BMC at several sites was detected after the age of 18 except for the skull, which continued to gain mass (Matkovic et al., 1994). In a longitudinal study (Recker et al., 1992), lumbar BMC increased by a median gain of 5.9 percent, and the forearm increased by 4.8 percent during the third decade of life. In a smaller study of 45 females aged 9 to 21 years, BMC of the whole body, spine, and femoral neck plateaued at age 16 years (Katzman et al., 1991). Bone density of the hip decreased after age 17 years (Matkovic et al., 1994; Theintz et al., 1992). In summary, the age of peak bone mass appears to vary with skeletal site and sex. Nevertheless, taken all together, the data indicate that the skeleton continues to accrete mass for approximately 10 years after adult stature is achieved.
Calcium Retention. A desirable level of calcium retention (the level of positive calcium balance) for the 19- to 30-year age group was set as the retention of calcium equivalent to the reported calcium accretion derived from studies of bone mineral accretion during the third decade (Peacock, 1991). The limitation of these data is that they were derived from metacarpal morphometry data (Garn, 1972). To date, there are no data on whole body bone mineral accretion using DXA technology nor for groups of subjects consuming variable amounts of dietary calcium. The accretion of calcium based on these data was 10 mg (0.25 mmol)/day for females and 50 mg (1.3 mmol)/day for males. This large discrepancy in calcium accretion between genders may reflect the older age at which males achieve peak bone mineral content velocity (Martin et al., 1997) or inaccuracies in the older bone densitometry methods.
The relationship between calcium intake and retention for this age group was computed from a compilation of balance studies in 163 young adults (26 males and 137 females), aged 18 to 30 years, from the literature between 1922 and 1992 as compiled by Matkovic and Heaney (1992). The non-linear regression approach of Jackman et al. (1997) was applied to these data () and the regression equation solved to determine the calcium intake at which a desirable daily calcium retention of 50 mg (1.3 mmol)/day for males and 10 mg (0.3 mmol)/day for females could be achieved. A value for sweat losses of 63 mg (1.6 mmol)/day (Charles et al., 1983) was added to the level of desired retention since these losses had not been corrected for in the calcium balance studies. The estimated calcium intake at which a desirable retention would be