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When you exercise on an empty stomach your energy comes from the fats and proteins your body has stored, instead of from the food you just ate. While this helps with fat and weight loss, it can also cause you to use energy from stored protein sources, which means that your body will then have access to less protein to build and repair your fatigued muscles after your workout.
It’s a little like driving a car on a near empty tank of fuel. As you rely on the last few drops of fuel, you are forced to drive economically and your ability to accelerate and hit top speed is compromised.
Similarly, if you want to improve fitness, build muscle, and burn calories, focus your efforts on what definitely works – regular moderate to high intensity workouts with the right type of fuel in your tank.
See these recommendations from the International Society of Sports Nutrition:
This study highlights the benefits of exercise in a fasting state to improve endurance: