Please listen to what I have to say... I was in the same situation years ago. At age 17 or so, I began to change my lifestyle, started working out everyday, quit smoking, and started to eat more healthfully. I kept a food diary, measured my portions, and counted fat and calories religiously. I don't know what happened, but I went from about 160 lbs (I'm 5'5") to 88 lbs. As I began to lose, I felt great. But the more I lost, the more determined I became to lose more and I got more restrictive and obsessed with diet and food and exercise. I will never fully understand how my attempt to reach a healthy weight of 120 lbs escalated into such a horrible downward spiral that wasted many years of my life and killed the relationship with the only guy I've ever really loved. When I went to the doctor one summer, she was shocked that I weighed 103 and so was I! I was scared but didn't know how to stop. And I lost more, eventually getting to 88 lbs. In college, my nutrition professor took my body fat and it was 5.5%. I knew I needed help. She referred me to a nutritionist who has helped me tremendously. I will be 22 next week and am at about 105 lbs now, still trying to gain a bit but it took professional help to get me back to a healthy frame of mind, and it is still tough at times. I do not want to scare you at all. But the reality of what COULD go from a reasonable, healthy diet to a life-controlling obsession is so scary. Watch her carefully! Look for warning signs! See what she knows about eating disorders. I never would have gotten help if it weren't for concerned teachers and friends and family. Be supportive, and don't accuse her of anything. You are awesome for being worried. I thank goodness for people like you!