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(Kovacs and Kronenberg, 1997; Kalkwarf, 1999; Prentice, 2003; Kovacs, 2005, 2008; Kovacs and Kronenberg, 2008). 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. Maternal BMD can decline 5 to 10 percent during the 2- to 6-month time period of exclusive breastfeeding. However, it normally returns to baseline during the 6 to 12 months post-weaning (Kalkwarf, 1999). Thus, in the long term, a history of lactation does not appear to increase the risk of low BMD or osteoporosis.

The physiological responses appear to be similar for lactating adolescents. In fact, an analysis using NHANES III data compared BMD from DXA measures in 819 women ages 20 to 25 years (Chantry et al., 2004), and found that young women who had breast-fed as adolescents had higher BMD than those who had not breast-fed, even after controlling for obstetrical variables. This suggests that the normal loss of BMD during lactation and the post-lactation recovery occurs in adolescents as well.

BONE MASS MEASURES ASSOCIATED WITH CALCIUM

Several key bone mass measures are commonly used in the context of calcium nutriture and related health outcomes. The accumulation and level of bone mass can be determined using the calcium balance method or, alternatively, the measurement of BMC or BMD based on DXA. The latter method relies on the assumption that about 32 percent of the measured bone mineral is calcium (Ellis et al., 1996; Ma et al., 1999). These methods are described below.

Calcium Balance

Calcium balance (positive, neutral, or negative) is the measure derived by taking the difference between the total intake and the sum of the urinary and endogenous fecal excretion. Balance studies embody a metabolic approach to examining the relationship between calcium intake and calcium retention and are based on the assumption that the body retains the amount of calcium that is needed. As such, measures of calcium balance (or of “calcium retention”) can reflect conditions of bone accretion, bone maintenance, or bone loss. Calcium balance analyses involve measuring as precisely as possible the intake and the output of calcium. Output is usually reflected by urine and fecal calcium; sweat calcium is not usually measured, but its inclusion adds to the precision of the estimates. Calcium balance studies are expensive and require considerable subject cooperation owing to the prolonged stays in metabolic wards. Measures of calcium balance have limitations and are generally cross–sectional in nature, and their



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