to elaborate on this would take a long time, as i would have to explain all the ways in which your body metabolises the three [or 4 if you count alcohol] forms of fuel, protien [from your meals, your muscles and your organs] carbs [from your meals and from your muscles and livers stores], and fats [from your meals anmd your bodies stores]
i did once write an enormous reply to a similar question detailing everything, but it was in CC's "cardio on an empty stomach" thread, which, unfortuneatly, james deleted by accident when removing a double<identical post.
basically, the "starve your body" school of thought, follows outdated and flawed thoery that because insulin inhibits the promotion of fat stores being mobilized for fuel/energy, eating before cardio would raise insulin levels and thereby inhibit your ability to burn fat as a source of energy.
#1 this can lead to an abundance of ketone bodies in your blood, ketosis, not good.
#2 insulin is the most powerful anabolic hormone produced by your body and high levels strongly counteract catabolism [and therefore save your body from muscle wasting]
#3 when your glycogen levels in your muscles are low, your liver will balence them out by releasing glucose into your bloodstream
#4 you will be able to exersize for longer and with more intensity having ingested carbs and some whey beofrehand - with fairly obvious benifits
#5 traiing at a certain intensity [admitedly difficult to moniter and find] can have you useing up to 75% of your energy source as fat, and carbs only 25%. this is usually around 40-55%MHR.
will reply to this in detail at a letr date if reminded, pushed for time now.