Red blood cell metallothionein as an indicator of zinc status during pregnancy

Effects of vitamin and mineral supplementation during pregnancy on maternal, birth, child health and development outcomes in low- and middle-income countries: A systematic review

Background

Almost two billion people who are deficient in vitamins and minerals are women and children in low- and middle-income countries (LMIC). These deficiencies are worsened during pregnancy due to increased energy and nutritional demands, causing adverse outcomes in mother and child. To reduce micronutrient deficiencies, several strategies have been implemented, including diet diversification, large-scale and targeted fortification, staple crop bio-fortification and micronutrient supplementation.

Objectives

To evaluate and summarize the available evidence on the effects of micronutrient supplementation during pregnancy in LMIC on maternal, fetal, child health and child development outcomes. This review will assess the impact of single micronutrient supplementation (calcium, vitamin A, iron, vitamin D, iodine, zinc, vitamin B12), iron-folic acid (IFA) supplementation, multiple micronutrient (MMN) supplementation, and lipid-based nutrient supplementation (LNS) during pregnancy.

Search Methods

We searched papers published from 1995 to 31 October 2019 (related programmes and good quality studies pre-1995 were limited) in CAB Abstracts, CINAHL, Cochrane Central Register of Controlled Trials, Embase, International Initiative for Impact Evaluations, LILACS, Medline, POPLINE, Web of Science, WHOLIS, ProQuest Dissertations & Theses Global, R4D, WHO International Clinical Trials Registry Platform. Non-indexed grey literature searches were conducted using Google, Google Scholar, and web pages of key international nutrition agencies.

Selection Criteria

We included randomized controlled trials (individual and cluster-randomized) and quasi-experimental studies that evaluated micronutrient supplementation in healthy, pregnant women of any age and parity living in a LMIC. LMIC were defined by the World Bank Group at the time of the search for this review. While the aim was to include healthy pregnant women, it is likely that these populations had one or more micronutrient deficiencies at baseline; women were not excluded on this basis.

Data Collection and Analysis

Two authors independently assessed studies for inclusion and risk of bias, and conducted data extraction. Data were matched to check for accuracy. Quality of evidence was assessed using the GRADE approach.

Main Results

A total of 314 papers across 72 studies (451,723 women) were eligible for inclusion, of which 64 studies (439,649 women) contributed to meta-analyses. Seven studies assessed iron-folic acid (IFA) supplementation versus folic acid; 34 studies assessed MMN vs. IFA; 4 studies assessed LNS vs. MMN; 13 evaluated iron; 13 assessed zinc; 9 evaluated vitamin A; 11 assessed vitamin D; and 6 assessed calcium. Several studies were eligible for inclusion in multiple types of supplementation. IFA compared to folic acid showed a large and significant (48%) reduction in the risk of maternal anaemia (average risk ratio (RR) 0.52, 95% CI 0.41 to 0.66; studies = 5; participants = 15,540; moderate-quality evidence). As well, IFA supplementation demonstrated a smaller but significant, 12% reduction in risk of low birthweight (LBW) babies (average RR 0.88, 95% CI 0.78 to 0.99; studies = 4; participants = 17,257; high-quality evidence). MMN supplementation was defined as any supplement that contained at least 3 micronutrients. Post-hoc analyses were conducted, where possible, comparing the differences in effect of MMN with 4+ components and MMN with 3 or 4 components. When compared to iron with or without FA, MMN supplementation reduced the risk of LBW by 15% (average RR 0.85, 95% CI 0.77 to 0.93; studies = 28; participants = 79,972); this effect was greater in MMN with >4 micronutrients (average RR 0.79, 95% CI 0.71 to 0.88; studies = 19; participants = 68,138 versus average RR 1.01, 95% CI 0.92 to 1.11; studies = 9; participants = 11,834). There was a small and significant reduction in the risk of stillbirths (average RR 0.91; 95% CI 0.86 to 0.98; studies = 22; participants = 96,772) and a small and significant effect on the risk of small-for-gestational age (SGA) (average RR 0.93; 95% CI 0.88 to 0.98; studies = 19; participants = 52,965). For stillbirths and SGA, the effects were greater among those provided MMN with 4+ micronutrients. Children whose mothers had been supplemented with MMN, compared to IFA, demonstrated a 16% reduced risk of diarrhea (average RR 0.84; 95% CI 0.76 to 0.92; studies = 4; participants = 3,142). LNS supplementation, compared to MMN, made no difference to any outcome; however, the evidence is limited. Iron supplementation, when compared to no iron or placebo, showed a large and significant effect on maternal anaemia, a reduction of 47% (average RR 0.53, 95% CI 0.43 to 0.65; studies = 6; participants = 15,737; moderate-quality evidence) and a small and significant effect on LBW (average RR 0.88, 95% CI 0.78 to 0.99; studies = 4; participants = 17,257; high-quality evidence). Zinc and vitamin A supplementation, each both compared to placebo, had no impact on any outcome examined with the exception of potentially improving serum/plasma zinc (mean difference (MD) 0.43 umol/L; 95% CI -0.04 to 0.89; studies = 5; participants = 1,202) and serum/plasma retinol (MD 0.13 umol/L; 95% CI -0.03 to 0.30; studies = 6; participants = 1,654), respectively. When compared to placebo, vitamin D supplementation may have reduced the risk of preterm births (average RR 0.64; 95% CI 0.40 to 1.04; studies = 7; participants = 1,262), though the upper CI just crosses the line of no effect. Similarly, calcium supplementation versus placebo may have improved rates of pre-eclampsia/eclampsia (average RR 0.45; 95% CI 0.19 to 1.06; studies = 4; participants = 9,616), though the upper CI just crosses 1.

