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calcium intake and pointed out that the data in their model reflected typical calcium intake between the 5th and approximately 95th percentiles for all boys and men 9 or more years of age, and between the approximately 25th and greater than 99th percentiles for all girls and women 9 or more years of age.

Hunt and Johnson (2007) also pointed out that most (but not all) studies with adults that indicate a positive influence of high total calcium in reducing the rate of bone remodeling were confounded by the presence of vitamin D as an experimental co-variable. In their study, the metabolic diets were similar to the estimated median intake of vitamin D by free-living young women. In short, the analysis may provide a reasonable approach for extracting meaningful data from calcium balance studies that are often confounded by multiple dietary factors. At this point, factorial methods should be briefly noted as the determination of calcium requirements has also made use of a factorial approach as noted in the 1997 DRI report (IOM, 1997). The factorial approach allows the estimate of an intake level that achieves the measured levels of calcium accretion/retention. The method combines estimates of losses of calcium via its main routes in apparently healthy individuals and then assumes that these losses represent the degree to which calcium intake, as corrected by estimated absorption, is required to balance these losses. The weakness in this method is that it is unusual for all of the necessary measurements to be obtained within a single study. Therefore, most calculations using the factorial approach are compiled from data in different studies and thus in different subjects; this can introduce considerable variation and confound the outcomes. This approach, as carried out in the 1997 IOM report on DRIs for calcium and vitamin D (IOM, 1997), where the interest was in desirable retention, is illustrated in Table 2-1.

Bone Mineral Content and Bone Mineral Density

BMC is the amount of mineral at a particular skeletal site, such as the femoral neck, lumbar spine, or total body. BMC is correctly a three-dimensional measurement, but when it is commonly measured by DXA, a cross-section of bone is analyzed, and the two-dimensional output is a real BMD (i.e., BMC divided by the area of the scanned region). True measurements of BMC (volumetric BMD) can be determined non-invasively by computed tomography. Throughout this report, the term “BMD” generally means areal BMD unless specified as volumetric BMD. Most importantly, any of these measures are strong predictors of fracture risk (IOM, 1997). Bone density studies can be considered to reflect average intakes of calcium over a long period of time. When available, such data likely provide

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