plasma (Miller et al., 1994; Sian et al., 1996). Once links to clinical, biochemical, or molecular effects of zinc deficiency have been achieved and appropriate cut-off levels for different age groups and gender have been defined, pool size and turnover measurements may be of value in future refinements of EARs. Even simpler models involving the measurement of plasma zinc clearance may be useful in assessing zinc deficiency, but dietary data derived by such a method are not available at this time (Kaji et al., 1998; Nakamura et al., 1993; Yokoi et al., 1994). More detailed model-based compartmental analyses, when specifically applied to the evaluation of dietary requirements, also have the potential to contribute to a more precise understanding of zinc requirements (Miller et al., 1998; Wastney et al., 1986).
While both plasma zinc concentration and serum zinc concentration are used as indicators of zinc status, plasma zinc concentration is preferable because of the lack of contamination of zinc from the erythrocyte. Homeostatic mechanisms are effective in maintaining plasma zinc concentrations for many weeks of even severe dietary zinc restriction (Johnson et al., 1993; Wada et al., 1985). A number of studies have reported no association between dietary zinc intake and plasma or serum zinc concentration (Artacho et al., 1997; Kant et al., 1989; Neggers et al., 1997; Thomas et al., 1988). Payette and Gray-Donald (1991) did observe a significant correlation between dietary zinc intake and serum zinc concentration in noninstitutionalized elderly; however, the correlation was positive for men and negative for women. Discernible relationships have been reported between plasma zinc concentration and habitual dietary zinc intake, even within the range typically occurring in North America. These relationships are of some utility in providing a supportive indicator of zinc requirements. For example, serum zinc concentrations of Canadian adolescent girls aged 14 to 19 years vary inversely with phytate:zinc molar ratios, and a greater percentage of lactoovo-vegetarians have serum zinc values below 70 μg/dL than do omnivores (Donovan and Gibson, 1995). Cut-off concentrations for lower limits have been established and depend on the time of day at which collections are made because of the substantial and cumulative effects of meals in lowering concentrations (King et al., 1994). The cut-off concentrations for prebreakfast samples is 70 μg/dL. Different cut-off concentrations are not considered necessary for different age groups or genders.