(32.3 mmol)/day, predicted urinary excretion would be 565 mg (18.2 mmol)/day. When urinary phosphorus is added to the mean accrued phosphorus for both boys and girls of 175 mg (5.6 mmol) (Table 5-5), there is a daily need for dietary phosphorus of 740 mg (23.9 mmol)/day. Absorption efficiency in the few balance studies performed in this age group (Greger et al., 1978; Lutwak et al., 1964) averaged 60 to 80 percent. This is consistent with the range of absorption efficiencies (60 to 65 percent) found in adults (Lemann, 1996). Using a midpoint value of absorption efficiency of 70 percent, ingested phosphorus to cover tissue accretion and urinary loss would need to be 1,055 mg (34 mmol)/day for both boys and girls. Although these phosphorus intakes cover tissue accumulation needs of the observed average adolescent, these intakes may not be optimal at the peak of the adolescent growth spurt.
The small amount of balance data available supports this estimate of phosphorus needs for children up to 14.5 years of age. Greger et al. (1978) studied 14 girls aged 12.5 to 14.5 years. Their balances averaged +82 ± 124 mg (+2.6 ± 4 mmol)/day on phosphorus intakes of 820 ± 10 mg (26.5 ± 0.3 mmol)/day. On phosphorus intakes of 925 mg (29.8 mmol)/day, 8 of 11 girls aged 12.5 to 14.2 years were in positive phosphorus balance with zinc intakes of 11.3 mg/day (Greger et al., 1979). In other studies, five adolescents aged 9 to 13 years and 18 adolescents aged 8 to 11 years were in positive phosphorus balance at intakes exceeding 1 g (32.3 mmol)/day (Lutwak et al., 1964; Sherman and Hawley, 1922). Of these 23 individuals, 4 boys and 13 girls aged 8 to 12 years were in positive phosphorus balance at daily phosphorus intakes above 34.5 mg (1.1 mmol)/kg body weight or 693 mg (22.4 mmol).
Serum Pi. An attempt was also made to estimate the EAR during adolescence by using the approach employed for adults, for example, using ECF Pi as the functional indicator, and estimating the intake needed to sustain ECF Pi at the lower limit of normal for the age concerned (see below for corresponding approach in adults). As Table 5-1 shows, the interpolated 2.5 percentile for Pi at age 9 is approximately 1.25 mmol/liter (3.9 mg/dl), falling to approximately 1.05 mmol/liter (3.3 mg/dl) by age 18. The curve in Figure 5-1 relates to adults and hence cannot be used directly. However, the lack of effective regulation of ECF Pi and the fact that it rises and falls with intake means that the rise in Pi for any given increment will be a function of the volume of the ECF into which the absorbed phosphorus is inserted, or in other words, a function of body size. Thus, a straightforward adjustment for body weight may permit a