I agree with the other posters that you may not be absorbing the Vit D in the milk well or there isn't enough in the milk as Perna said. Vitamin D is a 'fat soluble vitamin' that is assimilated in the lower intestine - if a person has an absorption problem, then the vitamin won't be used. Fat soluble means, the vitamin ends up being absorbed in fatty areas and stored in the body - vitamin D also needs some fat in the diet to be used well, which is why you should take it with dinner. JD mention other vitamins that help in absorption. Since vitamin D is fat soluble and stored in the body, we do have to remember it's possible to take or absorb 'too much' - therefore you should ask your doctor when you should have it checked again.

I had low vitamin D too even though I drink 3 glasses daily. Another reason why many of us are low, is because we've been bombarded about how sun is so bad and people are blocking the UV rays which help us make our own vitamin D. All it takes is 15 min. of sun on your arms and face, which isn't bad. Yes sunscreen is necessary but I don't over do it and slather it on every time my kids go out the door like some mom's do.

http://www.merckmanuals.com/professi...04/ch004k.html