In adults, with their generally stable skeletal size, change in both BMD and BMC are useful outcome measures. The ability of DXA to measure specific skeletal regions such as the spine, hip, and forearm adds a potentially valuable refinement to the determination of the calcium requirement derived from balance studies (see “Calcium Retention” below). This technique has already revealed that the patterns and timing of acquisition of peak bone mass vary by skeletal site, and that bone loss from trabecular- and cortical-rich sites occurs at different rates in women at menopause (see Table 4-1). Available evidence from supplementation trials in postmenopausal women also suggests that the calcium intake needed for maximal bone preservation differs by skeletal site (see Table 4-1). This is addressed more specifically in the section on requirements for adults over age 50.
Cross-sectional studies that relate dietary calcium intake to BMC or BMD are of modest value in establishing the calcium requirement. These studies are of limited value because calcium intake is not accurately measured, calcium intake at one point in time may not reflect lifetime calcium intake, and bone mass at a single point in time is the result of the lifelong influence of many confounding variables that are not measured.
In contrast, randomized, placebo-controlled calcium intervention studies that measure change in BMC or BMD provide valuable evidence for the calcium requirement. A major strength of such longitudinal studies is that the increment in calcium intake (the intervention) is known. In addition, their generally large sample sizes and subject randomization greatly reduce confounding of the results by other factors that influence bone mass. Limitations are that they require large sample sizes, are very expensive, and usually test only one or two doses of calcium per study. Assessment of dietary calcium intake (in contrast to the intervention) is subject to the usual inaccuracies.
An important consideration in the interpretation of longitudinal calcium intervention studies is the phenomenon of the bone remodeling transient, the one-time initial gain in bone mass that occurs in the first 3 to 12 months after increasing calcium intake (or administration of an antiresorptive drug) (Frost, 1973). Calcium supplementation trials of 1 to 2 years duration may overestimate the longer-term influence of calcium because of the effect of calcium (and all remodeling suppressers) in reducing the size of the remodeling