark, fat is now the primary fuel, as much of the stored muscle glycogen and blood glucose levels have significantly declined by that point.
With fasted aerobics, the idea is that you will be able to tap into fat stores much faster than usual, since the primary blockers of fat use, insulin and carbs as glycogen, are low. This makes sense from a physiological point of view since it's true that when you awaken, your stored glycogen levels are rather low, having been used during the hours you have slept. And because you haven't consumed any food, your insulin levels are also at a low point. This is important since when insulin levels in the blood are high, fat mobilization is blunted significantly. This is one reason why you wouldn't want to consume carbs during your workout if your goals are to lose body fat. But would eating carbs before the workout adversely affect fat usage? The answer will soon be apparent.
There is some evidence that doing aerobics or"cardio" in a fasted state does indeed promote changes in the body that favors fat used as a fuel source during exercise. For example, one 6-week study of fasting cardio showed a greater increase in intramuscular fatty acid binding protein and uncoupling protein-3 when doing the exercise fasted compared to doing it after eating a meal. The fatty acid-binding protein is used to transport fat into the mitochondria of cells, where the oxidation or "burning" of fat actually occurs. Having this increase in intramuscular fat stores would suggest that those fat stores are more easily used during exercise. Indeed, having larger intramuscular fat stores is considered desirable for athletes and those who exercise regularly. In those . . .