The aim of this investigation was to study the bone metabolism in early infancy by establishing the relationship between serum osteocalcin levels and the hormonal vitamin D status of exclusively breast-fed infants during their first month of life. Calcium, phosphorus, alkaline phosphatase, 25-hydroxycalciferol (calcidiol), 1,25-dihydroxycalciferol (calcitriol) and osteocalcin (BGP or GLA-protein) were measured in 22 healthy lactating women and their paired breast-fed infants before and after supplementation (400 IU vitamin D per day). Prior to supplementation calcidiol, calcitriol and osteocalcin remained unchanged. Following supplementation there was an increase in all the parameters with the exception of calcitriol. The administration of vitamin D to breast-fed infants should in fact have an effect on bone activity as reflected by the increase in osteocalcin levels.

http://www.ncbi.nlm.nih.gov/pubmed/3489605?dopt=Abstract