Importance of Zinc for Human Health

This is the third article in a series on zinc nutrition.Background information on zinc metabolism and dietary requirements was provided in part 1 and functional effects of zinc deficiency in part 2. Assessing the risk of zinc deficiency in a population and developing intervention programs are discussed here.

Assessment of the Risk of Zinc Deficiency

Assessing the nutritional status of a population is critical in developing intervention programs that enhance human health and well being. Regrettably, there are no simple, quantitative, biochemical or functional markers of zinc status available that are sensitive to identify mild or moderate zinc deficiency in individuals. Nevertheless, there is sufficient evidence to suggest that zinc deficiency may be common in many low-income countries. For example, animal foods that are particularly rich sources of zinc are inaccessible to many of the world’s poorer population. Diets based largely on cereals and legumes make it difficult to meet the zinc requirements because high phytate content reduces bioavailability of zinc. The evidence for widespread zinc deficiency in developing countries is also derived from the results of intervention trials in children, which showed that zinc supplementation improved growth among stunted children. Although other nutritional and environmental factors can also cause stunting, an elevated prevalence of this condition is considered as suggestive evidence of zinc deficiency in a population.

In recent years, efforts have been made to derive more precise estimates of the magnitude of the risk of zinc deficiency using direct measures of zinc status. These include assessment of dietary intakes of zinc and biochemical indicators of zinc status. Data available from ongoing diet surveys can be used to assess the risk of inadequate zinc intakes in a population. Serum zinc concentration is the most widely used biochemical indicator of zinc status. Suggested lower cutoffs for serum zinc concentration are based on data collected in the second National Health and Nutrition Examination Survey in US population (1). Although zinc concentrations may have limitations in validity and reliability for identification of mild or moderate zinc deficiency in individuals, this index is useful for assessing zinc status at the population level.

Table : Suggested lower cutoffs for assessment of serum zinc ?g/dl

Children <10 yr	65
Adult men	70
Non-pregnant women	66
Pregnant women	50

Intervention Strategies

It is clear from the numerous zinc supplementation trials described earlier (Part 2), that a wide range of health benefits can be realized by increasing the intake of zinc, where the diets are inadequate in zinc. These results argue strongly for the development of programs to improve zinc status in high-risk populations. However, because zinc deficiency rarely occurs in isolation, programs to address zinc deficiency should be incorporated into the existing health and nutrition programs. The three major intervention strategies are supplementation, fortification and dietary diversification. The choice of interventions depends upon the available resources and technical feasibility

Dietary intervention

Dietary diversification/modification represents a sustainable long-term approach to improving the intakes of several nutrients simultaneously. During the past few decades, home gardening and education interventions have been popular food-based strategies to address multiple micronutrients, especially vitamin A and iron. These interventions offer an opportunity for adding zinc as a target nutrient. More information is required on zinc content and zinc absorption modifiers in local indigenous foods to identify suitable sources of absorbable zinc.

Supplementation

Supplementation programs are useful for targeting vulnerable population subgroups, which are at greater risk. Based on the RDAs and experience with research trials, the recommended dosage levels of zinc are 5 mg/day for children between 7 months to 3 years and 10 mg for older children (2). It would be most feasible to include zinc in programs already delivering daily or weekly nutrient supplements for prevention of iron deficiency anemia and other micronutrient deficiencies. When formulating multi-nutrient supplements, it is recommended that salts that are readily absorbed, like zinc sulfate, zinc gluconate or zinc acetate should be used to avoid antagonistic interactions between zinc and other minerals.

Supplemental zinc is also recommended as an adjunct therapy during the treatment of diarrhea in children (3). The recommended daily dosage is equivalent to two times the age-specific RDA per day for 14 days - 10 mg/day for children under 3 years and 20 mg for older children. Several clinical trials have demonstrated that zinc supplements given during acute or persistent diarrhea reduce the severity and duration of diarrhea (4).

Fortification

Food fortification is a more cost-effective and sustainable strategy to overcome micronutrient malnutrition. Where the micronutrient deficiency is widely distributed in the population, universal or national level fortification of centrally processed foods is an appropriate intervention. An example of a country with a nationwide zinc fortification program is Mexico where zinc and other micronutrients are added to wheat and corn flour that are used in preparing bread and tortilla, the two principal staples in the country.

