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during pregnancy, decreases to the non-pregnant level in the puerperium, and remains at that level during lactation (Kent et al., 1991; Specker et al., 1994; O’Brien et al., 2006), then increases slightly during post-weaning compared with the level in non-pregnant or lactating women (Kalkwarf et al., 1996). Urinary calcium excretion also decreases (Allen et al., 1991; O’Brien et al., 2006) and may reach the lower end of the normal range, especially in women with low calcium intakes (Specker et al., 1994). This effect presumably is due to the influence of PTHrP, which stimulates renal calcium reabsorption.

Breast milk calcium content is homeostatically regulated and unaffected by maternal calcium intake. The evidence includes randomized trials in which supplemental calcium from 1 g/day (Kalkwarf et al., 1997) to 1.5 g/day in Gambian women whose habitual calcium intake was low (Jarjou et al., 2006) showed no effect on breast milk calcium content. These results are consistent with the notion that the calcium content of milk derives from resorption of the maternal skeleton and local regulation within mammary tissue. At least one study has confirmed that the breast milk output predicts the decline in maternal BMD during lactation, whereas calcium intake, breast milk calcium concentration, and VDR genotype have no effect (Laskey et al., 1998).

In lactating women, the albumin-corrected serum calcium as well as the ionized calcium levels are normal or slightly increased (Hillman et al., 1981; Specker et al., 1991). The mean ionized calcium level of exclusively lactating women is higher than that of normal controls (Dobnig et al., 1995; Kovacs and Chik, 1995). Also, mothers nursing twins have significantly higher total calcium levels compared with mothers nursing singletons (Greer et al., 1984).

These physiological responses appear to be similar for lactating adolescents. In fact, the largest and most reassuring data set from NHANES III (described previously), which obtained BMD using the DXA method in 819 women ages 20 to 25 years (Chantry et al., 2004) indicates that young women who had breast-fed as adolescents have higher BMD than those who had not breast-fed, even after controlling for obstetrical variables. This indicates that the normal loss of BMD during lactation and recovery afterward occur in adolescent women and may even lead to a higher BMD post-weaning.

Lactation: Vitamin D Breast milk is not normally a significant source of vitamin D for the infant. Because vitamin D (calciferol) is usually present in the circulation only for short intervals after meals, typically very little passes into breast milk. As discussed below, preliminary data may suggest that levels of vitamin D and 25OHD in breast milk can be increased by high

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