Zinc deficiency is a common problem with an estimated 1/3rd of world population suffering from zinc deficiency and is highly prevalent in Southeast Asia, sub-Saharan Africa, and other developing countries. It is excreted mainly through feces and in small amounts in urine and sweat. Zinc is mainly absorbed from proximal jejunum and distal duodenum and is perhaps facilitated by the presence of low molecular weight zinc binding ligands. Its absorption is hampered by the presence of phytates, calcium, and phosphates while chelating agents like EDTA and animal proteins increase its absorption from gut. Although cereals and legumes contain moderate amount of zinc, only 20–40% of the ingested metal is absorbed. Animal foods like meat, eggs, fish, and oysters are rich in zinc. The recommended daily allowance of zinc for an average adult male is 11 mg and the requirement increases from 8 mg/d to up to 12 mg/d in females during pregnancy and lactation.
As movement of zinc across various tissues is limited and there is no storage depot, the continuous external supply of zinc is important for metabolic needs, growth, and tissue repair.
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While about half of the total body zinc is in the bones, the skin contains nearly 6% of total body zinc.
Briefly, an average adult weighing 70 kg has a body zinc content of 1.4–2.3 gm, the highest tissue concentration (>500 μg/g dry weight) being in the prostate, seminal fluid, uveal tissue, and skin. It will be prudent to revisit the physiological aspects of zinc metabolism before discussing zinc deficiency states. Zinc Physiology and Zinc Deficiency States It has also been demonstrated to possess antiandrogenic properties as it causes modulation of 5 α-reductase type 1 and 2 activity. Zinc also possesses antioxidant property and has been found useful in preventing UV-induced damage and reducing the incidence of malignancies. It is also proposed that it is toll-like receptors mediated regulation of zinc homeostasis which influences dendritic cell function and immune processes. It inhibits the expression of integrins by keratinocytes and modulates the production of TNF- α and IL-6 and reduces the production of inflammatory mediators like nitric oxide. It activates natural killer cells and phagocytic function of granulocytes and stabilizes the plasma subcellular membranes especially the lysosomes. It maintains macrophage and neutrophil functions, natural killer cell activity, and complement activity. Zinc also plays an important role in maintaining the proper reproductive function, immune status, and wound repair via regulation of DNA and RNA polymerases, thymidine kinase, and ribonuclease.
An important family of zinc-finger proteins is the steroid or thyroid hormone receptors that bind hormones and facilitate their wide range of effects. It is involved in gene transcription at various levels, via participation in histone deacetylation reactions and via factors possessing the zinc-finger motifs. Zinc, a divalent cation, is an essential micronutrient for humans and its importance can be gauged from the fact that it is an essential component of more than 300 metalloenzymes and over 2000 transcription factors that are needed for regulation of lipid, protein and nucleic acid metabolism, and gene transcription.
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