(Zn) is another essential element in humans, vital for growth and
development, sexual maturity and reproduction, dark vision adaptation,
sense of smell and taste, insulin storage and release, and for a variety
of immune defenses. Even borderline deficiency or disturbances in Zinc
metabolism can have profound adverse health effects. Zinc deficiency has
been shown to impair immunity in many ways. Zinc deficiency has also
been shown to impair placental transport of antibodies from mother to
fetus. There is even preliminary evidence that Zinc can produce benefit
in some with rheumatoid arthritis. Diminished Zinc blood levels have
been reported in some with this disease. Zinc has demonstrated an
ability to inhibit lymphocyte reaction in some of these subjects.
Overall, more is being learned about this superstar in human nutrition
every day, and research efforts are continuing.
Zinc has structural, catalytic (enzymatic) and regulatory roles. About 1% of the human genome codes for zinc finger proteins, where zinc provides a structural role for regulatory functions. Over 60 enzymes require zinc for activity, including the RNA polymerases. Zinc is actively taken up by synaptic vesicles, supporting a role in neuronal activity and memory. Zinc metabolism is altered during disease and physical stress through hormones, cytokines and toxins, presumably as part of a host defense response.
IMPORTANCE: Is an antioxidant nutrient; necessary for protein synthesis; wound healing; vital for the development of the reproductive organs, prostate functions and male hormone activity; it governs the contractility of muscles; important for blood stability; maintains the body's alkaline balance; helps in normal tissue function; aids in the digestion and metabolism of phosphorus.
Deficiencies: An early sign of zinc deficiency in animals is decreased food intake. It is a type II deficiency since a reduction in growth occurs without an apparent reduction in tissue zinc. Reduced immune function, involving B cell and T cell depletion and/or reduced activity, and skin lesions associated with secondary infections are common findings. Chronic zinc deficiency in humans results in reduced growth (dwarfism) and sexual development which are reversible by raising zinc intake. Signs of zinc deficiency may reflect its involvement in cell proliferation and differentiation. Growth, behavioral abnormalities and cognition may respond to zinc supplementation in some populations. Many clinical findings that relate to depressed growth or immunity may have marginal zinc deficiency as a secondary cause. May result in delayed sexual maturity, prolonged healing wounds, white spots on finger nails, retarded growth, stretch marks, fatigue, decreased alertness, susceptibility to infections.
Clinical uses: Zinc is not widely used as a therapeutic agent except as an ingredient of topical medication. Oral zinc may be used to treat idiopathic skin lesions, some inflammatory conditions and depressed immunity. Zinc is usually indicated in rehabilitation therapy from malnutrition and/or malabsorption in children and adults, used in feeding programs for premature infants and neonates and is also a component of TPN solutions. Supplemental zinc reduces acute diarrhea and depressed immunity.
Diet recommendations: The Recommended Dietary Allowances (RDAs) are: infants, 5 mg/day; children <10 years, 10 mg/day; males >10 years, 15 mg/day; females >10 years, 12 mg/day; pregnancy, 15 mg/day; and lactation, 0-6 mo., 19 mg/day; 7-12 mo., 16 mg/day.
Food sources: Zinc is highly abundant in red and white meat and shellfish. Foods of plant origin except the embryo portion of grains, e.g., wheat germ, are low in zinc. Phytic acid in plants like soybeans binds zinc, forming an insoluble complex that lowers bioavailability. Other inhibitors of absorption are fiber, polyphenols and a high intake of calcium. Zinc from human milk is more absorbable than that from infant formulas or cow's milk.
Toxicity: Acute zinc toxicity is characterized by gastric distress, dizziness and nausea. Symptoms of chronic toxicity include gastric problems, decreased serum ceruloplasmin activity and hypocupremia, decreased lymphocyte stimulation to PHA and reduced HDL cholesterol. An emetic effect occurs at >150 mg Zn/day. Consumption of zinc supplements produces measurable cellular effects but the long term benefit/risk of zinc supplementation has yet to be determined.
Recent research: Experiments with transgenic and knock out mice are defining the role for zinc metalloproteins in metabolism, development and cytoprotection. Zinc as a component of an antioxidant system is being evaluated. Zinc as a factor in Ab amyloid protein aggregation leading to plaque formation found in Alzheimer's patients is under investigation. Supplemental zinc has been proven to be of benefit in treatment of acute diarrhea in infants and children. Fluorescent zinc indicators are in use to define zinc functions at the cellular level.