- Specific Aims: 1. To determine whether 12 months of high-dose vitamin D supplementation, compared to standard-dose vitamin D supplementation, increases areal bone mineral density z-scores at the lumbar spine. 2. To examine the effectiveness of high-dose vitamin D supplementation to increase trabecular and cortical bone mineral density at the radius. 3. To examine whether high-dose vitamin D supplementation has an effect on physiological determinants of bone mass (parathyroid hormone, activity of bone metabolism, muscle function).