Zinc [Zn, 30]
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Some of the reported symptoms of Zn deficiency in humans, especially in infants
and young children, include dwarfism (growth retardation), dermatitis (alopecia),
impaired neurology, decreased immune function, infections, and death. In industrialized countries, cases of mild Zn deficiency can be observed. The most common
symptoms include dry and rough skin, dull hair, brittle finger nails, white spots on
nails, reduced taste and smell, loss of appetite, mood swings, reduced adaptation to
darkness, frequent infections, delayed wound healing, dermatitis, and acne (Black
2008). Zn toxicity has been seen in both acute and chronic forms. In most cases, dermal exposure to Zn, or its compounds, does not result in any noticeable toxic effects.
Intakes of 150–450 mg of Zn per day have been associated with low Cu status,
altered Fe function, reduced immune function, and reduced levels of high-density
lipoprotein (Fraga 2005). A disproportionate intake of Zn in relation to Cu may
induce Cu deficiency, resulting in increased Cu requirements, increased Cu excretion, and impaired Cu status. Pharmacological intakes of Zn have been associated
with effects ranging from leukopenia and/or hypochromic microcytic anemia. These
conditions were reversible upon discontinuation of Zn therapy, together with Cu supplementation. Both Cu and Zn appear to bind to the same metallothionein protein;
however, Cu may displace Zn from these proteins.
Following longer term exposure to low Zn doses (about 0.5–2 mg Zn/kg/day),
the absorption of Cu from the diet decreased, leading to symptoms of its deficiency.
High-dose Zn administration has also resulted in reductions of leukocyte number
and function. Long-term consumption of excess Zn resulted in decreased Fe stores
(ATSDR 2005d).
Poisoning incidents, with symptoms of gastrointestinal distress, nausea, and diarrhea, may occur after a single or short-term exposure to concentrations of Zn in
water or beverages, at concentrations of 1000−2500 mg/L. Similar symptoms, occasionally leading to death, follow the inadvertent intravenous administration of large
Zn doses. Kidney dialysis patients, exposed to Zn through the use of water stored
in galvanized units, have developed symptoms of Zn toxicity, which are reversible
when the water is subjected to activated carbon filtration (WHO 2001d).
In humans, the absorption of Zn in the diet ranges widely. Bioavailability can
be affected by abnormalities in the gastrointestinal tract, in transport ligands or in
substances that interfere with Zn absorption. A decreased absorption was noted for
elderly persons. A significantly reduced absorption of Zn in humans and laboratory
animals was observed, after oral uptake of phytate (from grain and vegetable components), owing to the formation of insoluble Zn–phytate complexes in the upper
gastrointestinal tract. The absorption of Zn decreases with increasing gastric pH.
Other components that may reduce its availability are casein and its phosphopeptides
in dairy products; and Ca in the diet. Zinc is less bioavailable from whole grains
and legumes, due to inhibitory effects of phytic acid, fiber, and ligninon. It is relatively high bioavailable in meat, eggs, and seafood, and is present in all food groups
(Table 55.2).
In 1982, the Joint FAO/WHO Expert Committee on Food Additives proposed a
provisional maximum tolerable daily intake for Zn of 1 mg/kg bw. The daily requirement for adult man is 15–20 mg/day (WHO 2001d). The U.S. recommended dietary
allowance for Zn is as follows: for adult men, 11 mg/day, and for women, 8 mg/day.