Only a few occurrences of acute zinc poisoning have been reported. The
toxicity
signs are nausea, vomiting, diarrhoea, fever, and lethargy and have
been
observed after ingestion of 4-8 g (60-120 mmol) zinc. Long-term zinc
intakes
higher than the requirements could, however, interact with the
metabolism of
other trace elements. Copper seems to be especially sensitive to high
zinc
doses. A zinc intake of 50 mg/day (760 mmol) affects copper status
indexes, such
as CuZn-superoxide dismutase in erythrocytes (37, 38). Low copper and
ceruloplasmin levels and anaemia have been observed after higher zinc
intakes
450-660 mg/day (6.9-10 mmol/day) (39, 40). Changes in serum lipid
pattern and in
immune response have also been observed in zinc supplementation studies
(41,
42). Because copper also has a central role in immune defence, these
observations call for caution before large-scale zinc supplementation
programmes
are undertaken. Any positive effects of zinc supplementation on growth
or
infectious diseases could be disguised or counterbalanced by negative
effects on
copper-related functions.
The upper level of zinc intake for an adult man is set at 45 mg/day
(690
mmol/day) and extrapolated to other groups in relation to basal
metabolic rate.
For children this extrapolation means an upper limit of intake of 23-28
mg/day
(350-430 mmol/day), which is close to what has been used in some of the
zinc
supplementation studies. Except for excessive intakes of some types of
seafood,
such intakes are unlikely to be attained with most diets. Adventitious
zinc in
water from contaminated wells and from galvanized cooking utensils
could also
lead to high zinc intakes.
