If you believe your teen does have an eating disorder, you must take action. Most teens with eating disorders will be in denial about their issues, so it will be a challenge. However, for the sake of your teen's health -- physical and emotional -- you must do something quickly.
First, you need to determine if your teen has a true eating disorder, or is just making some bad health choices. While there are signs you can look for that indicate an eating disorder, such as anorexia or bulimia, your first step should be to enlist the help of a professional to diagnose your teen. Getting your teen to see and cooperate with a professional can be difficult, so try to open the conversation with your teen in a nonconfrontational way. Instead of focusing on your teen's behavior by using "you…" statements, which can make your teen defensive, start by focusing on your feelings and concerns. For example, you can say, "I'm concerned about how little you eat," or, "It must be so frustrating for you to constantly be counting calories." You want to show empathy for his/her struggle, not judgment. When you talk to your teenagers about their eating behavior, do be specific about your concerns. Then you can explain how you need them to visit with a professional so you can be comfortable that they're eating properly.
In addition to talking to your teens about what you see in their behavior that you find concerning, make sure to also mention to them how proud you are of their achievements. However, do stay away from praising (or criticizing) their appearance, weight, or eating habits. While this might seem beneficial, even the positive comments can reinforce your teen's feeling that his/her body is where he/she should find his/her self-esteem. The idea is to direct his/her self-esteem in other directions.
The most important thing, however, is to take your teen to visit a professional to get a formal diagnosis and assistance for your teen to overcome his or her eating disorder.