Total Requirement for Absorbed Iron. Because all components (basal iron loss, hemoglobin mass, and nonstorage iron) are not normally distributed (skewed), these components as shown in Table 9-10 can not be summed to accurately determine an EAR and RDA. After summing the components for each individual in the simulated population, the estimated percentiles of distribution were tabulated and are shown in Appendix Tables I-3 and I-4. The modeled distribution of iron requirements are used to set the EAR (fiftieth percentile) and RDA (ninety-seven and one-half percentile) with the assumption of an upper limit of 18 percent for iron absorption.
Dietary Iron Bioavailability. The upper limit of dietary iron absorption was estimated to be 18 percent and used to set the EAR based on the fiftieth percentile of absorbed iron requirements (see “Factors Affecting the Iron Requirement—Algorithms for Estimating Dietary Iron Bioavailability”).
Estimation of the Variability of Requirements. While Table 9-10 shows an estimate of median requirement, it is a simple summation and does not reflect the distributions. The distribution of requirements must be modeled using Monte Carlo simulation before the EAR and RDA can be estimated. This necessitates estimation of variability for components of requirements.
Basal or obligatory losses were derived from Green and coworkers (1968) with the assumption of proportionality to body surface area. To derive an estimate of variability of surface area, basal losses were computed with use of heights and weights reported in the USDA CSFII 1994–1996. Various transformations were then tested; a square root transformation approximated normality. The relative variability of surface area in this proxy data set was taken as an estimate of variability of basal iron loss. The observed CVs of proxy basal loss were 22.7 and 8.7 percent for boys aged 11 and 16, respectively, and 19.1 and 13.2 percent for girls aged 11 and 16, respectively. These CVs were applied to the square root of median iron loss, estimated on the basis of weight at ages 11 and 16 years (median loss shown in Table 9-10).
Estimating iron associated with change in hemoglobin mass requires consideration of rate of increase in blood volume and in hemoglobin concentration. Blood volume estimates were based on