In adults, a minimum serum 25(OH)D value of 75 nmol/L now defines optimal vitamin D status (8). This concentration is derived from dose-response studies of the relationship between 25(OH)D and PTH, where PTH plateaus in approximately the mid-normal range and also from studies where 25(OH)D concentrations between 90 and 100 nmol/L are positively related to bone mineral density (BMD) in young adults (9). A dose-response study to determine the physiological and functional response to vitamin D3 supplementation has not been conducted in infants and is necessary to confidently set DRI values for Canadian infants.