Zinc (Zn) is an
essential trace mineral. The human body has between 1.5?2.5 g Zn,
making it nearly as abundant as
iron. It is highly concentrated in specialized areas of the brain,
pancreas and adrenal gland, but is present in all cells, particularly in
the nucleus. 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. |