 Vitamin D intakes in children do not meet current US Dietary Reference Intake recommendations and emerging evidence suggests that a significant number of children, particularly those with darker skin pigmentation, have inadequate levels of serum 25-hydroxyvitamin D 25OHD. The optimum level of circulating 25OHD has not been clearly defined in children, nor is it known what functional outcome measures are ideal for defining this level, or if these requirements would differ by race. Graded doses of vitamin D3 supplementation will be used in this dual-site, 12-week trial. The investigators hypothesize that a dose-response relationship will be observed between vitamin D supplementation and intermediate endpoints of skeletal health, and that race will modify these responses. 