SECONDARY OBJECTIVES: When multiple doses achieve our primary objective of optimal 25(OH)D concentrations, the best dosage will be identified based on: (i) 25(OH)D-PTH dynamics (ii) incidence of adverse events, such as disturbed mineral homeostasis (e.g. hypercalcemia, hypercalciuria, etc), and (iii) alternate markers of bone health (e.g. bone mass, osteocalcin and N-telopeptide) TERTIARY OBJECTIVE: We will examine the quantitative relationships between primary and secondary outcome measures and the various prospectively collected predictor variables, including dose, vitamin D from breast milk or fortified formulae, gender, season of birth, ethnicity, baseline vitamin D stores and determine if the selected dosage maintains 25(OH)D over the first year of life.