Optimal 25(OH)D concentration for infants is unknown but is projected to be at least 75 nmol/L based on adult studies (8). This value is derived from well controlled dose-response studies of vitamin D3 supplementation in men and women, which demonstrate a reduction in serum PTH (8, 28, 29) with increasing 25(OH)D concentrations and a plateau in the mid-normal PTH reference range. However, current thinking suggests that the 25(OH)D-PTH dynamic is not the only instrument to determine recommendations, but should be supplemented with alternate biomarkers, such as bone mass and mineral metabolism, to more clearly define what is optimal (27). This is illustrated by a study of young adults in whom higher vitamin D concentrations (90-100 nmol/L) were associated with higher BMD when compared to those with lower concentrations (9). This would suggest that gains in 25(OH)D beyond those associated with the PTH plateau are important. Similarly, 25(OH)D concentration of 75 to 80 nmol/L in children 7 to 18 years of age are related to normalization of PTH (30-34) and has been selected as the target concentration for this age group as well. Concentrations of 25(OH)D at and above this value are associated with enhanced bone mineral accretion in one cohort (35) and in a randomized controlled trial (RCT) of vitamin D supplementation with 400 and 600 IU/d in adolescents (36). These studies again emphasize that the PTH-25(OH)D dynamic and bone mass are important determinants for establishing the RDA in pediatrics. Such dose-response studies of vitamin D3 do not exist for Canadian infants and it is possible that PTH should not be the only biomarker of adequacy during this life stage. There are limited dose-response studies [2] or RCT [1] examining daily oral vitamin D intake in infants (37-39). The details of these studies have recently been summarized as part of a NIH requested systematic review compiled by Canadian experts. The key message of this report is that there are no dose-response studies of vitamin D3 intake in infants, but only of vitamin D2 (9.5 x lower potency (12)) in dosages ranging from 100 to 1000 IU/d. These studies were of short duration and none were conducted in Canada. Additionally, not all studies measured PTH and none measured whole body bone mass. Clearly, a trial of vitamin D3 in Canadian infants is overdue.