menstrual loss of 0.45 mg/day of iron. Therefore, the requirement is increased by approximately 2.5 mg/day of iron.
Factorial modeling was used to calculate the EAR and RDA for adult men and women (see “Selection of Indicators for Estimating the Requirement for Iron—Factorial Modeling”). Requirements for maintaining iron requirements were derived by estimating losses. No provision is made for growth beyond age 19 years, and therefore there is no allowance for deposition of tissue iron.
Men. Basal iron loss was the only component used to estimate total needs for absorbed iron. Basal losses are based on the study by Green and coworkers (1968). Basal iron losses are taken as related to body weight (14 μg/kg/day), and for adult men, the requirement for absorbed iron is equivalent to the basal losses:
Basal losses (mg/day) = Weight (kg) × 0.014 mg/kg/day. (1)
There are insufficient data for estimating variability of basal losses in adult men. Therefore, the median and variability for basal losses were calculated by using the median and variability values for body weight reported in NHANES III. Because variability in body weight is needed for calculating the distribution of basal losses, the reference weights in Table 1-1 were not used. Recorded weights reasonably yield a normal distribution based on the square root of the median weight for men:
Weight 77.4 (kg)0.5 = 8.8 ± 0.84 kg. (2)
The distribution of basal losses, and therefore requirements in men, was obtained by combining equations (1) and (2). The estimated median daily iron loss in men living in the United States—and therefore the median requirement for absorbed iron—is 1.08 mg/day (77.4 kg × 0.014 mg/kg/day). The ninety-seven and one-half percentile of absorbed iron requirements is 1.53 mg/day.
The upper limit of dietary iron absorption was estimated to be 18 percent (see “Factors Affecting the Iron Requirement—Algorithms for Estimating Dietary Iron Bioavailability”). Using this value, the EAR is 6 mg/day (1.08 mg/day ÷ 0.18).