Vitamin D Metabolism in Breast-Fed Infants and their Mothers

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Abstract
The aim of this longitudinal study was to examine vitamin D metabolism in exclusively breast-fed infants. The four common vitamin D metabolites—25-hydroxyvitamin D (25OHD), 1,25-dihydroxyvitamin D [1,25(OH)2D], 24,25-dihydroxyvitamin D [24,25(OH)2D], and 25,26-dihydroxyvitamin D [25,26(OH)2D]—as well as vitamin D binding protein (DBP) were determined simultaneously in mothers and their children from delivery to several months of age. Maternal blood samples, drawn approximately 6 wk before the expected date of delivery, were also analyzed. At delivery, total vitamin D metabolites in maternal and fetal plasma were closely correlated, maternal levels being higher. Unbound (free) vitamin D metabolite concentrations were higher in fetal than in maternal plasma, with the exception of free 1,25(OH)2D levels, which were equal. This suggests a rapid placental transfer of 1,25(OH)2D. 24,25(OH)2D and 25,26(OH)2D levels both in mothers and children were closely correlated with the precursor sterol 25OHD. For 1,25(OH)2D, no correlation could be demonstrated with any of the other vitamin D metabolites. DBP concentrations in maternal plasma at the time of delivery were about twice the mean adult reference value. In cord blood, DBP levels were in the lower part of the adult reference range. Maternal total 1,25(OH)2D levels, which were twice the reference mean during pregnancy, fell sharply after delivery but free 1,25(OH)2D levels much less. Analogous to the biochemical changes in the mother, the infants' DBP levels fell after birth, as a result of the sudden disappearance of the estrogen stimulus. At the same time, the mineral supply via the placenta was cut off. These two factors are probably responsible for a stimulus to 1,25(OH)2D synthesis (and/ or inhibition of 1,25(OH)2D degradation), resulting in a sharp increase of 1,25(OH)2D and an even stronger increase of free 1,25(OH)2D. Concentrations of 25OHD and 1,25(OH)2D in breast milk were low. Such water-soluble metabolites as 25OHD-glucuronides were not detected. Judged by plasma 25OHD levels, the vitamin D stores of most children born to mothers with normal vitamin D status are depleted approximately 8 wk after delivery. Therefore, supplementation with an appropriate dose of vitamin D shortly after birth seems advisable, especially in winter.