Cardio on an empty stomach?
Everyone should experience and check the results in person. Far from settling the issue, I would like to point out some advantages and mistakes about it.
Aerobic exercise on an empty stomach can increase the amount of free fatty acids used as fuel. But, this is not because our glycogen deposits are empty in the morning. This would happen if we went to bed last night with low glycogen levels.
Almost all the energy we consume during sleep comes from free fatty acids. So glycogen deposits are not affected overnight. Next morning these fatty acids would be «free», ready as fuel for our cardio workout.
Low-intensity aerobic workout (50-70% of maximum) on an empty stomach, increases the insulin sensitivity after exercise, and the mobilization of fatty acids.
It is important to note that at a higher intensity (> 75% of maximum) we´ll get the opposite effect.
Moreover, not all are positive. Aerobic exercise is catabolic because it increases the production of cortisol. Cortisol levels are high in the morning, so this would lead to greater muscle wastage.
For greater efficiency of aerobic workout, your stomach should not be completely empty. I recommend a mixture: 5g of BCAA, 5g of glutamine and 5 grams of essential amino acids 15-30 minutes, before training.