Dawson-Hughes et al., 1990; Reid et al., 1995). The main results of these studies are shown in Table 4-1. These studies reveal that the effectiveness of calcium varies by skeletal site, by menopausal age, and with the usual calcium intakes of the study subjects. Apart from the initial bone remodeling transient in year 1, added calcium offers little benefit in BMD at the spine. In contrast, calcium generally has more impact on BMD at the more cortical-rich proximal radius, femoral neck, and total body. Late postmenopausal women tend to be more responsive to supplemental calcium than early postmenopausal women. In addition, late postmenopausal women with very low calcium intakes generally gain more from calcium supplementation than do women with higher usual calcium intakes (Dawson-Hughes et al., 1990, Elders et al., 1994).
The positive impact of supplemental calcium on BMD in women with low-to-moderate usual mean calcium intakes is generally consistent with the observation that increasing calcium intake improves calcium balance. Trials involving women with the highest usual calcium intakes are more useful in this context, and they demonstrate that increasing calcium intake above 750 mg (18.7 mmol) (Reid et al., 1995), 800 mg (20 mmol) (Prince et al., 1995), or 1,000 mg (25 mmol) (Riis et al., 1987) reduces loss of bone mineral from cortical-rich skeletal sites. Since 80 percent of the skeleton is comprised of cortical bone, one would expect changes in cortical bone to parallel balance changes. Trials in women with even higher usual calcium intakes are needed to test the balance study estimate of 1,200 mg (30 mmol)/day.
The AI for men and women ages 51 through 70 is set at 1,200 mg (30 mmol)/day based primarily on the clinical trial data in women which demonstrated a positive reduction of bone loss with calcium intakes over 1,000 mg (25 mmol)/day. In addition, balance studies in women (Hasling et al., 1990) and women and men (Selby, 1994), showed that calcium intakes up to 1,500 mg (37.5 mmol)/day (mean intakes of 1,116 mg [27.9 mmol]/day and 1,214 mg [30.4 mmol]/day, for the cited studies, respectively), were associated with higher calcium retention. Although a value of about 1,000 mg (25 mmol)/day was derived from the calcium retention model using balance studies in men, there were no data for calcium intakes between 800 and 1,200 mg (20 and 30 mmol)/day. For the reported balance studies in women, a plateau calcium retention value could not be derived. The AI of 1,200 mg (30 mmol)/day was chosen for this age group assuming that their needs would be somewhat high-