It is recommended that the selected food vehicle for zinc fortification be one that is widely consumed in stable and predictable amounts. There are several zinc compounds that are available for fortification. Of these, zinc oxide and zinc sulfate are least expensive and most commonly used by the food industry. The suggested levels for fortification of staples is 30-70 mg zinc/ Kg of flour (2). Zinc sulfate theoretically should provide more reliable absorption because of its greater solubility, although it is more expensive. Further information is required on the absorption of zinc, acceptability and cost of fortified products, when different chemical forms of zinc are added to different food vehicles.

Targeted fortification programs increase the intake of zinc in high-risk groups like infants and young children. In many countries, infant formulas and complementary foods are currently fortified with zinc and other micronutrients. Commercially available standard infant formulas contain zinc around 1 mg/ L, as per the current recommendations. The zinc levels in breast milk show a progressive decline during the course of lactation from 4 mg/L in the first week to 0.5mg/L at 9 months (5). Exclusively breast fed infants of mothers with adequate zinc nutriture can satisfy their zinc requirements for the first 4-5 months of life (6). Thereafter, complementary foods with high content of absorbable zinc are required to satisfy the growing needs. In many developing countries, cereals or tubers are used as a basis for such additional foods. Findings from a study on the nutrient adequacy of such foods suggest that even if strategies to improve the bioavailability of iron and zinc are employed, they are insufficient to overcome the deficits in these nutrients (7). Therefore, there is an urgent need for fortified foods. Many Latin American countries like Guatemala, peru and Mexico are currently using centrally processed complementary foods that are fortified with zinc and other micronutrients (8).

Interactions of micronutrients

Since zinc deficiency is often associated with other deficiencies, zinc is given as an additional component of multi-nutrient supplementation or fortification. Salts that are readily absorbed should be selected to avoid antagonistic interactions between zinc and other minerals. Interactions between zinc and calcium(9), zinc and iron (10), and zinc and copper (11) have been documented. Absorption of zinc also depends on whether the zinc supplement is consumed with or without food. Few zinc supplementation trials have provided the details of how the supplements were given. Further studies are needed to determine the implications of nutrient/food interaction on zinc absorption, and to assess the efficacy of various zinc dosage levels according to the method of administration.

See Also: Zinc, Part 1 and Zinc, Part 2.

References

1. 1. Pilch SM,Senti FR. Assessment of the zinc nutritional status of the US population based on data collected in the second National Health and Nutrition Examination Survey 1976-1980. Bethesda MD.
2. Hotz C and Brown KH. Assessment of the risk of zinc deficiency in populations and options for its control. Food Nutr Bull 2004; 25: 99 S.
3. Bahl R et al. Effect of zinc supplementation on clinical course of acute diarrhea. (Report of a meeting, New Delhi,May 2001). Health Popul Nutr 2001;19:338.
4. Bhutta ZA et al. Prevention of diarrhea and pneumonia by zinc supplementation in children in developing countries: pooled analysis of randomized controlled trials. J Pediatr 1999; 135: 689.
5. Simmer K et al.Breast milk zinc and copper concentrations in bangladesh. Br J Nutr 1990;63:91.
6. Krebs NF et al. Zinc requirements and zinc intakes in breastfed infants. Am J Clin Nutr 1995;41:571.
7. Gibson RS et al. Complementary foods for infant feeding in developing countries: their nutrient adequacy and improvement Eur J Clin Nutr. 1998 Oct;52(10):764.
8. Rivera JA et al. The potential role of processed complementary foods. In: Martorelle R, Haschke F eds. Nutrition and growth. 47th Nestle Nutrition Workshop Series. Philadelphia, PA. 2001.
9. Wood RJ et al. High dietary calcium intakes reduce zinc absorption and balance in humans. Am J Clin Nutr 1997; 65: 1803.
10. Solomons NW. Competitive interaction of iron and zinc in the diet: consequences for human nutrition. J Nutr 1986; 116: 927.
11. Fischer P et al. Effects of zinc supplementation on copper status in adult man. Am J Clin Nutr 1984:40: 743.

This material has been prepared by V. Reddy on behalf of the IFM's Advisory Committee on Child Health and Nutrition, July 2005.