4.0 METHODOLOGY: Dose Response Study 4.1 Population n=192 breast-fed infants from greater Montreal region will be studied. The rationale for studying breast-fed infants is that this is the current recommended optimal food for infants, along with vitamin D supplements, and the fact that 85% of women in Canada initially breastfeed their infant (44, 45). Beginning with breastfeeding allows confirmation that the best dose is suitable while breastfeeding and if this dose is also optimal later in infancy with introduction of foods and weaning to vitamin D fortified formula. Initially a pilot study will be employed with 24 infants to evaluate the recruitment strategy, study questionnaires and procedures that will be used in the larger study.