space, the portion of total bone mass that is currently being remodeled. The remodeling rate in trabecular bone is four to five times greater than that in cortical bone. As a result, the remodeling transient is more apparent at the more highly trabecular spine than it is at the more cortical radius or the even more dominantly cortical total skeleton. Because of the bone remodeling transient, the long-term or cumulative effect of calcium can best be evaluated by examining the effect of added calcium in the second and later years of an intervention (Table 4-1).
In estimating the intake requirement for calcium, it is important to recognize that calcium is unique in several respects. First, 99 percent of body calcium is located in the skeleton which has an essential structural function. To maximize skeletal size and strength, one must have adequate calcium retention to provide the substrate (along with other minerals) for bone mineral expansion during growth and maintenance after peak bone mass has been achieved. To a great extent, the retention of calcium in bone is under strong homeostatic control, which is regulated by genetics, calciotropic hormones and weight bearing exercise. The target intake of dietary calcium to achieve the desirable and optimal calcium accretion in bone is difficult to estimate because of all of the other factors which play a role in bone mineral homeostasis.
In this report, classic 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; and from this was derived an intake of calcium which would be adequate to attain a predetermined desirable calcium retention. This approach is a further refinement of an earlier approach suggested to determine the point at which additional calcium does not significantly increase calcium retention, called the plateau intake (Matkovic and Heaney, 1992; Spencer et al., 1984).
The predetermined desirable calcium retentions for adolescents and young adults were based on estimates of the calcium accretion in bone over either four years of adolescence or during the 3rd decade of life, to which were added estimates for sweat and other losses if not included in the experiment. For older adults, a value approximating zero balance was used for desirable retention assuming that no net positive accretion of bone at this age in replete individuals serves a functional advantage. Adults continue to lose bone despite high intakes of calcium for other reasons such as lack