Vitamin D activity in maternal plasma and milk in relation to gestational stage at delivery

Although the biochemistry of milk from mothers giving birth prematurely has been described in some detail, it was not known whether premature birth would affect vitamin D status of mothers, the vitamin D activity of milk or the concentration of vitamin D binding protein in milk. In the present study, plasma and milk samples were collected from eight mothers giving birth prematurely and compared to samples obtained from nine mothers giving birth at term. Vitamin D intake from milk and vitamin D supplements was at least 400 IU/day in all mothers. Complete 24-hour expressions of milk were collected between 14 and 21 days postpartum. No differences were observed in plasma D2 (5.1+0.7 and 3.7+1.0 ng/ml, mean±SEM) or milk D2 (0.19±0.06 and 0.12±0.02 ng/ml) in term versus preterm groups, respectively. In contrast, plasma vitamin D3 (0.7±0.1 and 2.7±0.5, ng/ml, p<0.01) and milk vitamin D3 (0.14±0.02 and 0.23±0.03, ng/ml, p<0.05) were significantly higher in preterm mothers. This was associated with higher plasma vitamin D binding protein (DBP) in preterm mothers (308±13 versus 463±30, ug/ml, p<0.01) but not preterm milk DBP. There were no differences in milk vitamin D sterols nor DBP content between the second and fourth week of lactation. In the case of the hydroxylated sterols, 25-hydroxyvitamin D2 and 25-hydroxyvitamin D3, no differences between mother groups could be discerned in plasma or milk. Differences in the synthesis and/or transport of vitamin D and DBP which are reflected in PT milk and plasma levels in the early postpartum period may relate to cutaneous synthesis or abbreviated gestational stage. Based on the vitamin D sterols measured in this study, the total vitamin D activity in human milk in the first month of lactation averaged 80±9 IU/L for preterm milk and 60±7 IU/L for term milk, both values which are in excess of the often quoted value for human milk vitamin D of 22 IU/L.