I'm not sure why I am bothering...but here it goes anyway.
first of all breaking down fat is a catabolic activity...there are several things you can do to maximize fat catabolism while sparing muscle.
cortisol levels are very high in the morning...they remain high if you dont eat...and get even higher if you train (cardio or lifting).
cortisol is VERY catabolic but not specific to muscle or fat...it will break down both.
the trick then would be to minimize muscle breakdown but not inhibit fat breakdown.
this can be accomplished by taking some BCAA's , glutamine, or a protein drink before morning cardio.
what you dont want to do is have carbs...as these will increase insulin and make it near impossible to burn fat at your most efficient rate.
again, we must remember individuality...I find that morning cardio does not burn off my muscle...only my fat.....and i do get bodyfat tested every week so it is not just speculation. Others may find that it burns off muscle and may want to use some protein or avoid doing it all together.
Cardio can be used in several different ways as well.
1. as a tool to create a calorie defecit.....if this is your only goal for cardio then it really doesnt matter when you do it or what is in your stomach.....you are just concerned about burning calories.
2. to amp up metabolism by stimulating the thyroid.....this would involve HITT type of intense training....again...who cares about what is in your stomach...eat if you want.
3.to burn as much fat as possible...in this case you want to maximize every avenue you can to ensure that fat will be burned as fuel rather than carbs or muscle.this would invole morning cardio on an empty stomach. This is where we have our argument.
IMO...if you are so concerned about losing muscle mass then I would do as little cardio as possible and control your diet to get the results you want...it may take longer but it is less likely to cause muscle wasting.
Or,,in Johnnny's cae....you can just take a laxative to lose weight....because I'm pretty sure any excess weight you have is because you are FULL OF SHIT.