Zinc

Zinc is an essential nutrient, required for numerous metabolic functions, including gene expression, growth and immunity. Manifestations of zinc deficiency include growth retardation, high rates of infection, skin lesions and impaired wound healing. Zinc deficiency is a major contributor to the burden of disease in developing countries. Seafood and red meat are good sources of bioavailable zinc. Dietary zinc from plant sources is less bioavailable due to the interaction with phytate. The recommended dietary intake for zinc is 8 and 14 mg/day for women and men, respectively. The Dietary Guidelines for Australian Adults identify red meat as a substantial source of zinc. Zinc is involved in a number of metabolic processes, including protein and nucleic acid synthesis, and for the synthesis and action of insulin. It is the most common catalytic metal ion in the cytoplasm of cells. Carbonic anhydrase was the first discovered zinc metalloenzyme; other enzymes include: carboxypeptidase, alkaline phosphatase, DNA/RNA polymerase and superoxide dismutase.1–3 As a component of enzymes, known as metalloenzymes, zinc participates in the reaction at the active site or provides structural integrity to the enzyme. Genes such as those involved in the regulation of redox state, fatty acid metabolism, signal transduction and platelet activation have been recognised as potential candidates for regulation by zinc: some genes are positively affected, and others negatively; whereas some are affected only by extremes of zinc status. Zinc-containing proteins in the human genome, known as 'zinc fingers', are able to interact with DNA and act as transcriptional mediators.1,2 Zinc serves as a structural component that allows for the coordinate binding of amino acids, mainly cysteine and histidine residues in the protein chain, to form a finger-like structure. Adult humans contain 1.5–2.5 g of zinc, which is distributed in all tissues but mostly in fat-free mass such as bones and muscles. Dietary zinc is absorbed mainly from the duodenum by active and passive mechanisms. Once absorbed, zinc is transported to the liver bound to albumin. The major route of zinc excretion is by the intestine, followed by the kidneys. Smaller amounts of zinc are shed in skin cells or secreted by the prostate gland.1–3 With this background, this paper discusses zinc deficiency, reviews food sources of zinc, and comments on the role of red meat in providing zinc in the overall diet. A number of conditions predispose to zinc deficiency and are related to: decreased intake; decreased absorption; decreased utilisation secondary to other conditions, such as alcoholism; increased losses in conditions such as diarrhoea and excessive vomiting; and increased requirement associated with growth, pregnancy and lactation.1–3 Zinc deficiency was first observed in Iran and Egypt during the 1960s.3 The first case of severe zinc deficiency was a 21-year-old man who resembled a 10-year-old boy. He displayed symptoms that included growth retardation, hypogonadism and delayed sexual maturation. Other manifestations of zinc deficiency reported subsequently include high rates of infection (e.g. pneumonia) and diarrhoea due to an immune deficiency, diverse forms of skin lesions (e.g. eczema and alopecia), impaired wound healing, loss of taste, and night blindness.1,2 The World Health Organisation (WHO) considers zinc deficiency to be a major contributor to the burden of disease in developing countries, especially in those with a high mortality rate.4 The combination of low zinc intake, usually from sources with poor bioavailability, and an increased requirement for zinc, for example in children, predisposes to zinc deficiency. The International Zinc Nutrition Consultative Group (IZiNCG) has provided estimates of the risk of zinc deficiency in 176 countries based on data from the Food and Agricultural Organisation's food balance sheets.5 Bioavailable zinc is calculated and compared with the estimated average requirement. Based on these estimates, it appears that 25% of the populations of South and South-East Asia and Latin America are at risk of inadequate zinc intake, compared with 10% of the population of Western Europe and North America. Data on a selected number of countries are shown in Table 1. Assessing zinc status is hindered by the absence of reliable biomarkers; however, as a step towards estimating the global prevalence of zinc deficiency, IZiNCG has suggested the use of stunting prevalence (height-for-age below the expected range) as it is the best known and easiest to measure of the symptoms of zinc deficiency. IZiNCG has also suggested the use of data on the rates of iron-deficiency anaemia as a crude indicator of zinc deficiency, because iron and zinc are oftenfound in the same foods and both nutrients have similar obstacles to their bioavailability.5 Zinc is found widely in the food supply, but its bioavailability from different foods is highly variable. Rich sources of zinc include: oysters, red meat, liver and cheese. Zinc in animal products, crustacea and mollusks is more readily absorbed than from plant foods. Cereal grains, legumes and nuts are rich in phytate (the main storage form of phosphorous in plants), which bind zinc in the intestine and reduce its absorption.6 The early cases of zinc deficiency