I recommend that this teenager be evaluated by her primary care doctor
. Body mass index, or BMI, is something that we use to see if people are at a healthy weight for their height. The teenager you are speaking about went from being at a body mass index of 23.4 (normal is considered 18.5-25), to being at a BMI of 17.2 (if she weighs 100 lbs) or 18.2 (if she weighs 106 pounds), and underweight is considered to be any BMI less than 18.5. It sounds like the weight loss
has been purposeful, and it is okay to lose some weight the way you are describing: with moderate exercise and reasonable caloric goals. That being said, it is not recommended that people be underweight, particularly young women. The reason for this is that being underweight puts people at risk for having low bone density, fractures, and having low hormone production in general. Sometimes young women who are underweight stop having regular periods, which is a warning sign that the weight loss is too much. In addition, young women are at risk for anorexia nervosa. I would be curious to know if this teenager has stopped having normal menstrual periods, if she has a fear of gaining weight, or if she has a distorted self image (feels fat when she is not). If any of those were the case, I would be concerned for anorexia nervosa, which is a serious medical and psychiatric condition. The best thing for this teenager is to see her primary care doctor in person and to have a full physical exam and evaluation. It is good that she is taking One a Day for Teen, and I would just verify that that has Vitamin D and Calcium to protect her bones.