I'm a 25yo guy, currently mulling over lack of excercise due to a slightly sprained back (atleast 3 days without excercise!).
I weigh 199lbs at the moment and I'm 6'2". Broad shoulders and some muscle here and there, I can see the top 2 of my sixpack but still need work and fat burning to dig out the last 4 (or 6, in reality it's an 8-pack, afterall).
Just to experiment I tried out this 500-700kcal/day diet and so far I've lost 8lbs in 4 days. Yeah, most of it's water but probably a pound or so is fat. I'll see how long I can keep it up (I eat normally on weekends and lightly during the week).
In a couple of days I can resume my training (running, gym work, krav maga, swimming & occasionally tennis) to which I'm totally addicted.
My gf (who is super slim herself and needs to eat more than I do just to maintain her feathery weight, lucky rascal!) says I'm borderline anorectic (anorexia nervosa being more an illness of the mind than of body) because I'm somewhat obsessive about lowering my body fat but I don't know if she's right, I don't feel sick, I just have a damn hard drive to train, train, and train some more.
That's about all the relevant info I can think of right now. I'll browse around the forums and see if I can find some info and maybe share some of the little knowledge I have.