Zinc is an essential trace element that is generally non-toxic and is essential for normal growth and development, wound healing and immunocompetence. It is also necessary for the activity of more than 70 metalloenzymes, e.g. carbonic anhydrase, alkaline phosphatise, RNA and DNA polymerases, thymidine kinase and carboxypeptidases.
Zinc is actively absorbed from the gut into epithelial cells, where it is stored as mucosal metallothionen or released into the plasma, where 80% is mainly bound to albumin. It is then transported to the liver, where it is stored by hepatocytes in metallothionen. Zinc is mainly stored in muscle, with bone, liver and plasma forming a small exchangeable pool. Twenty percent of body zinc is found in the skin, nails and hair.
Regulation of zinc absorption is thought to be controlled by the amount of metal free albumin. Zinc absorption decreases in the presence of dietary phytate, high dietary phosphate and excessive calcium. Coffee, dairy products and high fibre bread also reduce zinc absorption. Zinc is mainly excreted in the faeces, with small amounts being lost via the kidneys; urinary zinc increases in nephrosis, postalcoholic hepatic cirrhosis and hepatic porphyria. Increased excretion also occurs in total starvation and on administration of chelation agents. Large amounts of zinc can also be lost in sweat.
Increased zinc intake depresses copper absorption and conversely copper absorption is greatly increased in zinc deficiency. Metabolic interactions occur between zinc and cadmium, zinc and iron, and zinc and chromium. Cadmium and iron uptake are depressed by high zinc levels, while chromium and zinc are metabolised by a common pathway in the intestine and are mutually antagonistic.
In blood approximately 80% of zinc is in the red blood cells. Almost all of this is in carbonic anhydrase. About 3% is found in leucocytes, each leucocyte containing approximately 25 times the amount of zinc as each individual erythrocyte. The rest, approximately 20%, is found in the plasma. In the newborn, erythrocyte zinc levels are about half that of the adult, with levels progressively increasing until about 12 years of age.
Dietary intake: 6 mg/day for children, 2 mg/day for adults
Excretion: Bile, urine and faeces
Lipids/Trace Metals
PZN
Plasma MUST be separated from the cells within 4 hours of collection
5 days
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