That’s been the prevailing theory for a while, and it looks good on paper: Food provides fuel for energy, so if there’s nothing available in your stomach, it makes sense that your body would tap into its stored energy—mainly fat—to power exercise. The trouble is, the store it seems to go after first is protein, in the form of your hard-earned muscle mass. According to research published in Strength and Conditioning Journal in 2012, protein breakdown can actually double when you do cardio on an empty stomach.
Furthermore, studies have shown that fasted cardio did not burn fat more effectively than cardio performed after a meal. And since workout intensity directly affects how many calories you burn, not eating before a long and intense session could reduce the energy you have to train with and lead to a subpar workout.
But what about high-intensity interval training (HIIT)? That’s supposed to burn fat and spare muscle, right? The truth is, performing it on an empty stomach causes you to break down fat faster than your body can use it for energy, which results—sadly—in the fat being shuttled back into your fat cells!
The bottom line: Do fasted cardio if it makes you feel better—many don’t like to eat and then work out with food still sloshing around in their gut. But don’t think not eating is going to speed up the time till you see your abs. Eating less overall (i.e., adjusting your diet) is the best strategy for that.
I am sharing this from http://www.muscleandfitness.com/athletes-celebrities/pro-tips/facts-fasted-cardio. Thank you Adam B for the insightful information.