Breastfeeding is important for the development of the immune system of the infant. Emerging data suggest that vitamin D plays an important role in immunity as well. Exclusively breastfeeding mothers and their infants will be studied in a 3-month (4-study visit) pilot study of vitamin D supplementation versus placebo. Longitudinal effects of vitamin D status on breast milk composition and on the infant's immune system will be examined.
Additionally, exclusively breastfeeding mothers who are currently on vitamin D supplementation will be studied at a single visit, at which mothers will provide a single breastmilk sample and have a single blood sample obtained. These samples will be used to examine effects of vitamin D supplementation on breast milk composition.