I don't know the scientific answer, but what works for me is to have a small amount of food, then to work out, and then to follow it with some kind of protein and carbohydrate combination. So, usually I make a protein shake in the morning (banana, protein powder, yogurt, berries, oatmeal, and maybe some spinach leaves), drink 1/3 of it right away, get my kids ready to go, etc, then go work out. Afterwards, I drink the rest of the shake. Or, I'll go for a run at night a couple hours after dinner, and have some cheese and an apple after. I never go without some kind of fuel. This works for a lot of people, but it just makes me feel shaky and weak. I need to have at least half a banana or something if I'm going first thing in the morning.