As far as I'm concerned, the BDD is an associative disorder just as depression is.
It all usually comes hand in hand, as part of the territory.
I started out as bulimic, trying to lose some weight, then eventually found myself on a mission to pretty much outright kill myself through lack of weight. :l
Bad times. So, I figured I'd stop before I did any real damage.
I did - but ended up relapsing the year after, when I'd put weight back on. The BDD grew from there. Angry with the relapse, and apathetic and anxious about the future (will I have to fight this forever?) I grew into depression and self-harm. With things worsening, I took up Citalopram as a way of trying to help myself.

TLDR: It's an associative disorder. If you can beat the BDD, you can beat the Anorexia.