Janet C. King

Assessment of zinc status.

Plasma zinc concentration has been denigrated as a measure of zinc status because it responds to metabolic conditions unrelated to zinc status and because it is insensitive to changes in dietary zinc. The insensitivity of plasma zinc to reductions in dietary zinc reflects the tremendous capacity of the organism to conserve tissue zinc by reductions in zinc excretion and/or reductions in the rate of growth. Changes in plasma zinc concentrations do not seem to occur until the capacity to reestablish homeostasis by reducing excretion and/or growth has been exceeded. Reductions in dietary zinc beyond the capacity to maintain homeostasis lead to utilization of zinc from a small, rapidly turning over pool. This pool is located, at least in part, in the bone, liver, and plasma. Loss of a small, critical amount of zinc from this pool leads to the rapid onset of both biochemical and clinical signs of zinc deficiency. Thus, plasma zinc is a valid, useful indicator of the size of this exchangeable pool of zinc. Plasma metallothionein concentrations may prove useful for identifying poor zinc status. Plasma metallothionine concentrations reflect hepatic concentrations and, therefore, are reduced when the dietary zinc supply is low. However, since hepatic metallothionein concentrations rise in response to stress, plasma metallothionein concentrations are increased in this state when the plasma zinc concentrations are also likely to be low. Thus, measurement of both plasma metallothionein and plasma zinc concentrations could differentiate a low plasma zinc due to dietary zinc deficiency from a low concentration due to stress, infection, or other metabolic conditions.

Determinants of maternal zinc status during pregnancy

Zinc deficiency in pregnant experimental animals limits fetal growth and, if severe, causes teratogenic anomalies. Although the data from human studies are not consistent, similar outcomes have been observed and were associated with poor maternal zinc status. This paper reviews humans studies of zinc status and pregnancy outcome, describes the physiologic adjustments in zinc utilization during pregnancy to meet fetal needs while maintaining maternal status, and identifies dietary and environmental conditions that may override those physiologic adjustments and put the health of the mother and fetus at risk. Adjustments in intestinal zinc absorption appear to be the primary means by which zinc retention is increased to meet fetal demands. However, transfer of sufficient zinc to the fetus is dependent on maintenance of normal maternal serum zinc concentrations. Conditions that could interfere with zinc absorption include intake of cereal-based diets that are high in phytate, high intakes of supplemental iron, or any gastrointestinal disease. Conditions that may alter maternal plasma zinc concentrations and the transport of zinc to the fetus include smoking, alcohol abuse, and an acute stress response to infection or trauma. Supplemental zinc may be prudent for women with poor gastrointestinal function or with any of these conditions during pregnancy.

Zinc: an essential but elusive nutrient

Zinc is essential for multiple aspects of metabolism. Physiologic signs of zinc depletion are linked with diverse biochemical functions rather than with a specific function, which makes it difficult to identify biomarkers of zinc nutrition. Nutrients, such as zinc, that are required for general metabolism are called type 2 nutrients. Protein and magnesium are examples of other type 2 nutrients. Type 1 nutrients are required for one or more specific functions: examples include iron, vitamin A, iodine, folate, and copper. When dietary zinc is insufficient, a marked reduction in endogenous zinc loss occurs immediately to conserve the nutrient. If zinc balance is not reestablished, other metabolic adjustments occur to mobilize zinc from small body pools. The location of those pools is not known, but all cells probably have a small zinc reserve that includes zinc bound to metallothionein or zinc stored in the Golgi or in other organelles. Plasma zinc is also part of this small zinc pool that is vulnerable to insufficient intakes. Plasma zinc concentrations decline rapidly with severe deficiencies and more moderately with marginal depletion. Unfortunately, plasma zinc concentrations also decrease with a number of conditions (eg, infection, trauma, stress, steroid use, after a meal) due to a metabolic redistribution of zinc from the plasma to the tissues. This redistribution confounds the interpretation of low plasma zinc concentrations. Biomarkers of metabolic zinc redistribution are needed to determine whether this redistribution is the cause of a low plasma zinc rather than poor nutrition. Measures of metallothionein or cellular zinc transporters may fulfill that role.

Zinc and copper in Asian pregnancies–is there evidence for a nutritional deficiency?

Summary. In 92 Hindu Asians, 59% of them vegetarian, and 51 Europeans longitudinal measurements were made during pregnancy of the zinc and copper concentrations in plasma and hair together with urinary zinc excretion, as indices of their zinc and copper status. Maternal diets were assessed once at booking. Zinc intakes ranged from 3·1 to 16·9 mg/day, with average intakes least in vegetarian Hindus and most in Europeans. Average copper intakes ranged between 1·48 and 1·80 mg/day and were similar in the three patient groups. Both ethnic groups showed the pregnancy‐associated fall in the plasma concentration of zinc and rise in that of copper but throughout the study Hindus had statistically significant lower levels of zinc and higher levels of copper than Europeans. Urinary zinc excretion was not only significantly lower throughout the study in Hindus than in Europeans but the increase in excretion which occurred after 20 weeks gestation was smaller. There were no ethnic differences in the zinc content of hair. Urinary zinc excretion correlated with both plasma zinc levels and dietary zinc. Mean birth‐weight in the Hindus was 2912 g and 34% of infants were below the 10th centile, using the Aberdeen standards, compared with 6% of the European babies (mean birthweight 3349 g). No association was found between crude or adjusted birthweight and any of the measures of zinc or copper status in either ethnic group. The Hindus had an apparently lower average zinc status than the Europeans, but there was no evidence that this had acted as a nutritional constraint and was the cause of their slower rate of intrauterine growth.

Human Zinc Requirements

The dietary requirement for zinc is literally the amount required in the diet to maintain optimally the various metabolic and physiological functions of life (Smith et al. 1983). The dietary zinc requirement for a population is not a single value. Instead it varies over a wide range depending on the age and physiological state of the individuals and on composition of the diet, particularly with respect to the amount and proportion of organic and inorganic components of the diet which influence zinc absorption and utilization (Hambidge et al. 1986).