- Study Design: This is a parallel-group double-blind randomized controlled trial of 12 months duration on 60 children and adolescents aged 6 to 19 years with a clinical diagnosis of OI. One group of 30 participants will be randomized to receive vitamin D3 at a dose of 2000 international units per day ('high-dose group'). The other group of 30 participants will be randomized to receive vitamin D3 at a dose of 400 international units per day ('standard-dose group'). Randomization will be stratified according to pubertal status and bisphosphonate treatment status.