per cent of maximal retention, using the same data and statistical methodology as was included in the prepublication version (see Appendix E).

Where sufficient data were available, values from balance studies for individual subjects within specific age groups were applied to a nonlinear mathematical model recently used by Jackman et al. (1997) which describes the relationship between varying calcium intakes and retention. The equation derived from this model was then solved to determine the calcium intake required to achieve retention of the desirable amount of calcium. The desirable retention varied by age group but for the most part reflected accretion of calcium in bone based on bone mineral accretion data available for some of the age groups.

Another major approach considered by the DRI Committee to estimate intake needed to maintain calcium adequacy was the factorial method. This is based on combining estimates of losses of calcium via various routes by apparently healthy individuals and then assuming that these represent the degree to which calcium intake, as corrected by estimated absorption, will balance these losses. The weakness of using this approach alone is that the data come from different studies, in different subjects, and the variation in absorption, particularly depending on previous intake, may be significant. The third approach derives calcium requirements from the few available clinical trials in which additional calcium was given and changes in bone mineral content or density or in fracture rate were measured over time.

Comparison of the intakes needed to achieve desirable calcium retention or maintain minimal calcium loss using each of these three methods gave reasonable confidence and concordance to the levels of intake recommended as AIs.

The decision to set AIs rather than EARs for calcium was based on the following concerns: (1) uncertainties in the methods inherent in and the precise nutritional significance of values obtained from the balance studies that form the basis of the desirable retention model described in the previous paragraph, (2) the lack of concordance between observational and experimental data (mean calcium intakes in the United States and Canada are much lower than are the experimentally derived values required to achieve desirable calcium retention), and (3) the lack of longitudinal data that could be used to verify the association of the experimentally derived calcium intakes for achieving a pre-determining calcium retention with the rate and extent of long-term bone loss and its clinical sequelae, such as fracture. Taking all of these factors into consideration it



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