MARGINAL ZINC-DEFICIENCY IN OLDER ADULTS - RESPONSIVENESS OF ZINC STATUS INDICATORS

Objective: Suspicions that mild zinc deficiency is common among the el derly cannot be confirmed or refuted because definitive indicators of zinc status are lacking. The goal of this study was to document the cl inical responsiveness of parameters of zinc status in a group of older adults consuming a carefully controlled diet; first moderately low in zinc (3.97 mg/day for 15 days) and then high in zinc (28.19 mg/ day f or 6 days). Methods: Fifteen older adults (mean age = 66.6 yrs) volunt eered to Consume a marginally zinc-deficient diet for 15 days followed by 6 days of zinc repletion. Plasma concentrations of erythrocyte met allothionein and the enzyme 5'-nucleotidase, as well as levels of zinc , alkaline phosphatase, copper and ceruloplasmin were measured before and after zinc depletion and repletion. Results: Plasma zinc levels we re not altered during the study. Alkaline phosphatase (AP) values did not change in the expected direction, although a small decrease in AP following zinc repletion was statistically significant. Erythrocyte me tallothionein results followed a pattern similar to that of alkaline p hosphatase, little change, but a small, statistically significant drop after zinc repletion. As expected, there were no diet-associated chan ges in plasma copper and ceruloplasmin levels. In contrast, plasma con centrations of the enzyme 5'-nucleotidase decreased (p < 0.01) from 2. 7 +/- 0.5 to 1.1 +/- 0.5 U during zinc depletion and increased (p < 0. 05) to 2.2 +/- 0.4 U after 6 days of repletion. Conclusions: Mild zinc deficiency is difficult to detect. In this study, traditional indicat ors such as plasma zinc and alkaline phosphatase did not change as wou ld be expected in response to alterations in zinc intake. Likewise, er ythrocyte metallothionein did not respond to altered zinc intakes as e xpected but this factor may reflect long-standing or more severe zinc depletion and thus requires additional study. Activity of the enzyme 5 '-nucleotidase appears responsive to acute changes in zinc intake; how ever, more work is needed to define how well these activities will ref lect zinc intake in other types of subjects.