Extrapolation from Adults. As determined by the extrapolation method described in Chapter 2, the average requirement for boys and girls is 6.7 and 5.6 mg/day of zinc, respectively.

The RDA for zinc is set by using a CV of 10 percent (see Chapter 1) because information is not available on the standard deviation of the requirement. The RDA is defined as equal to the EAR plus twice the CV to cover the needs of 97 to 98 percent of the individuals in the group (therefore, the zinc RDA is 120 percent of the EAR). The calculated RDA is rounded to the nearest 1 mg.

Factorial Analysis. Endogenous losses are calculated as for younger age groups by using the reference weights (see Chapter 2) with the addition of 100 μg/day of zinc to allow for calculated average semen or menstrual losses (see “Adults Ages 19 Years and Older”, which follows). For this age group, a fractional absorption of 0.4 is used; it corresponds to the average “critical” value for adult men from the data sets used in estimating adult requirements (see below). Gender differences are sufficient at this age for boys and girls requirements to be calculated separately. As determined by the summation of average zinc losses and the zinc requirement for growth (Kuczmarski et al., 2000; Widdowson and Dickerson, 1964), the amount of absorbed zinc that is required for boys and girls is approximately 3.4 and 3.0 mg/day, respectively (Table 12-6). On the basis of a fractional zinc absorption of 0.4 that was derived for men