Authors' Conclusions

The findings suggest that MMN and vitamin supplementation improve maternal and child health outcomes, including maternal anaemia, LBW, preterm birth, SGA, stillbirths, micronutrient deficiencies, and morbidities, including pre-eclampsia/eclampsia and diarrhea among children. MMN supplementation demonstrated a beneficial impact on the most number of outcomes. In addition, MMN with >4 micronutrients appeared to be more impactful than MMN with only 3 or 4 micronutrients included in the tablet. Very few studies conducted longitudinal analysis on longer-term health outcomes for the child, such as anthropometric measures and developmental outcomes; this may be an important area for future research. This review may provide some basis to guide continual discourse around replacing IFA supplementation with MMN along with the use of single micronutrient supplementation programs for specific outcomes.

The Impact of Early Pregnancy and Exposure to Tobacco Smoke on Blood Antioxidant Status and Copper, Zinc, Cadmium Concentration-A Pilot Study

The aim of the study was to evaluate the impact of early pregnancy and exposure to tobacco smoke on antioxidant status and copper, zinc, and cadmium concentrations in the blood of non-smoking and smoking, as well as non-pregnant or pregnant women. The study included 213 women. More specifically, 150 women in first trimester of pregnancy and 63 non-pregnant women. Women were divided into subgroups according to exposure to tobacco smoke. Pregnancy significant influences higher copper and lower zinc concentration in the serum, whereas exposure to tobacco smoke during pregnancy is mainly associated with an elevation in cadmium and zinc concentration. It seems that metallothionein, superoxide dismutase, and glutathione peroxidase are the important antioxidants during early pregnancy, when exposure to tobacco smoke occurs, whereas the pregnancy itself is associated with a higher concentration of metallothionein and activity of catalase. Both pregnancy in the first trimester and exposure to tobacco smoke decrease glutathione concentration. In addition, active and passive maternal smoking have a similarly negative effect on antioxidant status in the first trimester. Early pregnancy as well as exposure to tobacco smoke is associated with significant alteration in antioxidant status and copper, zinc, and cadmium concentration. Due to a small number of smoking subjects (11 cases of non-pregnant, active smokers and 14 pregnant active smokers), the obtained results should be treated as a pilot, and this should be considered for future studies.

Zinc supplementation for improving pregnancy and infant outcome

BACKGROUND

It has been suggested that low serum zinc levels may be associated with suboptimal outcomes of pregnancy, such as prolonged labour, atonic postpartum haemorrhage, pregnancy-induced hypertension, preterm labour and post-term pregnancies, although these associations have not yet been established. This is an update of a review first published in 1997 and subsequently updated in 2007, 2012 and 2015.

OBJECTIVES

1. To compare the effects on maternal, fetal, neonatal and infant outcomes in healthy pregnant women receiving zinc supplementation versus no zinc supplementation, or placebo. 2. To assess the above outcomes in a subgroup analysis reviewing studies performed in women who are, or are likely to be, zinc-deficient.

