body size, and estimated median growth velocity is shown in Table 9-9. The algorithm for estimating iron need was presented earlier. For the purpose of modeling, blood volume as a proportion of body weight and rate of hemoglobin change as a function of age were taken as constants. The variability of iron need was attributed to variation in weight and weight velocity.

Based on reported percentiles of body weight in the Third National Health and Nutrition Examination Survey (NHANES III), normal distributions were fitted at 11 and 16 years of age for boys and girls. The fit was approximate only but acceptable for the present purpose. The resultant body weight distributions (kg) were 42.96 ± 12.47 and 70.30 ± 12.70 for boys aged 11 and 16, respectively, and 44.96 ± 9.96 and 61.36 ± 12.88 for girls aged 11 and 16, respectively.

The average weights differ from the median weights shown in Table 9-10. Estimates of weight velocity at ages 11 and 16 years were based on the analyses of longitudinal data reported by Tanner and coworkers (1966) (Table 9-9). Approximation of a normal distribution was assumed. The resultant distributions of weight velocities (kg/year) were used for modeling: 4.87 ± 1.65 and 2.75 ± 2.27 for boys aged 11 and 16, respectively, and 4.77 ± 2.06 and 1.63 ± 1.63 for girls aged 11 and 16, respectively. The variability of tissue iron deposition was based on the variability of body weight.

Values for iron hemoglobin concentration and altered hemoglobin concentration were estimated for these ages from the equations of Beaton and coworkers (1989), and variability in hemoglobin concentration was ignored. Variability arising from menstrual loss was estimated from the fitted regression of blood loss (ln blood loss = 3.3183 ± 0.6662 [SD]).

The EAR has been set by modeling the components of iron requirements, estimating the requirement for absorbed iron at the fiftieth percentile, and with use of an upper limit of 18 percent iron absorption and rounding (see Appendix Tables I-3 and I-4). For the EAR and RDA for girls, it is assumed that girls younger than 14 years do not menstruate and that all girls 14 years and older do menstruate.