I've got a 13 yesr old son who's pretty active in sports. He's a bigger kid in both height and weight, but he's broken 4 bones already playing sports. We had a bone density test and doc said all was normal, blood showed vitamin D at a very low 12 ng/ml. I started him on 5,000 iu's daily. My question is, how long does it take bone strength to improve after a vitamin D regimen has been put in place? He wants to get back out on the ice and play hockey but I'm fearful the kid's going to break another damn phone. Thanks for your comments guys.