were associated with high phytate-containing foods: unleavened bread from unrefined wheat flour as a dietary staple, and beans.3 The molar ratio of phytate to zinc in the diet has been proposed as a predictor of zinc bioavailability, and ratios greater than 15 have been associated with suboptimal zinc status. Three categories of bioavailability (Table 2) have been put forward by the WHO.7 To minimise the risk of zinc deficiency, it has been suggested that dietary diversification is required, including the increase in consumption of foods from animal sources and the introduction of food processing techniques, such as fermentation, that will reduce the zinc-chelating potency of phytate.5,8,9 The WHO proposed the use of zinc supplementation or fortification of complementary foods given to breastfed infants starting at six months of age as a means of meeting their requirement for zinc. It has been argued, however, that the inclusion of meat in complementary feeding in developed and developing countries will meet zinc requirements and alleviate the need for supplementation.10 In Kenyan children aged 6–14 years, the introduction of a meat-based snack over a two-year intervention resulted in increased growth and higher cognitive scores compared with children who consumed a milk- or fat-based snack.11 Such strategies will have the advantage of improving not only the dietary content and bioavailability of zinc, but also those of other nutrients, such vitamin B12 and iron. The concentration of zinc in plasma is the most commonly used diagnostic indicator of zinc status. However, zinc in this compartment represents less than 1% of the body pool of zinc, and hence, its measurement provides limited information about the zinc status of the individual.1 Unlike many other micronutrients, a sensitive biomarker of zinc status is yet to be identified. Australian dietary surveys have identified low zinc intakes in preschool children and adolescents,12,13 but biochemical screening of preschool children across a range of socioeconomic status showed that plasma zinc concentrations were within the normal range.14 Vegetarian men and women were shown to have lower zinc intakes than omnivores.15 The median intake of zinc in vegetarian female individuals was reported to be 5.8 mg/day, equivalent to 70% of the recommended dietary intake (RDI), and likely to be derived from sources with low bioavailability. Nevertheless, no clear differences in plasma zinc concentrations were observed. In the elderly, particularly in institutionalised individuals, zinc status is often compromised.16 A recent study of aged-care residents showed that plasma zinc concentrations were below the normal range in 46% of subjects, and that these individuals had lower functional mobility scores ('Timed Up and Go' test) than those with plasma zinc concentrations in the normal range.17 Zinc deficiency has been reported in Australian Indigenous children.18 Zinc supplementation, however, has not been shown to be effective in increasing linear growth19 or recovery from acute diarrhoea.20 In these circumstances, the data are suggestive of an underlying confounding factor such as parasitic infection. The Australian National Nutrition Survey in 199521,22 estimated the mean intakes of zinc in male and female individuals to be 14.4 and 9.7 mg/day, respectively. A considerable proportion of men in the age range of 25–64 years were consuming zinc at levels below the RDI (14 mg/day), and men aged >65 years were consuming zinc levels that were consistently below the RDI. A smaller percentage of women than men were consuming zinc levels below the RDI (8 mg/day) (Table 3). The National Nutrition Survey results showed that the category of 'meat, poultry and game products and dishes' provided 32% and 39% of the zinc in the diet of adult female and male individuals, respectively.22 Muscle meat was a major source of zinc for adolescents (contributing 14% of the total intake of zinc) and adults (15%), and a moderate source for children (9%). Moderate sources of zinc intake for all people aged two years and older were dairy milk, breads, and mixed dishes where beef, veal or cereals is the major ingredient. The proportion of the population recording low zinc intakes in the survey was inversely related to the consumption of red meat on the day of the survey.23 The Dietary Guidelines for Australian Adults (DGAA) encourage individuals to 'include lean meat, fish, poultry and/or alternatives'. The DGAA state that red meats (defined as muscle meat from cattle, sheep, goat and kangaroo) provide substantial amounts of zinc.24 The RDI for zinc is 14 mg/day for men and 8 mg/day for women, with an additional 2–3 mg for pregnancy and 3–4 mg per day for lactation.25 The current RDI for women is a third lower than the previous Australian RDI. The new figure for men is slightly higher. The upper limit of intake is 40 mg/day. Zinc is an essential nutrient, required for numerous metabolic functions, and its deficiency results in growth retardation, high rates of infection, skin lesions and impaired wound healing. Zinc deficiency is a major contributor to the burden of disease in developing countries. Seafood and red meat are good sources of bioavailable zinc. Dietary zinc from plant sources is less bioavailable due to the interaction with phytate.