normal subjects with somewhat similar age, body size, and gender characteristics were tested while consuming different dietary intakes. A key concern in such studies was the significant body store represented by the skeletal tissues for these nutrients and the fact that the balance method could easily fail to detect, due to systematic bias or error, small changes in mineral status. In addition, balance studies tend to err toward a positive balance since intake is usually overestimated and excretion is underestimated. With the advent of readily available noninvasive and fairly inexpensive methods to detect changes in bone mineral content and bone mineral density, additional information relative to small changes became available to augment information from balance studies.
In general, in order to account for possible errors introduced through the use of available balance data, criteria and methods for compiling balance data that could be used to develop DRIs for calcium and magnesium included the following:
Preferential use was made of studies on self-selected calcium intakes in order to avoid the bone remodeling transient as described in Chapter 4.
For every situation, data from more than one reported investigation of calcium balance were considered.
Only balance studies whose dietary periods were thought to be long enough to assure a reasonable degree of adaptation to the diet via urinary and fecal losses were used.
In some instances (as for children ages 9 through 13 years), comparable data from individual investigations were combined to create a larger sample size in order to facilitate use of a statistical model which describes the relationship between calcium intake and retention over a range of intakes. Using the equation derived from this model (see Appendix E), a prediction of calcium intake required to attain a desirable calcium retention could be obtained.
When investigators did not measure or estimate miscellaneous losses of calcium in balance studies, an adjustment for this was made in predicting the desirable calcium retention. When rate of expected growth or change in tissue mass was not accounted for in individual studies (particularly related to magnesium in children), adjustments for growth were made to the available balance data.