Adequate vitamin D is essential for proper infant growth and development. However, human milk is low in vitamin D, and most infants do not receive recommended supplementation. Our aim is to assess the feasibility of providing adequate vitamin D to breastfed infants through maternal vitamin D supplementation. Forty non-pregnant, lactating women at least 18 years of age with exclusively breastfed infants between the ages of 1 and 6 months will be randomized to receive oral vitamin D as either 5,000 IU daily for 28 days or 150,000 IU as a single dose. Maternal serum calcium, phosphorus, vitamin D and 25(OH)D; maternal urinary calcium; maternal milk vitamin D and 25(OH)D will be measured on days 0, 1, 3, 7, 14, and 28 of the study; and infant serum vitamin D and 25(OH)D will be measured on days 0 and 28.

Inclusion criteria

• Currently lactating mothers at least 18 years of age
• Willing to continue exclusively breastfeeding their infant throughout the study interval
• The infant is 1-6 months of age at the beginning of the study
• Willing and able to participate in all aspects of the study
• Mother and infant are in good health, as determined by the study investigator
• Have been provided with, understand, and have signed the informed consent for themselves and their child.

Exclusion criteria

• Have recently travelled (within the preceding 30 days) or plan to travel south of 35 degrees north latitude during the study interval
• Have recently or plan to engage in indoor tanning
• Are currently taking medications that affect vitamin D metabolism, like steroids, anticonvulsants, or barbiturates
• Are nursing multiple infants (e.g. twins)
• Are taking greater than the daily recommended intake of 1000 mg elemental calcium as calcium supplements
• Are taking greater than the standard daily dose of 400 IU of vitamin D found in prenatal vitamins
• Infant weight below 1.67 kg
• Mothers with baseline 25(OH)D levels >70 ng/ml, and/or infants with baseline 25(OH)D levels >70 ng/ml
• History of kidney stones