Many bodybuilders and fitness enthusiasts engage in fasting aerobics in the belief that this will lead to greater loss of body fat compared to exercising after a meal. This is based on the notion that glycogen levels are low when you first awaken, and because glycogen is the preferred energy substrate to power exercise, not having full stores of glycogen will cause the body to turn to an alternate fuel, stored body fat. This is particularly applicable to aerobics, because fat is burned more efficiently in the presence of oxygen and aerobics relies on a greater intake of oxygen, which is one reason why it's called "aerobics." In contrast, typical bodybuilding workouts are anaerobic, which means "without oxygen." That, of course, shouldn't be taken in a literal sense since some oxygen is always involved in any type of exercise. But what it refers to is that anaerobic exercise such as weight-training depends more on the use of circulating glucose and muscle glycogen, which is stored carbohydrate in muscle.
The concept of doing aerobics in the fasted state makes sense, but despite this studies that have examined the effects of fasted aerobics have been paradoxical, with some studies showing definite increased fat oxidation, while others show no difference in fat loss compared to consuming a meal prior to exercise. One study showed that doing aerobics while fasted increased 24-hour fat oxidation, while doing the same exercise after eating a meal didn't increase fat oxidation at all once the exercise ended. ]pullquote]Clearly, if fasting aerobics did increase 24-hour fat oxidation, it would provide definite benefits for those seeking to lose body fat[/pullquote] Clearly, if fasting aerobics did increase 24-hour fat oxidation, it would provide definite benefits for those seeking to lose body fat. A met-analysis, or compilation of previously published studies involving 27 studies that examined the effects of fasted aerobics and fat loss included 273 participants and found that doing low intensity aerobics, defined as doing the exercise at a heart rate between 40 and 70% of maximum heart rate in a fasted state, led to a greater rate of fat oxidation compared to doing the same exercise after a meal.
But how much fat you can oxidize while exercising in a fasted state depends on a number of factors. These include training status. Those with a greater history of training will be able to tap into stored fat more rapidly because they likely have more mitochondria in their cells, the site of fat oxidation, as well as a greater activity of enzymes related to fat oxidation. One problem with obese people new to training is that they usually lack such oxidative enzymes, and as a result burn more glucose than fat when they begin training, especially if the training is too intense. The lowering of glucose that results can increase hunger and make dieting compliance more difficult. One way around this problem . . .