The effect of various dosages of vitamin D on whole body/skeletal bone mineral accretion in the first year of life has not been reported. In the first year of life, it is expected that whole body bone mineral content (BMC) will almost triple in parallel to growth (24, 25). In formula fed infants, whole body BMC at 1, 3, 6, 9 and 12 months of age are 79, 130, 160, 200 and 235 g respectively as measured with dual energy x-ray absorptiometry (DXA) with a precision error of 4.5% (25). Since the largest increment is between 1 and 3 months (63%), this is an ideal time to compare outcomes with respect to effects of various dosages of vitamin D on bone mineral accretion. However, this has yet to be conducted and is an important objective of the present proposal. It is anticipated that optimal vitamin D status will yield greater bone mass, since in Geneva, Switzerland (46N latitude), even a 400 IU/d vitamin D3 supplement in infancy (median duration of 12 months) is associated with higher BMD at 7 to 9 years of age (4). In the girls who received a vitamin D supplement in infancy, BMD was 6% higher in the distal radius and 9% higher in the femoral neck, even after adjustment for size (4). These positive associations must be viewed in conjunction with studies identifying peak bone mass as a critical factor in bone health (26). Whether the positive effects of the supplement translate into higher peak bone mass, is not yet known. An additional limitation is that no such studies have been conducted in Canada, with its unique climate, culture, food supply, and policies related to infant sun exposure. Since optimal vitamin D requirements for Canadian infants are ill defined, the benefits to bone mass are also unclear. As such, recommended dietary intake of vitamin D3 for Canadian infants must be identified by rigorous, well-controlled, randomized designs with optimal vitamin D status as the primary outcome.