SEARCH METHODS

For this update, we searched Cochrane Pregnancy and Childbirth's Trials Register, ClinicalTrials.gov, the WHO International Clinical Trials Registry Platform (ICTRP) (3 July 2020), and reference lists of retrieved studies.

SELECTION CRITERIA

Randomised trials of zinc supplementation versus no zinc supplementation or placebo administration during pregnancy, earlier than 27 weeks' gestation. We excluded quasi-randomised controlled trials. We intended to include studies presented only as abstracts, if they provided enough information or, if necessary, by contacting authors to analyse them against our criteria; we did not find any such studies.

DATA COLLECTION AND ANALYSIS

Three review authors applied the study selection criteria, assessed trial quality and extracted data. When necessary, we contacted study authors for additional information. We assessed the certainty of the evidence using GRADE.

MAIN RESULTS

For this update, we included 25 randomised controlled trials (RCTs) involving over 18,000 women and their babies. The overall risk of bias was low in half of the studies. The evidence suggests that zinc supplementation may result in little or no difference in reducing preterm births (risk ratio (RR) 0.87, 95% confidence interval (CI) 0.74 to 1.03; 21 studies, 9851 participants; low-certainty evidence). Further, zinc supplementation may make little or no difference in reducing the risk of stillbirth (RR 1.22, 95% CI 0.80 to 1.88; 7 studies, 3295 participants; low-certainty evidence), or perinatal deaths (RR 1.10, 95% CI 0.81 to 1.51; 2 studies, 2489 participants; low-certainty evidence). It is unclear whether zinc supplementation reduces neonatal death, because the certainty of the evidence is very low. Finally, for other birth outcomes, zinc supplementation may make little or no difference to mean birthweight (MD 13.83, 95% CI -15.81 to 43.46; 22 studies, 7977 participants; low-certainty evidence), and probably makes little or no difference in reducing the risk of low birthweight (RR 0.94, 95% CI 0.79 to 1.13; 17 studies, 7399 participants; moderate-certainty evidence) and small-for-gestational age babies when compared to placebo or no zinc supplementation (RR 1.02, 95% CI 0.92 to 1.12; 9 studies, 5330 participants; moderate-certainty evidence). We did not conduct subgroup analyses, as very few studies used normal zinc populations.

AUTHORS' CONCLUSIONS

There is not enough evidence that zinc supplementation during pregnancy results in improvements in maternal or neonatal outcomes. Future research to address ways of improving the overall nutritional status of pregnant women, particularly in low-income regions, and not looking at zinc in isolation, should be an urgent priority.

Comparison of birth weights of neonates of mothers receiving vs. not receiving zinc supplement at pregnancy

Background

Zinc is an essential element for normal embryogenesis and embryonic and neonatal development. Therefore, we compared the birth weights of neonates born to mothers who consumed zinc supplement during pregnancy with that of neonates born to mothers who did not.

Methods

In a cross-sectional study, we divided 200 pregnant mothers into two groups: case group (mothers receiving zinc supplement during pregnancy) and control group (mothers not receiving zinc supplement during pregnancy) Then, the neonate’s cord zinc level and mother’s serum level were measured and neonate’s growth charts (weight, height and head circumference)were completed.

Results

In this study, both groups of mothers were observed to have zinc deficiency; 35% of the mothers who consumed zinc supplements and 81% of the mothers who did not consume zinc supplements (P < 0.001). Based on the results, maternal serum of zinc (P < 0.001), neonatal birth weight (P = 0.008), maternal age (P < 0.001) and parity (P < 0.01) in zinc-supplemented group were higher. Neonatal birth weight was associated moderately with mother’s zinc serum levels and poorly with neonatal serum zinc levels.

Conclusion

Zinc consumption during pregnancy increases serum zinc level of mother and neonatal weight. Neonatal weight has a higher correlation to maternal serum zinc level.

Prenatal Zinc and Vitamin A Reduce the Benefit of Iron on Maternal Hematologic and Micronutrient Status at Delivery in Tanzania

BACKGROUND

Zinc and vitamin A supplementation have both been shown to affect iron status, hemoglobin (Hb) concentration, and anemia in animal and human studies. However, evidence on their combined use in pregnancy, in the context of iron-folic acid (IFA) supplementation, remains limited.

