Is working out at 7Am on an empty stomach a bad thing?

So my work allows me to work out from 7AM to 8AM and more often then not, I don't have time to eat first.  I use either a treadmill at 3.5-4 mph at a steep incline or an eliptical with a resistance of 8-10.  I'll do either for 30-45 minutes and then some light calistetics.  I usually eat about 9am after I've cooled down from the workout.

If I haven't eaten yet, what is my body using for energy?  My fat or my muscle?  I have no idea.  Any idea?

From what I understand, this is actually an excellent time to due aerobic/cardio work to help maximize potential fat loss, provided you are not way under on your necessary calories.   Assuming that you are getting enough calories and your metabolism is not in a fat preservation mode, your body will turn primarily to stored fat for the necessary energy. 

To the extent that you can get in some resistance training at some other time of the day, I would recommend it.

 If intensity is kept in check you're relying on a slightly-higher-than-normal level of fat and leftover carbs from last night, supplemented with some muscle protein undergoing glucogenesis that converts it to glycogen if you go too high on the intensity scale.

 Not a whole lot of it though; while liver glycogen tends to deplete overnight muscle glycogen stays intact for longer so you don't actually see significant muscle protein->glycogen conversion unless you do high-intensity(anaerobic) exercise.

 When you exercise is completely irrelevant to fat loss though; substrate utilization during exercise has no effect on the outcome of your weight loss, only calorie balance does that.

 How about getting in a hit of milk before you get on the dreadmill? You don't need to eat solid food to have something nutritious in you while working out and a tall glass of milk would fit the bill nicely I'd say.

I've always learned to eat SOMETHING, anything before a workout. What about just a banana or a hand ful of nuts?  It'll keep you going.

As for working out in the morning vs other times of the day, I read a recent study that suggested that it doesn't really matter.  Some say that the morning your body gets started for the rest of the day, and others say working out later get better results bc the body's already warmed up.  I personally love to workout after work. I count most of my cals for the first part of the day, workout and have dinner and relax. When I worked out in the morning last year, I was always hungry the rest of the day....but that's just me.

Anyways...just keep some nutrients in you to keep moving.

I usually go for some carbs like a banana, handful of granola or a piece of toast beforehand, give myself 10 mins or so and then work out (I work out from 6-7am) and then I have protein afterwards, preferably protein powder & milk, but if I end up eating then I go with hard boiled eggs or something else high in protein.  I focus on the protein mainly because 90% of my workouts is weight lifting, so your mileage may vary.

So complicated you guys! I work out before eating all the time and live to tell about it. A lot of people can't for whatever reason, but I have a lot of energy in the morning

I've done some reading on fasted cardio...some experts suggest having some caffeine (perhaps in the form of a strong cup of coffee with just a small splash of milk?) before doing your 'fasted' cardio...you can google up on this...the caffeine is supposed to help your body shift to fat burning mode, rather than muscle burning when your stomach is empty. I also spoke to a trainer at the gym yesterday and he suggested that I keep my morning cardio short (not longer than 40mins) and to keep the intensity in check like Melkor said...keeping it at moderate intensity rather than high intensity to promote fat burning rather than muscle burning. Fasted cardio is still a controversial issue, but I still do it, mainly because I don't really have the time to work out later in the day, doing my cardio early in the morning makes me feel great and energized to start my day :)

My two cents.... I work out every morning at 4:00 am.  I am also the cardio queen.  (sorry melkor LOL).  I work out rather intensely for over an hour every morning.  Roll my butt outta bed, slam me some Diet Dew and off I go.  I could not imagine trying to eat something before I go.  I am no expert or anything but this has been working for me for about 1 year and 4 months.  I've lost nearly 90 pounds.  So, its working for me. 

Wow, thanks everybody for all the great responses!  I got a little of every possible answer in there but I get the general idea.  Keep it under 40 min and low intensity and try to eat something  if I can even if it's something small like a handful of nuts or a peice of toast but it won't kill me if I don't.  I figure if I prep it in the morning I can have a bottle of FF Milk with half a serving of Carnation Instant Breakfast ready to go and maybe a banana.  I just hate waking up any earlier than I have to so it's always my workout getting crunched.  That's where the sacrifice must come in, :) .

Thanks again everyone!

I used to work out hungry also, then a friend of mine suggested that I have a carb pre-workout and I actually feel the difference.  It doesn't matter if it's cardio or weights; my workouts have improved.

Try to eat a handful of nuts or a piece of fruit pre-workout and don't forget the protein and potassuim post-workout.  You don't need to wait until your body cools down--better to have the protein within 30 min of your workout. I don' know how you are able to wait so long post workout, I am usually ravenous afterwards. 

You could always boil a dozen eggs the night before and take a couple with you (and a banana) to work.  A better suggestion is a can of very-low sodium tuna.  It has 40.6-g Protein, 342.3-mg Potassium and 2.5-g Fat for only 177.5 calories, plus needs no refrigeration.