4.2.2 Study treatment groups: After obtaining written consent the infant will be randomized to a study group stratified for sex (see page 12) i for randomization process). At recruitment (2-4 weeks of age), baseline measurements will be conducted prior to providing treatment. Infants will then be randomized to receive 400, 800, 1200 or 1600 IU/d of vitamin D3 to 1 year of age. A placebo group is not included since the standard in Canada is 400 IU/d and a precedent for considering placebos unethical has already been set (37). The supplement will be double blinded to the investigators and all staff by coding of the supplements by Euro-Pharm who as agreed to provide the supplement in kind. This supplement will be stable for at least 3 months (the longest interval of measurements) and each dosage delivered in 2 ml volumes.