For the longest time, I didn't understand why I had more energy after fasting. I have my most productive hardcore workouts after fasting for 5-18 hours. If I ate anything in that 4-5 hour window before training, the workouts just weren't as intense. Ori Hofmekler explained where this "hidden" energy source came from…the Sympathetic Nervous System (SNS).
Go over and read more about the SNS.
As readers know, I have been working out fasted (18-30 hours) for a bit more than a year, now. I can't imagine doing it any other way. As an added benefit, take a cold plunge or sit in a totally cold bathtub after the workout, for as long as you can stand (in the summer, I somtimes remain in the 50ish degree water for 10 minutes). Then, don't eat until you're actually hungry, which, for me, is usually a couple of hours after the workout.