You must make sure that you consume adequate amounts of both carbohydrates and protein (protein shakes can help you achieve this). If your energy stores become drastically depleted or you experience lactic acid buildup, you may suffer temporary muscle fatigue. If you do not refill your glycogen stores before your next workout, your body may begin breaking down muscle tissue for the protein it needs for energy.
Directly before workout, consume food of high glycemic indexes to keep blood sugar sustained at an appropriate level to allow you to train intensively for longer periods of time.