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THE USE OF STABLE ISOTOPES IN STUDIES OF ZINC IN PREGNANCY AND LACTATION.

Manuel Ruz
Department of Nutrition, Faculty of Medicine, University of Chile
PO Box 13898-21, Santiago, Chile

Abstract

The expanding use of zinc stable isotopes in biology represents an important opportunity to advance in our knowledge on several events related to zinc metabolism and zinc homeostasis. This article reviews the applications of zinc stable isotopes methodologies in human pregnancy and lactation. Although there has been only few studies conducted in humans, it is expected a significant increase during the next years. The relevance of the findings already reported warrants further effort and resources be directed to these kind of studies.

Introduction

Over the past two decades there has been a steadily growing application of zinc stable isotope methodologies to investigate whole body human physiology. Two major factors have contributed to this expanding use. One is the safe use of stable isotopes as tracers in any age or physiological condition group, and the other is the availability of technical capabilities to perform accurate determinations. This article will analyze the applications of zinc stable isotope techniques to the study of a series of biologically relevant questions in human pregnancy and lactation.

General characteristics of zinc

Zinc is a flexible atom. It shows a notable capacity to form stable complexes with several molecules such as amino acids, peptides, and nucleotides. It has affinity for thiol and hydroxy groups and for ligands containing electron-rich nitrogen as a donor. It does not change its oxidation state. Because of its ability to participate in strong but readily exchangeable ligand binding and its coordination geometry flexibility, this element is very useful in biological systems [1].

Zinc is required for the activity of 200 enzymes in all six classes. Zinc participates in gene expression through the so-called zinc fingers. Zinc fingers proteins include transcription factors, hormone receptors, DNA repair enzymes, tumor suppressor genes. Zinc also plays a relevant role at the membranes. It is involved in cell signaling [1].

Zinc deficiency is characterized by: growth retardation, intercurrent infections, altered immune response, complications during delivery, malformations, delayed wound healing, abnormal dark adaptation, anorexia, and diarrhea, among others [2].

Stable isotopes of zinc

Zinc has five stable isotopes: 64Zn, natural abundance 48.89%; 66Zn, 27.91%; 67Zn, 4.11%; 68Zn, 18.57%, and 70Zn, 0.62%, respectively. Of these, 67Zn, 68Zn, 70Zn are used as tracers. Along with the availability of these isotopes for research, major technical improvements have been made to develop the capacity for accurate determinations of stable isotopes. Currently, the methods used on regular basis are ICP-MS and TIMMS [3].

General applications of zinc stable isotope methodologies

Zinc stable isotope methodologies have been used in several situations, such as: zinc absorption, endogenous zinc losses, assessment of zinc nutritional status, factors involved in zinc homeostasis, zinc turnover, and compartmental analyses [4, 5]. The intestinal tract is crucial for zinc homeostasis [6]. Hence, the attention on zinc absorption and excretion. The accuracy of simple techniques used to estimate fractional zinc absorption has been tested. The double isotopic tracer ratio performed in spot urine samples demonstrated to be the preferred choice [7].

Zinc stable isotopes studies in pregnancy

The importance of zinc for animals and humans has been established [2, 8]. There are some species-related differences not clearly explained. For instance, animals can double their intestinal capacity to absorb zinc during pregnancy, women apparently do at a much lesser extent [9]. Swanson et al. [10] performed the first study on human pregnancy using zinc stable isotopes. They used 70Zn to assess zinc absorption. No effect of pregnancy progression was observed. Fung et al. [11] studied zinc absorption in 13 women from preconception through pregnancy and lactation. Fractional zinc absorption changed from 14.6% from preconception to 18.9% at week 24-26 of gestation, 19.4% at week 34-36 of gestation, and 25.3% at week 7-9 postpartum. The only significant difference was between preconception and lactation.

As a result of previous studies conducted in Peruvian women receiving mineral supplementation, in which the potential consequences of the interactions iron-zinc was matter of concern [12, 13], a zinc stable isotope study was undertaken to evaluate the effects of prenatal iron supplements on zinc absorption [14]. Zinc absorption approached 20% in both iron (with and without zinc) supplemented groups, compared to 47% observed in the non-supplemented. These results are difficult to interpret because the subjects consumed their supplements at the same time when the zinc absorption tests were performed. In addition the tracer dose used was high, which may interfere with the test outcome.

Iron-zinc interaction has been focus of great attention. In the study by Fung et al referred above [11] , the authors identified four subjects (out of the thirteen studied) who were consuming iron supplements during the study. Despite the limitations of the conclusions obtained with such small number of cases, it was noteworthy that these women did not show the increased absorption during lactation as it was observed in the non-iron supplemented subjects.

In a non-pregnant non-anemic model, our group has been studying the effects of iron on zinc absorption and zinc status in Chilean women. Unlike most of other studies, iron supplements are provided between meals, and zinc absorption tests are conducted after three days of supplemental iron withdrawal. In the first study, the subjects consumed 55 mg of iron/day on average during three months [15]. Major findings were a significant increase of fractional zinc absorption on fasting conditions and a non-significant (although very close to the 0.05 probability level) decrease plasma zinc and the size of the rapidly exchangeable zinc pool. These were interpreted as indications of deteriorated zinc status as result of the iron supplementation. A second study, which preliminary results have recently been presented at the Trace Element in Man and Animal Meeting (TEMA-11), seems to add evidence in the same direction [16].

Zinc stable isotopes studies in lactation

Zinc requirements during lactation are greater than those for pregnancy, especially during the first weeks postpartum. It is not clear if populations with low zinc intakes are able to adapt to this demanding situation. It is also not known the effects of maternal zinc supplementation on child health and development [17]. The present article will be focused on the studies using stable isotopes directed to evaluate the effects on the mother. The effects on the infant are beyond the scope of this review.

The first study in lactating women using stable isotopes was conducted by Jackson and colleagues in 1988 in Brazil [18]. Seven women from a poor community in the Amazon were studied. The subjects had a zinc intake that represented 34% of the recommended intake. Zinc absorption, evaluated using 67Zn as a tracer, showed to range from 59% to 84%. These values look very high today and a potential technical problem should not be discarded.

A recent article conducted in rural Chinese women described how these women, despite their low zinc intake are able to maintain zinc homeostasis by a significant increase of zinc absorption and reduction of endogenous fecal zinc [19]. These same authors had provided earlier data comparing zinc absorption and excretion in non-lactating Chinese women from rural and urban locations [20]. In such study it was made evident the capacity of human beings to deal with adverse chronic conditions regarding low dietary zinc supply.

Zinc stable isotopes methodologies are a powerful tool to gain in our understanding on a variety of biological issues. For instance, applications of these techniques in longitudinal studies can provide quantitative data on the effectiveness of prevention programs such us zinc fortification and zinc supplementation programs, and the potential interference of related interventions.

Acknowledgements: Partially funded by FONDECYT, research project 1000896.

References

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