The global objective of this study is to conduct a dose-response study of vitamin D3 intakes and to provide information regarding the physiological and functional response to vitamin D supplementation in infancy, including: 25(OH)D concentrations and the relationship between 25(OH)D, PTH and bone mineral accretion. Once the best dosage is determined, funding for a larger study spanning all Canadian regions will be sought to more conclusively demonstrate that this dosage is appropriate for all Canadian infants. In the larger study, infants will be followed longitudinally to confirm that optimization of vitamin D status in infancy enhances peak bone mass.