We will evaluate patients at Children's Hospital Medical Center every 6 months for 2 years. During this 2-year period, participants in the study will take one multivitamin containing 400 IU (international units) of vitamin D and either 1,000 mg of calcium carbonate (Tums tablets) by mouth or a matching placebo once a day. We will check patients 6 and 18 months after the 2-year treatment period to find out if people in the Tums-treated group maintain any increases in bone formation that occurred during the 2-year treatment period.