Morning cardio training on an empty stomach or during fasting is a common practice in the bodybuilding community to ensure the lowest possible body fat before the competition.
The reason why this particular training approach is effective concerns the depletion of glycogen, the body’s first stored energy source. By freeing the body of this stored form of glucose, it forces it to burn fat or protein instead.
If you’ve fasted in the morning, the hormone cortisol is high and its only job is to break the appropriate tissue based on what other hormones are.” Insulin is elevated after eating and cortisol attacks muscles if you are fasting, insulin is low and cortisol will then go to body fat stores.
As for the types of cardio, you should do, 15-20 minutes or less of high-intensity interval training or low-to-medium intensity exercise should do you good.