OBJECTIVE

This study determined the effects of prenatal zinc, vitamin A, and iron supplementation on maternal hematologic and micronutrient status at delivery in Tanzania.

METHODS

We analyzed 2 large randomized controlled trials, using generalized estimating equations, and examined the effect of daily zinc (25 mg) and vitamin A (2500 IU) supplementation starting in the first trimester of pregnancy compared with placebo (n = 2500), and separately evaluated the safety and efficacy of daily iron (60 mg) supplementation among iron-replete pregnant women (n = 1500). Blood samples from baseline and delivery were tested for Hb, serum ferritin, soluble transferrin receptor, plasma zinc, and zinc protoporphyrin.

RESULTS

Zinc and vitamin A supplementation were associated with lower Hb concentrations at delivery of  -0.26 g/dL (95% CI: -0.50, -0.02 g/dL) and -0.25 g/dL (95% CI: -0.49, -0.01 g/dL), respectively. Vitamin A increased mean ferritin concentrations at delivery (14.3 μg/L, 95% CI: 1.84, 29.11 μg/L), but was associated with increased risk of severe anemia (RR: 1.41; 95% CI: 1.06, 1.88). Among women who were iron replete at baseline, iron supplementation reduced the risk of iron depletion at delivery by 47% (RR: 0.53; 95% CI: 0.43, 0.65). There was no effect of zinc or iron supplements on plasma zinc concentrations.

CONCLUSIONS

Our findings support existing WHO guidelines on prenatal iron, vitamin A, and zinc supplementation among pregnant women. In this setting, scaling uptake of prenatal iron supplements is warranted, but prenatal zinc and vitamin A supplementation did not benefit maternal hematologic status at delivery. In settings where vitamin A deficiency is endemic, the efficacy and safety of the WHO recommended prenatal vitamin A supplementation require further evaluation.

Effects of supplementing late-gestation sow diets with zinc on preweaning mortality of pigs under commercial rearing conditions

The objective of this experiment was to determine preweaning survival of pigs when sows were supplemented with 3 dietary levels of zinc (Zn) in late gestation. Gilts and sows (n = 339) were assigned to 1 of 3 dietary treatments based on parity. Dietary treatments were 1) Control—sows fed a corn–soybean meal-based diet containing 125 ppm total supplemental Zn supplied by ZnSO₄ (75 ppm Zn) and AvailaZn (50 ppm Zn, CON); 2) Intermediate—as Control + 240 ppm supplemental Zn as ZnSO₄ (INT); and 3) High—as Control + 470 ppm supplemental Zn as ZnSO₄ (HI). Final supplemental Zn concentrations of the 3 dietary treatments were 1) CON—125 ppm; 2) INT—365 ppm; and 3) HI—595 ppm. Sows received dietary treatments from about day 85 of gestation until farrowing. Individual piglet birth weights were recorded within 12 h of parturition. Instances of piglet mortality were recorded daily. The statistical model considered fixed effects of treatment and random effects of parity. Piglets from sows fed the INT diet had heavier (P < 0.05) birth weights than those fed CON (1.42 vs. 1.38 kg, respectively), while offspring from sows fed HI tended to have heavier (P < 0.10) birth weights (1.40 kg) than pigs from INT sows. Furthermore, incidence of low birth weight pigs was less (P < 0.05) for sows consuming INT compared with sows fed CON and HI. Despite differences in birth weight, there were no differences (P > 0.05) in total pigs born, born alive, or weaned, nor differences in individual piglet gain or weaning weight across treatments. Mortality of low birth weight pigs was lowest (P < 0.05) for offspring from sows fed HI (28.1%) compared with offspring from sows fed INT (36.1%) and CON (38.3%). Similarly, overall piglet mortality tended to decrease (P < 0.10) as dietary Zn content increased (CON: 15.0%, INT: 13.2%, and HI: 12.2%). A subset of pigs (n = 420, n = 140/treatment) were selected at weaning to evaluate effects of dietary treatment on postweaning performance. There were no significant effects of sow Zn supplementation on final body weight, days to market, or carcass characteristics of market pigs. Overall, effects of supplemental dietary Zn at 365 and 595 ppm in late gestation improved preweaning survival of low birth weight piglets and reduced overall preweaning mortality of piglets.

