Zinc

Introduction

Zinc was shown to be an essential nutrient in the mid-1930s (Todd et al, 1934). It was not until 30 years later, however, that widespread zinc deficiency was identified as the underlying cause of stunted growth and delayed sexual maturation in adolescent boys in western Asia and the Middle East (Prasad et al, 1963). Zinc deficiency is now known to occur in many population groups in developing countries, and is increasingly felt to be an important public health problem (Hambidge, 1997). The main effects of zinc deficiency are poor child growth, delayed maturation, poor appetite and impaired immune function.

Zinc has a role in a large number of metabolic synthetic reactions and is essential for all forms of life. Periods of rapid growth such as infancy, adolescence and late pregnancy, when requirements are highest, are most susceptible to zinc deficiency. Dietary sources of zinc are protein-rich foods including meat, fish and shellfish, and whole grains. Roots and tubers are low in zinc, while zinc in cereal staples is most often poorly bioavailable.

Magnitude and Causes

The prevalence of zinc deficiency is unknown; mild and moderate forms are likely to be widespread and until recently largely overlooked
(WHO, 1996).

Zinc deficiency may contribute significantly to growth stunting in young children in many regions (Sandstead, 1991). What has been termed 'protein energy malnutrition', especially low height-for-age, may be due to poor diet quality, including low levels of bioavailable zinc, rather than an inadequate quantity of either protein or energy. Zinc supplementation has been shown to correct growth failure in Canadian young children (Gibson et al., 1989). Linear growth in low-income preschool children in the US has also responded to supplementation (Walravens et al., 1983). Male children may be more at risk than young girls. Zinc deficiency may also contribute to the major causes of morbidity in young children, as shown by studies in India and elsewhere (Sazawal et al., 1996).

A high proportion of pregnant women in developing countries are also likely to be at risk because of habitually inadequate zinc intakes (Gibson, 1994). Zinc deficiency may be particularly widespread among African women. Studies in Egypt (Kirksey et al., 1994), Nigeria (Mbofung et al., 1987) and the former Zaire have all reported low zinc status among pregnant women. In Malawi (Huddle et al., 1998), zinc deficiency among pregnant women, which was confirmed by biochemical indicators, was associated with low intakes of poorly bioavailable zinc, frequent reproductive cycling, and malarial infection. Low energy intakes partly explained the low intakes of zinc in these Malawian women. Other micronutrients were also low, even after adjusting for energy content, confirming the generally poor diet quality of these women. Some zinc supplementation trials have confirmed that zinc deficiency during pregnancy causes poor foetal growth, delivery complications and increased mortality in mothers and their babies (Tamara and Goldenberg, 1996).

Zinc deficiency may be an underlying cause of the very high rates of maternal mortality in Sub-Saharan Africa.

Prevalence of zinc deficiency is probably similar to that of nutritional iron deficiency because the same dietary pattern induces both. Where diets are plant-based and intakes of animal foods tow, the risk of inadequate intakes of both zinc and iron is very high, even when energy and protein intakes meet recommended levels (Gibson, 1994). In these circumstances bioavailability, rather than amount, is the critical factor. Strategies to address zinc deficiency by increasing intake of flesh foods or by decreasing the content of phytates through cereal fermentation, will also improve iron nutrition in deficient populations.

Assessment Issues

There is as yet no single sensitive and specific indicator for zinc deficiency. Dietary sources of zinc need to be quantified, as well as dietary phytate/zinc molar ratios. Work has been frustrated by incomplete information on food composition. Zinc in some plant staples reflects soil levels. Probability estimates for risk of zinc deficiency can be calculated from dietary data atone. However, to determine the severity of zinc inadequacies, dietary information needs to be combined with biochemical and functional indices (Gibson, 1997). Changes in these indices, though, are not specific for zinc (WHO, 1996c). Thus, focusing on zinc as a single deficiency will be particularly problematic in populations where undernutrition is a public health concern. Response to zinc supplementation at physiological levels will more reliably diagnose zinc deficiency; however, this approach is prohibitively costly at the population level in most circumstances.

Recent Research

Important research on the impact of zinc supplementation has been carried out in recent years in a range of settings in developing countries. This research has yielded valuable findings that may be incorporated into advocacy messages and will help to bring zinc higher up on the nutrition agenda. Zinc supplementation has been shown to reduce the prevalence, severity and duration of diarrhoea (Roy et al., 1992; Sazawal et al., 1995). This has significant implications for mortality reduction in many countries. Certain types of malarial morbidity may be reduced by zinc supplementation. Field research done in Papua New Guinea showed that both clinic attendance and fever were tower in a zinc-supplemented group of children followed for ten months (Shankar, unpublished). The incidence of acute respiratory illness and pneumonia is also responsive to zinc supplementation (Xuan Ninh et al., 1996). Treatment regimes for severely malnourished children produce more rapid weight gain when zinc levels are adequate (Waterlow, 1992, p. 164).

Activity levels and frequency of childrens' play, important for long-term developmental and cognitive outcomes, have increased with zinc supplementation. In an Indian study, zinc-supplemented children spent significantly more time moving about and exploring their environment (Sazawal et al., 1996). Work in China has documented positive effects on neuromotor and cognitive functions in school-aged children living in urban areas (Penland et al., 1997).

Summary

In summary, zinc deficiency is likely to be widespread where animal foods are priced out of reach of the poor and where zinc absorption from plant foods is tow due to high phytate. Prevalence rates are not known and work in this area is hampered because there is no reliable indicator for zinc deficiency. However, a rich research base has accumulated in recent years which will facilitate advocacy efforts as well as help to improve the design of effective community-based strategies to control zinc deficiency. Comprehensive guidelines for zinc supplementation, fortification and dietary-based interventions are in press (Gibson and Ferguson, in press). Dietary modification interventions include improved cereal varieties, addition of enhancers of zinc absorption, and reduction of phytic acid content via phytase hydrolysis induced by germination and fermentation, as well as via non-enzymatic degradation by soaking.