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precision is difficult to ascertain. However, if well conducted, they provide valuable information on calcium requirements relative to the typical intake of the population under study. Long-term balance studies for calcium are generally not carried out because of the difficult study protocol. Calcium balance can also be estimated by using stable isotopes to trace the amount of calcium absorbed, usually in infants from a single feeding (Abrams, 2006).

Calcium balance outcomes that are positive are indicative of calcium accretion and are sometimes referred to as net calcium retention; neutral balance suggests maintenance of bone, and negative balance indicates bone loss. The relevance of the calcium balance state varies depending upon developmental stage. Infancy through late adolescence are characterized by positive calcium balance. In female adolescents and adults, even within the normal menstrual cycle, there are measurable fluctuations in calcium balance owing to the effects of fluctuating sex steroid levels and other factors on the basal rates of bone formation and resorption. Later in life, menopause and age-related bone loss lead to a net loss as a result of calcium due to enhanced bone resorption.

In the 1997 IOM report that focused on calcium DRIs (IOM, 1997), metabolic studies of calcium balance were used to obtain data on the relationship between calcium intakes and retention, from which a non-linear regression model was developed; from this was derived an intake of calcium that would be adequate to attain a predetermined desirable calcium retention.4 The approach used in 1997 was a refinement of an earlier approach suggested to determine the point at which additional calcium does not significantly increase calcium retention, called the plateau intake (Spencer et al., 1984; Matkovic and Heaney, 1992).

The balance studies included in the 1997 IOM report (IOM, 1997) met

4

A footnote to the 1997 IOM report (IOM, 1997) explains the decision not to base considerations on maximal calcium retention: The 1997 committee intended to use a recently described statistical model (Jackman et al., 1997) to estimate an intake necessary to support maximal calcium retention and from which to derive an EAR, and did so in the prepublication of the report. In the original paper by Jackman et al. (1997), an estimate was made of the lowest level of calcium intake that was statistically indistinguishable from 100 percent maximal retention in some individuals. However, the Standing Committee on the Scientific Evaluation of Dietary Reference Intakes (DRI Committee) reviewed the approach in the pre-publication of the report and adopted a different interpretation of the data for the purpose of establishing an AI. The 1997 committee was subsequently advised that there were both statistical and biological concerns with the application of the percent maximal retention model (presented in Appendix E of the 1997 IOM report [IOM, 1997]). The final print of the 1997 report retained the statistical model described by Jackman et al. (1997), but applied it to determine, from the same calcium balance data as was used in the pre-publication report, an estimate of the calcium intake that is sufficient to achieve a defined, desirable level of calcium retention specific to the age groups considered.



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