Context

Routine vitamin D supplementation (400 IU/day) of breastfed infants has been recommended in North America for >50 years.1 Historically, the practice was advocated to prevent rickets; recently, there has been greater emphasis on its role in maintaining serum 25-hydroxyvitamin D concentrations (25(OH)D) above conventional thresholds of sufficiency (eg, 50 nmol/L).2 Yet, some breastfeeding advocates have argued that this policy conflicts with the message that mother's milk is a complete source of nutrient requirements in the first 6 months of life. The recognition that breast milk vitamin D inadequacy reflects maternal vitamin D insufficiency has prompted efforts to define maternal postpartum vitamin …