Insufficient and inconsistent data exist for plasma or serum zinc concentrations in apparently normal subjects whose habitual dietary zinc intakes straddle the vicinity of the average requirement, and therefore use of those concentrations for estimating an average requirement is limited. Furthermore, plasma and serum zinc concentrations do not seem to be sufficiently sensitive to serve as a subsidiary indicator.

Erythrocyte zinc concentration is depressed at moderately severe levels of dietary zinc restriction (Thomas et al., 1992), but the sensitivity of this assay is inadequate to provide more than a secondary supportive indicator of dietary zinc requirements. Sample preparation may account for some of the lack of sensitivity. Results from experimental depletion studies (Baer and King, 1984; Bales et al., 1994; Grider et al., 1990; Ruz et al., 1992; Thomas et al., 1992) have been mixed, and the value of erythrocyte zinc concentrations as an indicator of zinc nutritional status is not well defined.

Associations between low zinc concentration in hair and poor growth have been documented (Ferguson et al., 1993; Gibson et al., 1989; Hambidge et al., 1972; Walravens et al., 1983). In three of these studies, low zinc concentration in hair was used as a criterion for zinc supplementation in children and resulted in increased growth velocity. Low zinc concentrations in hair have been reported in Canadian children with low meat consumption (Smit-Vanderkooy and Gibson, 1987). Subjects whose habitual diets are high in phytate or who have very high phytate:zinc molar ratios have also been noted to have relatively low zinc concentrations in hair. However, there is a lack of uniformity in apparently low zinc concentrations in hair, and no lower cut-off values have been defined clearly for any age group or either gender. The use of zinc in hair as a supportive indicator for establishing zinc requirements needs further research.

With the large number of zinc-dependent enzymes that have been identified, it is perhaps remarkable that no single zinc-dependent enzyme has found broad acceptance as an indicator of zinc status or