approach are discussed under “Findings by Life Stage and Gender Group—Adults Ages 19 Years and Older”. A sufficient number of metabolic studies of zinc homeostasis have been reported to permit an estimation of dietary zinc requirements in adults.
The first step in this approach is to calculate nonintestinal losses of endogenous zinc, that is, losses via the kidney and integument with smaller quantities in semen and menstrual losses. Although urinary zinc excretion decreases markedly with severe dietary zinc restriction (Baer and King, 1984), extensive data indicate that excretion by this route is unrelated to dietary zinc intake over a wide range (4 to 25 mg/day) that is certain to encompass the dietary zinc requirements for adults. Data regarding this lack of relation between intake and integumental and semen losses of zinc are more limited. Therefore, nonintestinal losses of endogenous zinc have been treated as a constant in response to varied zinc intake.
In contrast to excretion of zinc via other routes, excretion of endogenous zinc via the intestine is a major variable in the maintenance of zinc homeostasis and is strongly correlated with absorbed zinc. The second step in estimating dietary zinc requirements is to define this relationship (Figure 12-1). After it has been defined and adjusted by the constant for other endogenous losses, one can calculate the minimum quantity of absorbed zinc necessary to offset endogenous zinc losses (Figure 12-1).
The dietary zinc intake corresponding to this average minimum quantity of absorbed zinc is the EAR. This value has been determined from the plot of the asymptotic regression analysis of absorbed zinc versus ingested zinc (Figure 12-2).
Theoretically, given the results described in detail for adults below, balance could also be used as an indicator. However, review of all published data on zinc balance (and net [apparent] absorption) studies in young adult men (excluding those studies that have included tracer data and are being utilized for the current factorial calculations) collectively revealed no correlation with dietary zinc. Presumably this lack of correlation reflects the vagaries of balance studies. The factorial calculations for adults are based on tracer/ metabolic studies in which participants were fed diets from which the bioavailability of zinc was likely to be representative of typical diets in North America or, in some instances, possibly greater than average.