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 Committee to estimating 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 addditional 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. Thus the recommended AI for each life stage group is an approximation of the calcium intake that would appear to be sufficient to maintain calcium nutriture for almost all the individuals in the specific group. It is also recognized that the ability to maximize calcium retention may not be limited by calcium intake alone since there are many other factors that affect calcium retention, such as growth velocity (in children), hormonal status, gender and ethnic backgrounds, other diet components, and genetic patterns. Evidence to support this is cited in the study by Jackman et al. (1997), which demonstrated that the further into puberty the teenage girls were, the lower their relative calcium retention was even though calcium intake remained the same. In addition, calcium retention would be expected to oscillate above and below a mean value at the calcium intake levels tested, which often were intended to approximate or exceed the subjects' usual intakes. Additional consideration of the approach used is included in Chapter 4.