I'm naturally very thin too - although my body is nowhere near as good as hers (smaller boobs and I don't do any much exercise, so don't have the six pack).
Despite my BMI being between 16.5 and 17.5, I too, like a previous poster, have somehow managed to give naturally give birth to two perfectly healthy, full term babies via very easy labours.
I eat perfectly well, and have never had an eating disorder, yet was bullied at school for being too thin and because I was back in my size 8 jeans within a week of giving birth, everyone assumed I'd either exercised a lot or hadn't eaten, when in reality, I'd done nothing and it is just the way my body is made.
I get that it is concerning that stick thin is held up as the ideal body image, but it would be nice if the issue could be debated without the collateral damage of those of us that are naturally thin being accused of 'looking awful and quite unwell'.
We would never dream of tackling the obesity epidemic (which is much bigger than the eating disorder epidemic caused by the size zero nonsense) by calling fat people 'heart attacks in waiting'.