What vitamins does a stick-thin young girl need?

Thin girl; vitamins?: Generally, a healthful diet including lean protein, good fats, low glycemic carbs, plenty of colorful fruits & veggies, whole grains should be adequate, but a good multivitamin (including iron if your menstruating) like centrum et al won't hurt. See your fp about basic tests to rule out overactive thyroid, type 1 diabetes, or other metabolic causes for your thinness. Don't smoke.

Everyone needs D: Beyond that, it depends on the individual's diet. If she is thin, but not underweight (medically underweight, not just "i don't like it when she looks so skinny"), and she eats a healthy diet with lots of vegetables, fruits, and whole grains, then she's fine and doesn't need anything more.