During the 28-day study interval, both cholecalciferol-dosing regimens appeared safe. However, the mild increase in renal calcium excretion raises concern for an increased risk of nephrolithiasis. The mechanism for the increased calcium excretion is unclear. Parathyroid hormone erelated peptide produced by the lactating breast and low estradiol concentrations contribute to mobilization of calcium from bone.Calcium lost in breast milk also contributes to a negative calcium balance during lactation that is not prevented by calcium supplementation.We didnotobserveasignificant increase in 1,25(OH)2D concentrations to suggest that cholecalciferol increased intestinal calcium absorption,and urinary calcium excretion was un related to1,25(OH)2D concentrations.In a large population study,nephrolithiasis was not associated with serum25(OH)D values.