It's not unusual for adolescents to experiment with different diet restrictions, as they take cues from peer groups or react to other social influences. From a nutritional and health point of view, children can thrive on a carefully planned vegan diet.
The key is careful, informed planning. Unfortunately, becoming vegan can be more about fashion than health. Many kids think a vegan diet just means substituting soy burgers for regular hamburgers. They end up eating a highly processed and unbalanced diet. Some vegan teens actually refuse to eat vegetables.
There's another more worrisome possibility: many eating-disordered adolescents adopt a vegan diet as another way to impose more food restrictions. They go from vegetarian to vegan, and may add some imagined food intolerances along the way. Then they start restricting even the vegan foods until there's little left to eat.
How should a parent react? Clearly, if you suspect your child is using a vegan diet to mask an eating disorder, arguing with your child about the diet will not be helpful. You need to find professional help for your child, starting with a therapist who is qualified to work with eating disordered patients. You may also need to schedule a checkup with your pediatrician, if your child has lost significant weight. Keep in mind, this type of behavior is not limited to girls. Boys can also develop eating disorders and use a vegan diet to justify the behavior.
Even if your teen isn't developing an eating disorder, adopting a vegan diet can cause a parent to worry. Ask your child to do some research on the nutritional aspects of vegan diets, so you can understand it better. This will also help your child learn that vegan meals are more than just soy burgers. With this knowledge, your teen should help plan the vegan meals, including grocery shopping and cooking. A teen who wants to follow a vegan diet should not expect parents to become short order cooks.
Working with your child to learn more about vegan meals and recipes can be a fun shared experience. It might also inspire family members to eat healthier meals, with more legumes, whole grains and vegetables. There's no downside to that.