Assessment of zinc status.

Plasma zinc concentration has been denigrated as a measure of zinc status because it responds to metabolic conditions unrelated to zinc status and because it is insensitive to changes in dietary zinc. The insensitivity of plasma zinc to reductions in dietary zinc reflects the tremendous capacity of the organism to conserve tissue zinc by reductions in zinc excretion and/or reductions in the rate of growth. Changes in plasma zinc concentrations do not seem to occur until the capacity to reestablish homeostasis by reducing excretion and/or growth has been exceeded. Reductions in dietary zinc beyond the capacity to maintain homeostasis lead to utilization of zinc from a small, rapidly turning over pool. This pool is located, at least in part, in the bone, liver, and plasma. Loss of a small, critical amount of zinc from this pool leads to the rapid onset of both biochemical and clinical signs of zinc deficiency. Thus, plasma zinc is a valid, useful indicator of the size of this exchangeable pool of zinc. Plasma metallothionein concentrations may prove useful for identifying poor zinc status. Plasma metallothionine concentrations reflect hepatic concentrations and, therefore, are reduced when the dietary zinc supply is low. However, since hepatic metallothionein concentrations rise in response to stress, plasma metallothionein concentrations are increased in this state when the plasma zinc concentrations are also likely to be low. Thus, measurement of both plasma metallothionein and plasma zinc concentrations could differentiate a low plasma zinc due to dietary zinc deficiency from a low concentration due to stress, infection, or other metabolic conditions.