Zinc is an essential element, necessary for sustaining all life. It is estimated that 3000 of the hundreds of thousands of proteins in the human body contain zinc prosthetic groups. 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. Intriguingly, brain cells in the mammalian forebrain are one type of cell that secretes zinc, along with its other neuronal messenger substances. 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, whole grains, pumpkin seeds and sunflower seeds. 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, but the effect is extremely small and not likely to be clinically important. 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 includes hair loss, skin lesions, diarrhea, wasting of body tissues, and, eventually, death. 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.
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. <ref>Muyssen et al., (Aquat Toxicol. 2006)</ref> Swallowing an American one cent piece (98% zinc) can also cause damage to the stomach lining due to the high solubility of the zinc ion in the acidic stomach. <ref>Bothwell and Mair, PEDIATRICS 2003</ref> 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. <ref>Stowe CM, Nelson R, Werdin R, et al: Zinc phosphide poisoning in dogs. JAVMA 173:270, 1978</ref>
Zinc salts are effective against pathogens in direct application. Gastrointestinal infections are also 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.
The direct effect of zinc (as in lozenges) on bacteria and viruses is also well-established, and has been used since at least 2000 BC, from when zinc salts in palliative salves are documented. However, exactly how to deliver zinc salts against pathogens without injuring one's own tissues is still being investigated.