a better snapshot of long-term calcium intake than does the combination of accretion/retention data.

In children, change in BMC is a useful indicator of calcium retention; change in BMD is less suitable, because it overestimates mineral content as a result of changes in skeletal size from growth (IOM, 1997). In adults, with their generally stable skeletal size, changes in either BMD or BMC are useful measures. In the context of longitudinal calcium intervention trials that measure change in BMC, the measures can provide data on the long-term impact of calcium intake not only on the total skeleton, but also on skeletal sites that are subject to osteoporotic fracture (IOM, 1997). However, because DXA does not distinguish between calcium that is within bone and calcium on the surface (e.g., osteophytes, calcifications in other tissues) or within blood vessels (e.g., calcified aorta), an increase in BMC or BMD, particularly in the spine, may result in false positive readings suggesting high bone mass (Banks et al., 1994).

In DXA, fan beam dual-energy X-ray beams are used to measure bone mass, with correction for overlying soft tissue. Data are converted to BMC and the area represented is measured. The BMD measurement is annotated in grams of mineral per square centimeter. BMC represents the amount of mineral in a volume of bone without consideration of total body