narche, calcium intakes required to achieve desirable retention do not necessarily fall because calcium absorption efficiency decreases. This menarcheal change in absorption was not observed in African American girls (Abrams and Stuff, 1994). Measures of sexual maturity are better predictors of calcium retention than is chronological age during this developmental period.
In a cross-sectional evaluation in 136 males and 130 females aged 4 to 27 years, BMD of total body, lumbar spine, and femoral neck increased significantly with age until 17.5 years in males and 15.8 years in females (Lu et al., 1994). The later timing of peak BMD in boys may relate in part to the fact that BMD is more strongly correlated with weight than with age (Ponder et al., 1990; Teegarden et al., 1995).
Calcium Retention. The desirable level of calcium retention for children in this age group was based upon new information on whole body bone mineral accretion for 228 children followed over 4 years between the ages of 9 and 19 years (Martin et al., 1997). The average peak velocity of bone mineral content which occurs between the ages 9.5 to 19.5 years was 320 g/year in boys and 240 g/year in girls. Using the assumption that bone mineral is 32.3 percent calcium, these values correspond to a daily calcium retention of 282 mg (7.1 mmol) in boys and 212 mg (5.3 mmol) in girls. One limitation of these data is that they do not provide information as to whether peak bone mineral accretion would be greater at higher calcium intakes than that consumed by the children studied; the mean intake for boys was 1,045 mg (26 mmol)/day at ages 10 to 12 years and 1,299 mg (32.5 mmol)/day at ages 13 to 15 years while for girls it was 903 mg (22.5 mmol)/day at 10 to 12 years and 954 mg (23.8 mmol)/day at ages 13 to 15 years (Martin et al., 1997). However, because the intakes of the children in the study were based on 24-hour recall data over 4 years, they are subject to under-reporting as previously observed (Livingstone et al., 1992), so actual intakes may have been higher.
In order to derive an estimate of calcium intake which would allow for the level of accretion of calcium in bone as derived above, a model for describing the relationship between calcium intake and retention was adopted. It had been applied to one set of calcium balance studies in girls (Jackman et al., 1997) (the method is described in detail in Appendix E).
The majority of the balance studies to which the model was ap-