Serum 25(OH)D. During lactation, small and probably insignificant quantities of maternal circulating vitamin D and its metabolites are secreted into human milk (Nakao, 1988; Specker et al., 1985a). Although there is no reason to expect the mother's vitamin D requirement to be increased during lactation, some investigators have determined whether the infant can be supplemented via the mother's milk. Ala-Houhala (1985) and Ala-Houhala et al. (1986) evaluated the vitamin D status of mothers and their infants supplemented with vitamin D. Healthy mothers delivering in January received either 50 µg (2,000 IU)/day, 25 µg (1,000 IU)/day, or no vitamin D. Their infants were exclusively breast-fed and received 10 µg (400 IU)/day of vitamin D if their mothers received none. After 8 weeks of lactation, 25(OH)D concentrations of infants who were breast-fed from women receiving 50 µg (2,000 IU)/day of vitamin D were similar to those of infants supplemented with 10 µg (400 IU)/day. The serum 25(OH)D levels in the infants from mothers receiving 25 µg (1,000 IU)/day were significantly lower. None of the infants showed any clinical or biochemical signs of rickets, and all infants showed equal growth. Although it was concluded that postpartum maternal supplementation with 50 µg (2,000 IU)/day of vitamin D, but not 25 µg (1,000 IU)/day, seemed to normalize serum 25(OH)D concentration in infants fed human breast milk in the winter, the maternal 25(OH)D level increased in the two groups of mothers receiving 50 or 25 µg (2,000 or 1,000 IU)/day of vitamin D compared with mothers who received no vitamin D supplementation.

There is no scientific literature that has determined a minimum vitamin D intake to sustain serum 25(OH)D concentration in the normal range during lactation, and there is no evidence that lactation increases a mother's AI for vitamin D. Therefore, it is reasonable to extrapolate from observations in nonlactating women that when sunlight exposure is inadequate, an AI of 5.0 µg (200 IU)/day is needed. However, an intake of 10 µg (400 IU)/day, which is supplied by postnatal vitamin supplements, would not be excessive.