of genes (McMahon and Cousins, 1998). Zinc has been shown to influence both apoptosis and protein kinase C activity (McCabe et al., 1993; Telford and Fraker, 1995; Zalewski et al., 1994), which is within the regulatory function. The relationship of zinc to normal synaptic signaling processes also falls within the regulatory role (Cole et al., 1999). The most widely studied MTF-regulated gene is the metallothionein gene. An unequivocal function has not been established, but this metalloprotein appears to act as a zinc trafficking molecule for maintaining cellular zinc concentrations (Cousins, 1996) and perhaps as part of a cellular redox system for zinc donation to zinc finger proteins (Jacob et al., 1998; Roesijadi et al., 1998). Upregulation of metallothionein by specific cytokines and some hormones suggests a function that is critical to a stress response. Induction of metallothionein by changes in dietary zinc intake has received considerable attention in experiments with both animals and humans (reviewed in Chesters, 1997; Cousins, 1994). Erythrocyte metallothionein concentrations decreased rapidly in humans fed a phytate-containing diet of very low zinc content (Grider et al., 1990). Erythrocyte metallothionein concentration appears to be a measure of severe zinc depletion, and the extent of a change in concentration can distinguish between low and adequate levels of zinc intake under experimental conditions (Thomas et al., 1992). Erythrocyte metallothionein and monocyte metallothionein messenger RNA concentrations increase with elevated zinc intake levels such as those encountered with dietary supplements (Grider et al., 1990; Sullivan et al., 1998). Studies of metallothionein concentration in blood cells or plasma during large human dietary trials have not been undertaken. Consequently, the use of metallothionein as a static or functional indicator of zinc status needs further study.
While knowledge of the biochemical and molecular genetics of zinc function is well developed and expanding, neither the relationship of these genetics to zinc deficiency or toxicity nor the function(s) for which zinc is particularly critical have been established. For example, explanations for depressed growth, immune dysfunction, diarrhea, altered cognition, host defense properties, defects in carbohydrate utilization, reproductive teratogenesis, and numerous other clinical outcomes of mild and severe zinc deficiency (Hambidge, 1989; King and Keen, 1999) have not been conclusively established.
Zinc is widely distributed in foods. Because virtually none of it is present as the free ion, bioavailability is a function of the extent of