The Rolling Stone interview I read with her (years ago) did reveal that she did go through anorexia. But, I highly disagree with the idea that Paper Bag is in support of anorexia. Paper Bag is one of my favorite fiona songs and I've listened to this song (and related to it) enough to know that it's about love. To be specific, it's a song about false hope, realizing that the guy you like isn't willing to deal with your problems and that he's nothing but a fool. No matter how much you hope and wish, the only way to break away from that hunger for love is to keep him away... because "it's cost too much to love."
The use of words may have come from her anorexia, but I don't see how just taking that one line to say that it's "pro-ana."
http://www.songmeanings.net/lyric.php?lid=2059