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Med J Aust 2000; 173 Suppl 6 November: S98-S99

Zinc plays a central role in metabolism, including the assimilation of metabolic fuel and maintenance of immune function. It is a component of many enzymes, often located at the active site;¹ it helps to stabilise cell membranes and can function as a site-specific antioxidant;² and zinc "finger" proteins have sequence-specific DNA-binding domains and act as transcriptional mediators.³ Biochemical abnormalities in the absence of zinc are wide ranging and include reduced protein synthesis, resistance to infection, collagen synthesis and platelet aggregation.^1,4 The major cause of zinc deficiency in First World countries is a lack of bioavailable zinc in the diet.

Dietary factors that can improve immune function

The immune system is a series of multifaceted, interrelated events. Given the broad spectrum of activities which are involved in immunity, several nutrients play a significant role, including essential amino acids, essential fatty acids, vitamins (A, E, B₆, C) and minerals (calcium, magnesium, zinc, iron, manganese, copper, selenium, chromium, iodine).⁵

The role of zinc in immunity

Zinc stimulates the production of interleukin-1 and appears to interact with other nutrients in this response.⁶ In zinc deficiency, there is a suppressive effect on thymic function, T-lymphocyte development, lymphocyte proliferation, and T-cell dependent B-cell functions, leading to a decrease in resistance to respiratory and gastrointestinal infections. While it has been shown that improving zinc status is effective in reducing the incidence and severity of childhood diarrhoea,⁷ a meta-analysis of clinical trials, including one from Australia,⁸ concluded that zinc lozenges (4.5-24mg) are ineffective against the common cold.⁹ Hence, the utility of zinc needs to be considered in the context of the nutritional status of the individual. In the broader view of immunity, zinc helps to maintain the barrier function of the skin and aids in wound healing.¹

Are women aged 40 and over consuming enough zinc?

Based on data from the National Nutrition Survey,¹⁰ and compared with the RDI (12mg/day),¹¹ zinc intake in Australian women is low. Further, while information on zinc bioavailability is not readily available from the National Nutrition Survey, the high contribution of plant foods to the diet of Australians¹⁰ suggests that zinc bioavailability is low to medium. Thus, a considerable proportion of women are likely to be at risk of zinc deficiency. Even when there are no overt signs of deficiency, as measured by the classical symptoms (growth retardation and hypogonadism),⁴ the possibility of marginal deficiency and the associated subtle symptoms, such as hypogeusia (loss of taste),¹² can not be dismissed. As there are no known biomarkers of zinc status, degrees of zinc deficiency can not be quantified.¹³

Another potential factor is that the absorption of zinc is hindered by calcium -- recommended daily doses of calcium (1000mg) reduce the bioavailability of zinc.^12,14

Is it possible to get too much zinc?

Increasing the intake of zinc through high-dose supplementation (300mg) decreases immune function,¹⁵ as indicated by reduction in lymphocytic stimulation response, chemotaxis and phagocytosis, possibly because of an adverse interaction between high doses of zinc and copper metabolism.¹⁶ The antioxidant effect of zinc supplementation helps to decrease the oxidation of lipids in people with diabetes,¹⁷ but not in healthy people.¹⁸ By contrast, supplements may increase the risk of heart disease by increasing low density lipoprotein cholesterol concentration and decreasing high density lipoprotein.¹⁵

Thus, to overcome possible zinc deficiency, Australian women should increase their intake of zinc through food sources such as oysters, lean red meat and fortified breakfast cereal (see Box 2, below).

Summary Zinc plays a central role in metabolism, including the assimilation of metabolic fuel and maintenance of immune function. Zinc intake in Australian women is low. Women should increase their intake of zinc through food sources. Zinc supplementation greater than 25mg/day should be avoided.

References

1. Samman S. Zinc and copper. In: Mann JI, Truswell AS, editors. Essentials of human nutrition. Oxford: Oxford University Press, 1998: 151-159.
2. Bray TM, Bettger WJ. The physiological role of zinc as an antioxidant. Free Radic Biol Med 1990; 8: 281-291.
3. Cousins RJ. Zinc. In: Ziegler EE, Filer LJ, editors. Present knowledge in nutrition. Washington, DC: ILSI Press, 1996: 293-306.
4. Prasad AS. Discovery and importance of zinc in human nutrition. Fed Proc 1984; 43: 2829-2834.
5. Ferguson A, Griffen GE. Nutrition and the immune system. In: Garrow JS, James WPT, Ralph A, editors. Human nutrition and dietetics. Edinburgh: Churchill Livingstone, 2000: 747-764.
6. Prasad AS. Effects of zinc deficiency on immune functions. J Trace Elem Exp Med 2000; 1: 1-20.
7. Sazawal S, Black RE, Bhan MK, et al. Zinc supplementation in young children with acute diarrhea in India. N Engl J Med 1995; 333: 839-844.
8. Douglas RM, Miles HB, Moore BW, et al. Failure of effervescent zinc acetate lozenges to alter the course of upper respiratory infection in Australian adults. Antimicrob Agents Chemother 1987; 31: 1183-1187.
9. Jackson JL, Lesho E, Peterson C. Zinc and the common cold: A meta-analysis revisited. J Nutr 2000; 130: 1512S-1515S.
10. Australian Bureau of Statistics. National nutrition survey. Nutrient intakes and physical measurements, Australia 1995. Canberra: AGPS, 1998.
11. National Health and Medical Research Council. Recommended dietary intakes for use in Australia. Canberra: AGPS, 1991.
12. Sandstead HH. Zinc: growth, development, and function. J Trace Elem Exp Med 2000; 1: 41-49.
13. Thompson RPH. Assessment of zinc status. Proc Nutr Soc 1991; 50: 19-28.
14. Argiratos V, Samman S. The effect of calcium carbonate and calcium citrate on the absorption of zinc in healthy female subjects. Eur J Clin Nutr 1994; 48: 198-204.
15. Chandra RK. Excessive intake of zinc impairs immune responses. JAMA 1984; 252:1443-1446.
16. Samman S. Dietary versus cellular zinc: the antioxidant paradox. Free Radic Biol Med 1993; 14: 95-96.
17. Faure P, Benhamou PY, Perard A, et al. Lipid peroxidation in insulin-dependent diabetic patients with early retina degenerative lesions: effects of an oral zinc supplementation. Eur J Clin Nutr 1995; 49: 282-288.
18. Gatto LM, Samman S. The effect of zinc supplementation on plasma lipids and LDL oxidation in males. Free Radic Biol Med 1995; 19: 517-521.

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