Influence of tobacco smoke on zinc, cadmium, iron, iron-binding proteins, and low-weight anti-oxidant status in pregnancy

Pregnancy and tobacco smoking (TS) each can cause increases in reactive oxygen species (ROS) production; this, in turn, can lead to disorders in iron management and disruption of the pro- and anti-oxidant balance. The aim of the study was to analyze the influence of TS and Cd on Fe, Zn, and anti-oxidant levels (i.e. glutathione [GSH], metallothionein [MT]) in the blood of pregnant women. The study reported here evaluated 110 blood samples from pregnant women in their 1st, 2nd and 3rd trimester. Concentrations of ferritin and transferrin were measured in the serum; Zn, Fe and cotinine in the plasma, that of Cd in whole blood, that for glutathione in red blood cell lysates, and levels of metallothionein both in the plasma and in lysates prepared from isolated erythrocytes. The results indicated there was a decrease in Zn and increase in Cd and metallothionein levels in pregnant women smokers as compared to in nonsmoking counterparts. Differences in intracellular MT concentration were noted both in smoking and nonsmoking women during pregnancy while there were no changes in extracellular MT level. A decline in circulating ferritin and a rise in transferrin during pregnancy was observed in all groups. Based on the results, it was concluded that exposure to TS-associated xenobiotics like Cd could result in higher MT levels in erythrocytes and in pregnant smokers, the major anti-oxidant mechanism that is in place is one being mediated by MT and not by reduced GSH.

Effect of maternal zinc supplementation on the cardiometabolic profile of Peruvian children: results from a randomized clinical trial

Zinc is an essential micronutrient for the development of the fetal renal, cardiovascular and metabolic systems; however, there is limited evidence of its effects on the postnatal cardiometabolic function. In this study, we evaluated the effect of maternal zinc supplementation during pregnancy on the cardiometabolic profile of the offspring in childhood. A total of 242 pregnant women were randomly assigned to receive a daily supplement containing iron+folic acid with or without zinc. A follow-up study was conducted when children of participating mothers were 4.5 years of age to evaluate their cardiometabolic profile, including anthropometric measures of body size and composition, blood pressure, lipid profile and insulin resistance. No difference in measures of child cardiometabolic risk depending on whether mothers received supplemental zinc during pregnancy. Our results do not support the hypothesis that maternal zinc supplementation reduces the risk of offspring cardiometabolic disease.

Fluctuation of potential zinc status biomarkers throughout a reproductive cycle of primiparous and multiparous sows

Fluctuations in Zn metabolism throughout gestation and lactation might affect Zn requirements. However, scientific data on Zn requirements for breeding sows are limited. The objective of the present study was to assess the Zn status of primiparous and multiparous sows using different Zn status biomarkers, to identify periods of critical Zn status throughout the reproductive cycle at different parities. Blood samples were taken after overnight fasting before feeding in the morning from five primiparous and ten multiparous sows at fixed time intervals during gestation (days - 5, 0 (insemination), 21, 42, 63 and 84), around parturition (days 108, 112, 115 (parturition) and 118) and during lactation (days 122, 129 and 143 (weaning)). At parturition, blood samples were collected from two randomly selected piglets per sow before colostrum intake. Plasma was analysed for Zn and Cu contents, whereas serum was analysed for alkaline phosphatase, metallothionein and albumin concentrations. Independently of parity, all biomarkers fluctuated differently during gestation and lactation (P< 0·050). This reflects their different roles in Zn metabolism, and suggests that the choice of a Zn status biomarker necessitates careful consideration. Low average plasma Zn concentration at the end of gestation and throughout lactation seem to be replenished towards weaning.

Zinc supplementation for improving pregnancy and infant outcome

BACKGROUND

It has been suggested that low serum zinc levels may be associated with suboptimal outcomes of pregnancy such as prolonged labour, atonic postpartum haemorrhage, pregnancy-induced hypertension, preterm labour and post-term pregnancies, although many of these associations have not yet been established.

OBJECTIVES

To assess the effects of zinc supplementation in pregnancy on maternal, fetal, neonatal and infant outcomes.

SEARCH METHODS

We searched the Cochrane Pregnancy and Childbirth Group's Trials Register (31 October 2014) and reference lists of retrieved studies.

SELECTION CRITERIA

Randomised trials of zinc supplementation in pregnancy. We excluded quasi-randomised controlled trials.

