These physical and mental changes can create doubts and incomprehension and can change ideals; each young person reacts to this stage of life in his/her own way, according to his/her personality and his/her past experiences.
Unfortunately, today we are stressed from messages suggesting that slim and beautiful models are normal. Those same messages tell us how we should dress, behave, and be fashionable. We are all deeply influenced, consciously or unconsciously, by the mass media, and this is especially true for teen-agers. It's normal and unavoidable that in that phase of life, values change and move on to what society proposes as ideals. You can't avoid it, but you need to understand these behaviours in order to prevent fights and incomprehension feelings with your daughter.
Every stage of human life has some characteristics for which that period is worth to be lived, and the only thing that is really important when someone is deep in a difficult life crisis, is to have someone who demonstrates his/her love to him/her; someone who helps and who understands.
It's important to never give up our educational role as parents, but is equally important to try and make teen-agers understand the motivation of the advice (or prohibitions) we give them in order to let them feel understood and cared for.