Well, first we have to go all the way over to Italy, to hear from researchers who had a study published in the International Journal of Sports Nutrition Exercise and Metabolism (21:48-54, 2011).
The Italian researchers wanted to see what would happen when eight healthy young men did the following workouts:
Workout A: 36 minutes of slow cardio at 65% maximum heart rate in the morning without eating
Workout B: 36 minutes of slow cardio at 65% maximum heart rate in the morning AFTER a small meal
24-hours after exercise, the results found that eating before cardio increased both calorie burning AND fat burning.
That’s right, eating before the workout increased fat burning in the 24-hours after exercise.
The scientists concluded that “fasting before exercise does not enhance lipid utilization; rather, physical activity after a light meal is advisable.”
The truth about fasted cardio is that it is NOT a magical way to boost fat burning. It's really all about what keeps your ability to really utilize the training time in an effective manner. Some people seem to do well with fasted training. I find that eating a fruit about 30 minutes before my workout keeps the numbers higher (and if you've been here for any length of time, you know that the amount of progress is the only thing that really concerns me.) It's a cool concept, but it's really subjective to the practitioner.