I work with teens and adults as a therapist and having teenagers on a diet is a fast track to disordered eating. There is much greater risk of harm happening to her developing brain and body by having her fast. The teenage years are very important for bone strength and calcium absorption, something that stops in your twenties and is extremely difficult to recover from in later adulthood. In addition, if she isn't eating enough, our bodies will eventually pull energy from our muscles, including the heart which can lead to significant long-term problems and potential heart failure, even if she appears to be "overweight", her organs can be in danger of failing if she is starving herself and her body isn't getting what she needs. It also will disrupt her metabolism in ways that can take several years to repair. If you're interested in more information on health risks and other options, the book Intuitive Eating 4th edition is a great more clinical read or the F*k It Diet book is also a great resource. If you're concerned about her health in the meantime, it can be beneficial to encourage her to find movement she enjoys and that feels good to her and to practice cooking a variety of foods, including pizza, pasta, but also vegetables, fruits and a variety of other things and encourage her to notice what feels good to her body. I know there's a lot of frightening information in this, but this is the time where you can help her prevent a lifetime of potential serious mental health concerns but also physical health concerns. It sounds like you are already such an awesome and supportive parental figure to her!