birth and left uncorrected will more readily lead to neonatal hypocalcemia and the postnatal development of rickets.

Lactation: Calcium Key physiological changes in the female adolescent or adult occur to meet the calcium demands of lactation that are higher than those in pregnancy, but the adaptations differ from those that occur during pregnancy (Kovacs and Kronenberg, 1997, 2008; Kalkwarf, 1999; Prentice, 2003). Maternal bone resorption is markedly up-regulated (Specker et al., 1994; Kalkwarf et al., 1997), and it appears that most of the calcium present in milk derives from the maternal skeleton. This bone resorption is driven by low estradiol and high plasma PTH-related protein (PTHrP) levels (and possibly other factors), which act through osteoblasts to up-regulate osteoclast number and activity. Maternal BMD can decline 10 to 45 percent during 2 to 6 months of exclusive breastfeeding, but it normally returns to baseline over the succeeding 6 to 12 months post-weaning (Kalkwarf, 1999).

The effect of dietary intake of calcium on the skeletal resorption that occurs during lactation has been examined through randomized trials and in observational studies comparing North American and Gambian women. The consistent finding is that calcium intakes ranging from very low (< 500 mg/day) to supplemented well above normal (1.0 to 2.5 g/day) have no effect on the degree of skeletal demineralization that occurs during lactation, but calcium supplementation does increase urinary calcium excretion (Cross et al., 1995; Fairweather-Tait et al., 1995; Prentice et al., 1995; Kalkwarf et al., 1997; Laskey et al., 1998; Polatti et al., 1999).

The effect of calcium intake on skeletal recovery after weaning has not been rigorously studied. In one RCT that enrolled 95 lactating women prior to weaning, use of a 1 g/day calcium supplement resulted in a 5.9 percent increase in lumbar spine BMD compared with a 4.4 percent increase in women who took a placebo, as well as a 2.5 percent increase in non-lactating women compared with a 1.6 percent increase in women who took a placebo (Kalkwarf et al., 1997). These studies suggest that a higher calcium intake during post-weaning recovery might be beneficial for ensuring restoration of skeletal mineral content; conversely, a low calcium intake during post-weaning might be expected to impair skeletal recovery. However, skeletal recovery was complete in Gambian women with habitually very low calcium intakes. Moreover, the large associational studies mentioned above found no effect (and some found a protective effect) of a history of lactation or the number of months that a mother recalled breastfeeding her child on BMD, osteoporosis, or fracture risk later in life (Sowers, 1996; Kovacs and Kronenberg, 1997). Thus, in the long term, a history of lactation does not increase the risk of low BMD or osteoporosis.

The efficiency of intestinal calcium absorption, which is up-regulated