DATA COLLECTION AND ANALYSIS

Three review authors applied the study selection criteria, assessed trial quality and extracted data. When necessary, we contacted study authors for additional information. The quality of the evidence was assessed using GRADE.

MAIN RESULTS

We included 21 randomised controlled trials (RCTs) reported in 54 papers involving over 17,000 women and their babies. One trial did not contribute data. Trials were generally at low risk of bias. Zinc supplementation resulted in a small reduction in preterm birth (risk ratio (RR) 0.86, 95% confidence interval (CI) 0.76 to 0.97 in 16 RCTs; 16 trials of 7637 women). This was not accompanied by a similar reduction in numbers of babies with low birthweight (RR 0.93, 95% CI 0.78 to 1.12; 14 trials of 5643 women). No clear differences were seen between the zinc and no zinc groups for any of the other primary maternal or neonatal outcomes, except for induction of labour in a single trial. No differing patterns were evident in the subgroups of women with low versus normal zinc and nutrition levels or in women who complied with their treatment versus those who did not. The GRADE quality of the evidence was moderate for preterm birth, small-for-gestational age, and low birthweight, and low for stillbirth or neonatal death and birthweight.

AUTHORS' CONCLUSIONS

The evidence for a 14% relative reduction in preterm birth for zinc compared with placebo was primarily represented by trials involving women of low income and this has some relevance in areas of high perinatal mortality. There was no convincing evidence that zinc supplementation during pregnancy results in other useful and important benefits. Since the preterm association could well reflect poor nutrition, studies to address ways of improving the overall nutritional status of populations in impoverished areas, rather than focusing on micronutrient and or zinc supplementation in isolation, should be an urgent priority.

The influence of biological and environmental factors on metallothionein concentration in the blood

The concentration of metallothionein (MT), a low-molecular-weight protein, is regulated by many factors, primarily metals (zinc, cadmium, copper), cytokines, glucocorticoides and free radicals. These factors are determined by such aspects of human biology as gender, pregnancy and age, as well as by environmental factors including the use of oral contraceptives and cigarette smoking, all which may affect MT levels in the body. The aim of this study was to investigate the influence of these biological and environmental factors on MT concentrations in erythrocyte lysate and in plasma. MT concentrations were determined by a two-step direct enzyme-linked immunosorbent assay. Evaluation of exposure to cigarette smoking was performed by checking cotinine levels in the plasma of subjects. The studies showed higher MT concentrations in both the erythrocyte lysate and plasma of women when compared to men. Furthermore, pregnancy causes an increase of MT concentration in plasma, while oral contraceptives cause an elevated concentration of MT in erythrocyte lysate. Age impacts plasma MT concentrations in men, whereas it does not affect concentrations of MT in erythrocyte lysate.

Zinc supplementation for improving pregnancy and infant outcome

BACKGROUND

It has been suggested that low serum zinc levels may be associated with suboptimal outcomes of pregnancy such as prolonged labour, atonic postpartum haemorrhage, pregnancy-induced hypertension, preterm labour and post-term pregnancies, although many of these associations have not yet been established.

OBJECTIVES

To assess the effects of zinc supplementation in pregnancy on maternal, fetal, neonatal and infant outcomes.

SEARCH METHODS

We searched the Cochrane Pregnancy and Childbirth Group's Trials Register (30 September 2011) and reference lists of retrieved studies.

SELECTION CRITERIA

Randomised trials of zinc supplementation in pregnancy. We excluded quasi-randomised controlled trials.

DATA COLLECTION AND ANALYSIS

Three review authors applied the study selection criteria, assessed trial quality and extracted data. When necessary, we contacted study authors for additional information.

MAIN RESULTS

We included 20 randomised controlled trials (RCTs) reported in 51 papers involving over 15,000 women and their babies. Trials were generally at low risk of bias. Zinc supplementation resulted in a small but significant reduction in preterm birth (risk ratio (RR) 0.86, 95% confidence interval (CI) 0.76 to 0.97 in 16 RCTs; 16 trials of 7637 women). This was not accompanied by a similar reduction in numbers of babies with low birthweight (RR 0.93, 95% CI 0.78 to 1.12; 14 trials of 5643 women). No significant differences were seen between the zinc and no zinc groups for any of the other primary maternal or neonatal outcomes, except for induction of labour in a single trial. No differing patterns were evident in the subgroups of women with low versus normal zinc and nutrition levels or in women who complied with their treatment versus those who did not.

