Training on an empty stomach is something you must teach to your body in the beginning, but once it learns how to function on a dead empty stomach, it will function just fine. Its never good to train on a full stomach, when you eat a big meal, blood rushes to your digestive organs in order to process the consumed food, and that takes blood away from your brain and your skeletal tissues. If there is very little blood around your digestive organs (aka empty stomach), more blood will be available for your brain, lungs, and skeletal tissues, resulting in better performance. As stated earlier, if youre going to have anything before a training session, make sure its BCAA's