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