It depends. If training without eating first negatively impacts your energy or blood sugar, always eat first. If, on the other hand, you feel fine working out before breakfast, you’ll need to consider your goals and body type.
If your priority is fat loss then you’ll get a slight bump to your fat-burning efficiency by doing fasting state cardio first thing in the morning. It will however, come at the cost of adopting a sub-maximal environment for muscle development. For endomorphs and mesomorphs—who have an easier time building or holding onto muscle mass—this is less of a concern, and they should take advantage of that early morning fat-burning window (again assuming fat loss is the priority). Regardless of your body type, if you choose to exercise first, always eat as soon as possible after your workout. For ectomorphs, muscle is more precious as your body type has to work harder to hold onto it. It’s not unusual for ectomorphs who have gained weight to develop the “skinny fat” look—soft belly, narrow legs and arms. If this describes you, fasting state cardio is not for you. Instead, you’ll want to allocate adequate time for resistance training (which will help develop your arms and legs). Some cardio will help with fat loss, but don’t do it in a fasting state.