The prevalence of marginal zinc deficiency in human populations is unknown because of the lack of a good means of assessing zinc status. Measurement of plasma zinc is straightforward, but it does not serve as a reliable indicator of zinc status. Plasma zinc is a quantitatively minor pool that can be easily influenced by minor shifts in tissue zinc. Plasma concentrations do not fall with decreasing dietary intake, except at very low intakes. Plasma zinc can also be affected by factors unrelated to zinc status (e.g., time of day, stress, and infection). Cellular components of blood can be assayed, but erythro-cyte concentrations of zinc are maintained in deficient states and variable results have been found with leucocytes. Hair zinc concentrations may reflect available zinc but will also depend on the rate of hair growth.
Several different zinc-dependent enzymes have been investigated as potential markers of zinc status, but none have proved reliable. MT in blood cells has been suggested as a useful indicator of zinc status, assayed at either the protein or the mRNA level. MT expression is likely to be regulated by factors other than zinc and therefore may lack the specificity required of a good indicator. The gene-array approaches that have recently been used to determine the global effects of zinc deficiency within a tissue would appear to offer hope for the identification of an appropriate functional marker of zinc status.
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