I tried that - didn't work too well. Other than for cardio purposes, it's rather pointless to wake up that early in the morning to workout because a) you're brain isn't functioning as well as it will be later on in the day and b) your carb supply is empty - yea sure you can eat breakfast but it's a lot more beneficial to workout later on in the day with a couple meals under your belt and your carb supply high and c) it's better to workout when your muscles are warm - and your body is at it's warmest around mid-afternoon.
that very much depends on what your tryig to do..... now, the reason I think that isnt the best adviec for most people is:
Most people (maybe not most but ALOT) at the gym are there to lose weight while building muscle. If you have just eaten , or eaten twice, your body will draw on the protein and carbs in the food, instead of the carb store in fat. If your aimnig to lsoe weight, from what ive read, the best solution is to work out on an empty stomache (but make sure your hyudrated) and make sure you get a high prtoein low carb meal or supplement in soon after the work out... plus, it has the added benefit or revving your metabolism for the rest of your active day.