2.6 Theoretical Calculation of Vitamin D Intakes To Establish Study Dosage Groups As described in 2.1, the RDA is defined as the EAR + 2 SD and covers requirements of 97% of the population. If the target is 75 nmol/L for all infants, this means that the 2.5 percentile must be at this concentration. Using data from the PI's research of 83 cord blood samples, the mean value of 25(OH)D was 35 with SD of 15 nmol/L (full dataset from Weiler et al. (6)). Thus the lower 2.5 percentile is 5 nmol/L. To elevate this to 75 nmol/L, a gain in 70 nmol/L would be required (97 % of infants above target). Unfortunately, there is little available data as to the dose-response dynamics of mammalian cholecalciferol (D3) in infants, and studies of plant ergocalciferol (D2) may be misleading due to differences in bioavailability, which may be as much as a whole order of magnitude. The preliminary data from Drs. Taback and Weiler using daily supplements of either 400 IU or 2000 IU of D3 in a small number of Winnipeg infants (n=4 in each group) are the only data for us to base our estimates. Although too small a sample to confidently estimate measurement variance, they observed a rise of 44 and 115 nmol/L in the respective mean 25(OH)D concentrations in the first 3 months of supplementation. Thus, the 400 IU dose of vitamin D3 is expected to achieve a mean concentration of 75 nmol/L by 3 months of age. 800 IU D3 will be used in this study because this is currently recommended for Canadian infants. Because 2000 IU D3 was felt to be excessive based on the modest numbers of infants given this dose and their short follow up, we elected to select 1200 and 1600 IU as intermediate yet likely candidate RDA doses.