Should I be concerned about my daughter's extreme Dieting?

She's 17, she's not underweight, at least not yet, but she's obsessed with dieting and will only eat organic foods and is totally obsessed about where her food comes from and what's in it. She is really into her looks and is very vain and she looks at herself in the mirror so much. She's very beautiful and healthy but she just acts obsessive over certain things. Sometimes she insists on only eating once a day. She is also really concerned with never getting any type of sickness or disease and living as long as knowingly possible, which I think is kind of an unusual concern for someone her age. Do you think I should be concerned? What should I do?