To determine the effects of vitamin D3 1000 IU/d on serum 25-hydroxyvitamin D [ 25 ( OH ) D ] , PTH , and markers of bone turnover ( osteocalcin and collagen type 1 cross-linked C-telopeptide ) in black children and white children , and to explore whether there is a threshold level of 25 ( OH ) D associated with maximal suppression of serum PTH concentration .