Well, if you won't argue then I won't argue...
I can't say that I know much about metabolisms, but my basic understanding was that your body got nutrients out of food, processed the food, and then got rid of the leftover product that it had been unable to process. That is pretty much it.
Several friends of mine took ballet when they were younger, and one had severe anorexia that nearly killed her. Whenever I see skinny people I worry about their health -- I'm sorry, but I can't help it. When I said that the girl was unhealthily thin I wasn't attempting to be rude either to her, the artist, or whomever viewed the comments.
I was not knocking her, actually I love to photograph thin people and I would work with her in a minute. But I realize you had no way to know that.
There is an artist here in town who is also thin and no matter what he eats to gain weight it simply does not happen.