AUTHORS' CONCLUSIONS

The evidence for a 14% relative reduction in preterm birth for zinc compared with placebo was primarily represented by trials involving women of low income and this has some relevance in areas of high perinatal mortality. There was no convincing evidence that zinc supplementation during pregnancy results in other useful and important benefits. Since the preterm association could well reflect poor nutrition, studies to address ways of improving the overall nutritional status of populations in impoverished areas, rather than focusing on micronutrient and or zinc supplementation in isolation, should be an urgent priority.

Maternal zinc intakes and homeostatic adjustments during pregnancy and lactation

Zinc plays critical roles during embryogenesis, fetal growth, and milk secretion, which increase the zinc need for pregnancy and lactation. Increased needs can be met by increasing the dietary zinc intake, along with making homeostatic adjustments in zinc utilization. Potential homeostatic adjustments include changes in circulating zinc, increased zinc absorption, decreased zinc losses, and changes in whole body zinc kinetics. Although severe zinc deficiency during pregnancy has devastating effects, systematic reviews and meta-analysis of the effect of maternal zinc supplementation on pregnancy outcomes have consistently shown a limited benefit. We hypothesize, therefore, that zinc homeostatic adjustments during pregnancy and lactation improve zinc utilization sufficiently to provide the increased zinc needs in these stages and, therefore, mitigate immediate detrimental effects due to a low zinc intake. The specific questions addressed are the following: How is zinc utilization altered during pregnancy and lactation? Are those homeostatic adjustments influenced by maternal zinc status, dietary zinc, or zinc supplementation? These questions are addressed by critically reviewing results from published human studies on zinc homeostasis during pregnancy and lactation carried out in different populations worldwide.

Effect of zinc supplementation on pregnancy and infant outcomes: a systematic review

Poor maternal zinc status has been associated with foetal loss, congenital malformations, intra-uterine growth retardation, reduced birth weight, prolonged labour and preterm or post-term deliveries. A meta-analysis completed in 2007 showed that maternal zinc supplementation resulted in a small but significant reduction in preterm birth. The purposes of this analysis are to update that previous review and expand the scope of assessment to include maternal, infant and child health outcomes. Electronic searches were carried out to identify peer-reviewed, randomised controlled trials where daily zinc supplementation was given for at least one trimester of pregnancy. The co-authors applied the study selection criteria, assessed trial quality and abstracted data. A total of 20 independent intervention trials involving more than 11,000 births were identified. The 20 trials took place across five continents between 1977 and 2008. Most studies assessed the zinc effect against a background of other micronutrient supplements, but five were placebo-controlled trials of zinc alone. The provided dose of supplemental zinc ranged from 5 to 50 mg/day. Only the risk of preterm birth reached statistical significance (summary relative risk 0.86 [95% confidence interval 0.75, 0.99]). There was no evidence that supplemental zinc affected any parameter of foetal growth (risk of low birth weight, birth weight, length at birth or head circumference at birth). Six of the 20 trials were graded as high quality. The evidence that maternal zinc supplementation lowers the risk of preterm birth was graded low; evidence for a positive effect on other foetal outcomes was graded as very low. The effect of zinc supplementation on preterm birth, if causal, might reflect a reduction in maternal infection, a primary cause of prematurity. While further study would be needed to explore this possibility in detail, the overall public health benefit of zinc supplementation in pregnancy appears limited.

The relationship between zinc intake and serum/plasma zinc concentration in pregnant and lactating women: a systematic review with dose-response meta-analyses

Recommendations for zinc intake during pregnancy and lactation vary widely across Europe. Using data on zinc intake and biomarkers of zinc status reported in randomized controlled trials (RCTs) and observational studies can provide estimates of dose-response relationships that may be used for underpinning zinc reference values. This systematic review included all RCTs, prospective cohort studies, nested case-control studies and cross-sectional studies in healthy pregnant and lactating populations published by February 2010 which provided data on zinc intake and biomarkers of zinc status. An intake-status regression coefficient (βˆ) was calculated for each individual study and calculated the overall pooled βˆ and SE (βˆ) using random effects meta-analysis on a double log scale. The pooled dose-response relationship between zinc intake and zinc status found that a doubling of zinc intake was associated with an increase in serum/plasma zinc status by 3% in pregnant women and by 1% in lactating women. These modest associations are likely to reflect the low-moderate zinc bioavailability dietary patterns and the widespread use of other micronutrients in the populations included in this review, physiologic adjustments of zinc homeostasis, insensitivity of serum/plasma zinc as a biomarker of zinc status, and wide heterogeneity between study results which reflect real uncertainty in the current evidence base. Although this review provides useful information for dietary zinc requirements in populations vulnerable to zinc deficiency, it also highlights a need for further studies in pregnant and lactating women with different dietary patterns in order to provide useful complementary evidence that can be utilized when setting zinc recommendations as a basis for nutrition policies in Europe.

