My dd went through this at the same age. She would even go as far as to read nutritional labels before eating anything and hiding food in fear she would get fat. She would constantly tell me she was fat when in fact she was wearing a size smaller than she should have been at that age. I started talking to her about healthy eating and 'sometimes' foods.

It took quite a while and loads of reassurance but I finally got her out of it.

I do think this is a phase many young girls go through but it really needs to be addressed early before it becomes a habit.