The specific aims of this study are to determine levels of vitamin D supplementation to achieve goal serum 25(OH)D levels of 30 ng/mL, and to define serum 25(OH)D levels required to achieve suppression of parathyroid hormone in preterm newborn infants hospitalized in the Nebraska Medical Center Newborn Intensive Care Nursery (NICU). In this randomized, controlled trial, infants 23 weeks gestational age or greater will be randomized to two different levels of vitamin D supplementation: 400 IU vitamin D3/day, or 800 IU vitamin D3/day. As an exploratory aim, vitamin D binding protein levels (VDBP) will also be quantified in these infants.