constituents is accentuated at the near neutral pH in the intestinal lumen. The exact nature of the form in which zinc is needed for uptake has not been established. Some transporters responsible for transcellular zinc movement may require the free ion, but cotransport with small peptides and nucleotides has not been ruled out. Absorption of zinc, when consumed as a chelate, has not been investigated extensively. The option for zinc to be absorbed by the paracellular route adds to the lack of a unified form or path of zinc absorption from foods. Furthermore, the methods used to assess zinc absorption have varied widely, including balance studies, intestinal perfusion, responses of plasma zinc to single meals or aqueous doses, and tracer studies with intrinsically or extrinsically stable or radioactive zinc isotopes (Sandstrom and Lonnerdal, 1989).
Daily intake of iron at levels such as those found in some supplements could decrease zinc absorption (O’Brien et al., 2000; Solomons and Jacob, 1981; Valberg et al., 1984). This relationship is of some concern in management of iron supplementation during pregnancy and lactation (Fung et al., 1997). Recent studies of the mechanism of nonheme iron absorption suggest that upregulation of an iron transport protein occurs in iron deficiency (Gunshin et al., 1997). The comparable affinity of this transporter for zinc suggests that, during low iron intake, zinc absorption may be stimulated and suggests one possible locus for a zinc-iron interaction. The influence of heme iron on zinc absorption has not received much attention. The activity of other divalent metal transporters may also affect zinc absorption.
The importance of calcium in the diet and the mass of the element that must be consumed daily to maintain maximum bone density suggest that special attention should be given to its potential inhibitory effect on zinc absorption. Nutrition experiments with swine have shown conclusively that excess dietary calcium produces a decrease in zinc absorption, which leads to a skin condition called parakeratosis. Experiments in humans have been equivocal, with calcium phosphate (1,360 mg/day of calcium) decreasing zinc absorption (Wood and Zheng, 1997) and calcium as the citrate-malate