Zinc is a moderately reactive, blue gray metal that tarnishes in moist air and burns in air with a bright bluish-green flame, giving off plumes of zinc oxide. It reacts with acids, alkalis and other non-metals. If not completely pure, zinc reacts with dilute acids to release hydrogen. The one common oxidation state of zinc is +2. From 100 °C to 210 °C (212 °F to 410 °F) zinc metal is malleable and can easily be beaten into various shapes. Above 210 °C (410 °F), the metal becomes brittle and will be pulverized by beating. Zinc is nonmagnetic.
Zinc is an essential element, necessary for sustaining all life. It is estimated that 3,000 of the hundreds of thousands of proteins in the human body contain zinc prosthetic groups, one type of which is the so-called zinc finger. In addition, there are over a dozen types of cells in the human body that secrete zinc ions, and the roles of these secreted zinc signals in medicine and health are now being actively studied. Zinc ions are now considered neurotransmitters. Cells in the salivary gland, prostate, immune system and intestine are other types that secrete zinc.
Zinc is an activator of certain enzymes, such as carbonic anhydrase. Carbonic anhydrase is important in the transport of carbon dioxide in vertebrate blood. It is also required in plants for leaf formation, the synthesis of indole acetic acid (auxin) and anaerobic respiration (alcoholic fermentation).
Zinc is found in oysters, and to a far lesser degree in most animal proteins, beans, nuts, almonds, whole grains, pumpkin seeds and sunflower seeds. A turkey's neck and beef's chuck or shank also contain good amounts of zinc. Phytates, which are found in whole grain breads, cereals, legumes and other products, have been known to decrease zinc absorption. Clinical studies have found that zinc, combined with antioxidants, may delay progression of age-related macular degeneration. Significant dietary intake of zinc has also recently been shown to impede the onset of flu. Soil conservation analyzes the vegetative uptake of naturally occurring zinc in many soil types.
The (US) recommended dietary allowance of zinc from puberty on is 11mg for males and 8mg for females, with higher amounts recommended during pregnancy and lactation.
Zinc deficiency results from inadequate intake of zinc, or inadequate absorption of zinc into the body. Signs of zinc deficiency include hair loss, skin lesions, diarrhea, and wasting of body tissues. Eyesight, taste, smell and memory are also connected with zinc. A deficiency in zinc can cause malfunctions of these organs and functions. Congenital abnormalities causing zinc deficiency may lead to a disease called Acrodermatitis enteropathica.
Obtaining a sufficient zinc intake during pregnancy and in young children is a very real problem, especially among those who cannot afford a good and varied diet. Brain development is stunted by zinc insufficiency in utero and in youth.
Zinc deficiency as a cause of anorexia nervosa
Zinc deficiency causes a decrease in appetite -- which could degenerate in anorexia nervosa (AN). Appetite disorders, in turn, cause malnutrition and, notably, inadequate zinc intake. The use of zinc in the treatment of anorexia nervosa has been advocated since 1979 by Bakan. At least 15 trials showed that zinc improved weight gain in anorexia. A 1994 randomized, double-blind, placebo-controlled trial showed that zinc (14 mg per day) doubled the rate of body mass increase in the treatment of anorexia nervosa (AN). Deficiency of other nutrients such as tyrosine and tryptophan (precursors of the monoamine neurotransmitters norepinephrine and serotonin, respectively), as well as vitamin B1 (thiamine) could contribute to this phenomenon of malnutrition-induced malnutrition.
Even though zinc is an essential requirement for a healthy body, too much zinc can be harmful. Excessive absorption of zinc can also suppress copper and iron absorption. The free zinc ion in solution is highly toxic to plants, invertebrates, and even vertebrate fish. The Free Ion Activity Model (FIAM) is well-established in the literature, and shows that just micromolar amounts of the free ion kills some organisms. A recent example showed 6 micromolar killing 93% of all daphnia in water. Swallowing a post 1982 American one cent piece (97.5% zinc) can also cause damage to the stomach lining due to the high solubility of the zinc ion in the acidic stomach. Zinc toxicity, mostly in the form of the ingestion of US pennies minted after 1982, is commonly fatal in dogs where it causes a severe hemolytic anemia. In pet parrots zinc is highly toxic and poisoning can often be fatal.
There is evidence of induced copper deficiency at low intakes of 100–300 mg Zn/d. The USDA RDA is 15 mg Zn/d. Even lower levels, closer to the RDA, may interfere with the utilization of copper and iron or to adversely affect cholesterol..
- See also: Zinc gluconate
Zinc salts are effective against pathogens in direct application. Gastroenteritis is strongly attenuated by ingestion of zinc, and this effect could be due to direct antimicrobial action of the zinc ions in the GI tract, or to absorption of the zinc and re-release from immune cells (all granulocytes secrete zinc), or both.
In clinical trials, both zinc gluconate and zinc gluconate glycine (the formulation used in lozenges) have been shown to shorten the duration of symptoms of the common cold. The amount of glycine can vary from two to twenty moles per mole of zinc gluconate.
Zinc oxide is perhaps the best known and most widely used zinc compound, as it makes a good base for white pigments in paint. It also finds industrial use in the rubber industry, and is sold as opaque sunscreen. A variety of other zinc compounds find use industrially, such as zinc chloride (in deodorants), zinc pyrithione (anti-dandruff shampoos), zinc sulfide (in luminescent paints), and zinc methyl or zinc diethyl in the organic laboratory. Roughly one quarter of all zinc output is consumed in the form of zinc compounds.
Metallic zinc is not considered to be toxic, but free zinc ions in solution (like copper or iron ions) are highly toxic. There is also a condition called zinc shakes or zinc chills (see metal fume fever) that can be induced by the inhalation of freshly formed zinc oxide formed during the welding of galvanized materials. Excessive intake of zinc can promote deficiency in other dietary minerals.
- "Zinc content of selected foods per common measure" (pdf). USDA National Nutrient Database for Standard Reference, Release 20. USDA. Retrieved 2007-12-06.
- "Age-Related Eye Disease Study Research Group". www.pubmed.gov. Retrieved 2007-11-13.
- "Neurobiology of Zinc-Influenced Eating Behavior". Retrieved 2007-07-19.
- Muyssen et al., (Aquat Toxicol. 2006)
- Bothwell and Mair, PEDIATRICS 2003
- Stowe CM, Nelson R, Werdin R, et al: Zinc phosphide poisoning in dogs. JAVMA 173:270, 1978
- See, for example, this list of common parrot illnesses and their causes.
- Zinc toxicity by GJ Fosmire, American Journal of Clinical Nutrition.
- Aydemir, T, B.; Blanchard, R.K.; Cousins, R.J (2006). "Zinc Supplementation of Young Men Alters Metallothionein, Zinc Transporter, and Cytokine Gene Expression in Leucocyte Populations". PNS. 103 (3): 1699–1704.
- Valko, M; Morris, H.; Cronin, MTD (2005). "Metals, Toxicity and Oxidative stress". Current Medicinal Chemistry (12): 1161–1208.
- Godfrey JC, Godfrey NJ, Novick SG. (1996). "Zinc for treating the common cold: Review of all clinical trials since 1984". PMID 8942045.
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