O.k. boys, I didn't want to do it, but you leave me no other choice but to pull out THE BIOCHEMISTRY BIG GUNS. It's late and I don't feel like sifting through my THICK Biochem textbook, but let me just summarize from memory the following:
The reaction involved in "fat loss" at the biochemical level is the breakdown of the Triacylglyecrol molecule to it's component glycerol + 3 fatty acids. These in turn are used to produce ATP energy through other reactions. All of these reactions release H20 molecules as intermediaries. The accumulation of these is what we perseve as sweat.
My point? For significant fat loss at a fast rate, one must sweat... the more, the better. later