This fitness myth has been around for a long time, but it's still not true. While you sleep your body burns through about 80 percemt of the stored carbohydrates in your liver. When you wake up and exercise before you eat, you burn through the remaining 20 percent pretty quickly and your blood sugar falls rapidly. However, this mainly deprives your brain of fuel, not your muscles, because they have their own glycogen stores that weren't depleted overnight.
So, instead of your muscles burning an increased percentage of fat, they're busily burning the carbohydrate they stored after dinner last night while you're brain is starving. As a result, you feel nauseous and lightheaded and the quality of your workout suffers. This means you have trouble maintaining a high pace or intense effort during cardio workouts, or that you struggle to maintain the focus to complete effective strength training workouts.
What I'd rather see athletes do is wake up and immediately eat a small amount of carbohydratea glass of juice and bagel or energy bar works very well for most people. This boost to your blood sugar is typically enough to power your brain through a 45 to 60 minute morning workout. If you feel yourself getting sluggish and lightheaded or nauseous before the end of your morning workout, increase the amount of carbohydrate you're eating when you wake up. This can be tricky balancetoo much and you'll feel full and bloated, too little and you'll run out of brain fuel before the end of the workout.