Acute depletion is specific to the intensity of the workout. During energy depletion, amino acids may be sought out for energy assistance (typically be seen more during endurance training). But this is not necessarily a good thing! Inadequate carbohydrate consumption can be detrimental. Proteins and fats alone cannot serve as energy. When all ATP, Creatine, and Glucose reserves are lost, so is the energy. For example, lipid metabolism only provides half of the power that carbs provide, and without an optimal consumption of carbs, you can slow down your metabolic rate. And if amino acid proteins are being used for energy, the body would be in a catabolic state (exercise would have diminishing returns).