The primary effect appears to be a decreased absorption of zinc at an iron:zinc ratio of 3:1 when the iron was administered in water. However, when iron was administered during a meal, no such effect was found. Similarly, when iron was present as heme iron, no effect was noted. One study found a 56 percent decline in iron absorption when the zinc:iron ratio was 5:1 and was administered in water (Rossander-Hulten et al., 1991). However, when this ratio of zinc and iron was administered in a hamburger meal, no effect on iron absorption was noted.

Other Endpoints. No evidence was found of reproductive effects in humans from zinc intake. There is one case report of three premature deliveries and one stillborn infant after excess zinc intake during pregnancy (Kumar, 1976). Because details on other contributing factors were not provided, interpretation of these results is limited. There is insufficient evidence of carcinogenicity from human or animal studies.

Although there are no data indicating adverse interactions between zinc and other nutrients when zinc is found in food, adverse nutrient interactions are present after feeding zinc in the form of dietary supplements. The adverse effect of excess zinc on copper metabolism (i.e., reduced copper status) was chosen as the critical effect on which to base a UL for total daily intake of zinc from food, water, and supplements in humans. This selection is based on (1) the consistency of findings from studies measuring the interaction of zinc and copper (Fischer et al., 1984; Samman and Roberts, 1988; Yadrick et al., 1989), (2) the sensitivity of ESOD activity as a marker for this effect, and (3) the quality and completeness of the database for this endpoint. The data on the effects of zinc on HDL cholesterol concentration were not consistent from study to study and therefore were not used to derive a UL.

Data Selection. Data on reduced copper status in humans were used to derive a UL for zinc (Table 12-7). Studies measuring ESOD activity (which is a sensitive indicator of copper status) or other indicators