Zinc is an essential trace mineral that is known to be a co-factor for more enzymes than all the other metals combined. With such a wide range of enzymes dependent on zinc for their function, the metabolic roles for zinc as well as the deficiency symptoms that result from inadequate intake, are diverse and widespread. The symptoms of zinc deficiency range from severe to mild, and can even be undetectable clinically in the short-term. Severe zinc deficiency results in dermatitis, alopecia, diarrhoea, emotional disorder, weight loss, infection, hypogonadism in males, neurosensory disorders and problems with frequent and non-healing ulcers. Because zinc is essential, failure to add zinc to the diet results in death. In contrast, moderate zinc deficiency results in hypogonadism, rough skin, poor appetite, lethargy and delayed wound healing. Severe zinc deficiency is common throughout the developing world and marginal zinc status is common throughout the developed world.
Zinc deficiency can affect the immune system because zinc is required for cell mediated immunity, phagocytosis as well as for the function of neutrophils, macrophages and natural killer cells. Zinc deficiency also causes apoptosis of T and B cells. Reducing human zinc intake to 3-5mg per day causes a decrease in serum thymulin, a zinc dependent thymic hormone, in just 12 weeks. In the same subjects, natural killer cell lytic activity was also reduced. Following zinc supplementation, these changes were reversed. Zinc can also decrease the severity of diarrhoea in children in developing countries, possibly because correction of a zinc deficiency increases the absorption of water and electrolytes by the intestine, and allows a faster regeneration of the gut. Supplementation of elderly subjects (55 to 87 years) with 45mg of elemental zinc caused a decrease in the incidence of infection and reduced oxidative stress.
Supplementation of zinc lozenges have resulted in a reduction in duration and severity of symptoms of the common cold (mean duration of cold 4.0 versus 7.1 days in treatment and control, respectively). Evidence also suggests that zinc supplementation can reduce inflammation. Some studies using glycine or citrate as ligands for the zinc lozenges have shown negative results, which is problematic because these ligands prevent the release of zinc from the lozenge. Suitable ligands for zinc lozenges include acetate and gluconate, which allow adequate zinc to be released from the lozenge at physiological pH. Trials using zinc lozenges also show that the use of supplements must start within 24 hours of the onset of cold symptoms to be effective, and the total dose of zinc must be at least 75mg per day. Use of smaller doses of zinc in some trials reporting negative findings suggests that the dose of zinc is important in relation to fighting infection.