Maternal zinc supplementation during pregnancy affects autonomic function of Peruvian children assessed at 54 months of age

Maternal prenatal zinc supplementation improved fetal autonomic regulation in a nutrient-deficient population in Peru. To evaluate whether differences in autonomic regulation existed in early childhood, we studied 165 children from a zinc supplementation trial (80% of original sample) as part of a comprehensive evaluation at age 54 mo. Electrocardiogram (ECG) data were collected from the children at rest and while they underwent a cognitive testing battery following a standardized protocol. Of these, 79 were born to mothers receiving 25 mg/d zinc in addition to 60 mg/d iron and 250 μg/d folic acid during pregnancy, and 86 were born to mothers receiving iron and folic acid only. Derived cardiac measures included heart period (HP), range, HP variability (HPV), mean square of successive differences (MSSD), and a measure of vagal tone (V). Children in the zinc supplementation group had greater HP (i.e. slower heart rate), greater range, higher time-independent (HPV) and time-dependent (MSSD) variability in HP, and higher V (P < 0.05) during baseline. Analyses conducted across the cognitive testing period revealed similar effects of prenatal zinc supplementation on cardiac patterns. Concurrent child zinc plasma concentration was also associated with longer HP, greater variability, and marginally higher range and V (P < 0.10). Differences in cardiac patterns due to prenatal zinc supplementation were detectable in children at 54 mo of age during conditions of both rest and challenge, indicating that supplementing zinc-deficient pregnant women has beneficial long-term consequences for neural development associated with autonomic regulation.

Zinc supplementation for improving pregnancy and infant outcome

BACKGROUND

It has been suggested that low serum zinc levels may be associated with suboptimal outcomes of pregnancy such as prolonged labour, atonic postpartum haemorrhage, pregnancy-induced hypertension, preterm labour and post-term pregnancies, although many of these associations have not yet been established.

OBJECTIVES

To assess the effects of zinc supplementation in pregnancy on maternal, fetal, neonatal and infant outcomes.

SEARCH STRATEGY

We searched the Cochrane Pregnancy and Childbirth Group's Trials Register (February 2007).

SELECTION CRITERIA

Randomised or quasi-randomised trials of zinc supplementation in pregnancy.

DATA COLLECTION AND ANALYSIS

Two review authors applied the study selection criteria, assessed trial quality and extracted data. When necessary, study authors were contacted for additional information.

MAIN RESULTS

We included 17 randomised controlled trials (RCTs) involving over 9000 women and their babies. Zinc supplementation resulted in a small but significant reduction in preterm birth (relative risk (RR) 0.86, 95% confidence interval (CI) 0.76 to 0.98 in 13 RCTs; 6854 women). This was not accompanied by a similar reduction in numbers of babies with low birthweight (RR 1.05 95% CI 0.94 to 1.17; 11 studies of 4941 women). No significant differences were seen between the zinc and no zinc groups for any of the other primary maternal or neonatal outcomes, except for a small effect favouring zinc for caesarean section (four trials with high heterogeneity) and for induction of labour in a single trial. No differing patterns were evident in the subgroups of women with low versus normal zinc and nutrition levels or in women who complied with their treatment versus those who did not.

AUTHORS' CONCLUSIONS

The 14% relative reduction in preterm birth for zinc compared with placebo was primarily in the group of studies involving women of low income and this has some relevance in areas of high perinatal mortality. There was no convincing evidence that zinc supplementation during pregnancy results in other useful and important benefits. Since the preterm association could well reflect poor nutrition, studies to address ways of improving the overall nutritional status of populations in impoverished areas, rather than focusing on micronutrient and or zinc supplementation in isolation, should be an urgent priority.