There have been several studies recently showing that people who work out in the morning without eating breakfast burn slightly more fat during cardio workouts than people who do eat breakfast.
The reason is simple: When you're sleeping, you're fasting. By the time your alarm goes off, your body has already burned up most of its main fuel source—. Head out on your morning run on an empty stomach and your body starts to burn its next favorite fuel—.
But according to a study from the Strength and Conditioning Journal, what those other studies don't show is that if you make a habit of it, your body will not only burn up fat, it'll start burning up protein in your muscles, thus shrinking those bulges you worked so hard to build.
What's more, if you don't have some food in your system, you can't work out nearly as hard. So sure, you might burn up more fat for one workout, but you're not advancing your overall fitness level, which can ultimately lead to gaining more weight down the road.
Still, you don't need a huge carb-loaded breakfast to hit the trail. A small glass of juice, or a banana, or a scoop or two of peanut butter should do the trick.