The biology of zinc is linked extensively to hormone metabolism. Notable examples are the zinc finger motifs of regulatory proteins required for hormonal signals to regulate gene transcription (Cousins, 1994; Klug and Schwabe, 1995). Zinc has been reported to have roles in the synthesis, transport, and peripheral action of hormones. Low dietary zinc status has been associated with low circulating concentrations of several hormones including testosterone (Prasad et al., 1996), free T4 (Wada and King, 1986), and IGF-1 (Ninh et al., 1996). Zinc supplementation has been associated with an increase in both circulating IGF-1 concentration and growth velocity (Ninh et al., 1996). However, no studies have directly related hormone concentrations to decreases or increases in zinc intake.
Reductions in retinol binding protein, albumin, and pre-albumin concentrations have been reported with moderate dietary zinc restriction (Wada and King, 1986). Serum zinc and retinol binding protein concentrations are significantly correlated in zinc-deficient Thai children (Udomkesmalee et al., 1990). Changes in circulating concentrations of these proteins with changes in dietary zinc may serve as minor supportive indicators. The relationship of such indicators to general malnutrition or to dietary deficiency that is not related to zinc status supports their being minor indicators for zinc requirements.
Bioavailability of zinc can be affected by many factors at many sites. The intestine is the major organ in which variations in bioavailability affect dietary zinc requirements. These effects occur through two key regulatory processes: absorption of exogenous zinc and reabsorption of endogenous zinc. Dietary factors that affect bioavailability can have an impact on each of these processes (Cousins, 1989b; Lonnerdal, 1989).
Zinc absorption from foods and supplements has received extensive study. The environment within the gastrointestinal tract drastically influences zinc solubility and absorptive efficiency. The propensity of zinc to bind tenaciously to ligands provided by dietary