I bet this is Anna Nicole Smith all over again. Word is, he became addicted to pain killers following his burn accident years ago. Jacko was probably on a dozen different meds when he died and had been for years; his frail little heart just couldn't take it anymore. Wasn't he also a vegetarian? 
I agree. He was pretty out there in his more recent interviews, and I'm sure whatever he was on had been taking it's toll.
If I remember correctly, his weight/height on his mugshot from years back showed him being 6'2"-6'3" and only 120# or so? Not sure of the exact numbers, but one could look at him and didn't have to be a rocket scientist to observe he was borderline anorexic. Perhaps he was, and covered it up with a facade (vegetarianism?) or similar?
I overheard a person saying "he was so skinny, how could he die of cardiac arrest?!" Well, Karen Carpenter was skinny too, and while the anorexia was what put her in her physical shape the time of death, she apparently died of cardiac arrest, because she'd put on 8lbs within a very short time frame, and her weakened heart couldn't take the weight gain.
Perhaps MJ suffered something similar?
