The variance in requirements cannot be determined from the available data. Thus, a CV of 10 percent (1 SD) is assumed, resulting in an RDA of 500 mg (16.1 mmol)/day.

During the rapid growth period of adolescence, the most logical basis for estimating the phosphorus requirement would be from observation of the balance plateau, that is, the intake level above which no further phosphorus retention occurs, just as was done for the calcium requirement (see Chapter 4). Unfortunately, only a few phosphorus balance studies have been conducted in this age group, and insufficient data exist across a range of intakes to determine maximal retention. As an alternative, the same approach using tissue accretion used for the 1 through 3 and 4 through 8 years age groups was employed.

Accretion. Phosphorus intakes necessary to meet the needs for the addition of bone and soft tissue during this period of rapid growth can be calculated and adjusted for by urinary output and absorptive efficiency. The main limitation of this approach for this age category is that tissue accretion values are not available for adolescents beyond 14 years; thus, predicted needs for older adolescents may not be optimal to support any growth spurts beyond this age.

To estimate phosphorus requirement from tissue accretion, longitudinal data and a large database of cross-sectional data are available and represent very recent databases. Slemenda and colleagues (1994) conducted a 3-year longitudinal study in 90 white children aged 6 to 14 years at baseline. They showed increases in weight during this time of 10.9 ± 4.3 kg for the 44 prepubertal children, 19.6 ± 0.9 kg for the 38 peripubertal children, and 6.98 ± 4.54 kg for the 8 postpubertal children. The growth rate is higher and later in boys than in girls. Weight gain during this period amounts to approximately 50 percent of the ideal adult weight. However, there is enormous variability in the timing and extent of this growth acceleration.

To estimate tissue accretion of phosphorus, knowledge of both