According to Dr. Suzanne Kaseta, a pediatrician,"excessive amounts of milk, which was more like 3or 4 or more cups a day was associated with a decrease feritin level and feritin is associated with your iron stores in your body."
Getting enough iron is important for children because it plays a critical role in early brain development. Low iron can also increase your risk for anemia.
Milk isn't the only way to get vitamin D. fatty fish like salmon is a good source, so is the sun.
The study points out some children with dark skin may need supplements because their body doesn't make enough vitamin D when exposed to sunlight.