Introduction

Heavy metals widely exist in our environment because they are difficult to degrade. Heavy metals may cause adverse health effects when converted into the food chain and accumulate in our body (Brender et al. 2006; Kolachi et al. 2011; Lin et al. 2014). Recent studies have revealed that lead (Pb) and mercury (Hg) exposure may contribute to an increased prevalence of dyslipidemia (Liu et al. 2022) whereas cadmium (Cd) exposure is related to the prevalence of coronary heart disease and stroke burden (Kim et al. 2022) in adults. For glucose balance, metals are involved in glucose metabolism, and arsenic (As) and Pb may alter glycemic regulation and are related to diabetes in women (Wang et al. 2020a). In addition, early pregnancy with exposure to barium (Ba) and Hg is associated with altered post-load glucose concentrations in later pregnancy (Zheng et al. 2021), and Cd has been related to diabetes (Filippini et al. 2022) and gestational diabetes (Zhou et al. 2022). A review article has highlighted that Pb, Cd, As, and Hg may produce reactive oxygen species; induce oxidative stress in the trophoblastic placental tissue; and lead to preterm birth (Singh et al. 2020). In addition, a case–control study showed that pregnant women exposed to chromium (Cr) and As exhibit an excessive association with preeclampsia (Wang et al. 2020c). Reducing the exposure of women to metals, especially Cd and Pb, during pregnancy is important because of its possible health impact (Osorio-Yanez et al. 2021). Pb exposure is harmful to the reproductive system in male and female; it can cause female infertility, abortion, premature birth, gestational hypertension, and inhibit embryonic growth and development (ATSDR 2019). In addition, As exposure of pregnant women might cause abortion and influence embryonic development (Vahter 2008). Meanwhile, a cohort study reported that maternal exposure to Hg, Cd, and cesium (Cs) in the first trimester is related to thyroid-stimulating hormone levels in mothers and newborns (Wang et al. 2020b). Moreover, the levels of As, Cd, Pb, Hg, aluminum (Al), tin (Sn), manganese (Mn), and antimony (Sb) are significantly higher in offspring/newborns with neural tube defects than in those without these conditions (Brender et al. 2006; Ovayolu et al. 2020; Ozel et al. 2019). Therefore, measuring the blood metal concentrations in reproductive-age women is important to promote the health of pregnant women and their newborns.

Human biomonitoring (HBM) is a method of measuring the chemicals, metabolites, or degraded compounds in body fluids, including blood, urine, saliva, sweat, excrement, hair, and nail; as it represents the total burden of the body after exposure and reflects the overall metabolism of the body dependent on personal exposure, metabolization, and excretion (Ladeira and Viegas 2016; WHO 2015). The National Health and Nutrition Examination Survey (NHANES) is the largest national survey for HBM in the United States, and their data showed an over 70% decrease in blood Pb levels in the general population in the past 20 years (CDC 2017). The Canadian Health Measures Survey also observed decreasing trends for Pb, Cd, Hg, and methylmercury in blood and As in urine from 2007 to 2019 (Health Canada 2013; 2015; 2019; 2021). In Europe, a study called “Consortium to Perform Human Biomonitoring on a European Scale” in 17 countries performed HBM in the urine of 1844 children (5–11 years of age) and their mothers in 2011–2012. Results showed that their biomarker concentrations are below the health-based guidance values and that the Hg and Cd levels are highly correlated between the mothers and their children (Joas et al. 2012; Den Hond et al. 2015). In Japan, a follow-up study measured Pb, Hg, Cd, Mn, and selenium (Se) concentrations in the blood of 20,000 pregnant women, and results showed that the blood Pb levels decreased 5–10 folds in the past 25 years (Nakayama et al. 2019). In China, the levels of Hg, Cd, and As in the plasma of women showed a 30%–40% decrease from 2005 to 2012 in a historically contaminated area in China (Jin et al. 2020). Overall, a decreasing trend of blood element levels was found in most countries worldwide.

The factors influencing high Hg body burden are agriculture and high-frequency water product consumption; high consumption of fresh fruits and eggs is also associated with high levels of Cd in the plasma (Jin et al. 2020). High seafood consumption contributes to elevated levels of As and Hg in the ovarian follicular fluid (FF) of women undergoing in vitro fertilization; in addition, diet source might contribute to As, Hg, Cd, and Pb in FF (Butts et al. 2020). A previous study showed that fish consumption is a predictor of blood Hg level (Lee et al. 2012; Nakayama et al. 2019). In addition, maternal age and alcohol drinks are predictors of blood Pb, whereas maternal age and smoking are predictors of Cd concentration (Nakayama et al. 2019). Overall, the dietary pattern of pregnant women is responsible for metal accumulation in maternal exposure and affects the performance of newborns.

The primary public health concern is related to maternal exposure to metals and how the high exposure to metals can be linked to offspring development, especially when these metals are ingested and bio-accumulated through different types of food, such as fish and other marine foods, which are popular and frequently consumed in Taiwan. Therefore, this study aims to evaluate the potential factors associated with metal accumulation in mothers or reproductive-age women. Preventive risk communication and educational strategies can be applied to these subgroups in view of food safety and public health concerns.

Materials and Methods

Participant Enrollment

This study recruited 849 women of reproductive-age between November 2019 and October 2020 from four medical centers and two universities around Taiwan. Inclusion criteria included living in the current area over one year and aged 20–49 years on the day of the questionnaire interview. Exclusion criteria included those with occupational exposure or living near industrial areas with metal exposure. Setting of this exclusion criteria according to the occupational and environmental exposures have been well-known to be linked to bioaccumulation in human body (Jin et al. 2020), therefore, the aim of this study was to try to find the major influencing factors in metals accumulation through food consumption. Finally, 837 women completed the questionnaire interviews and blood samplings in this study.

A stratified random sampling design was applied to recruit participants. The stratification was based on geographic regions in Taiwan, which were divided into northern, central, southern, and eastern Taiwan. The sample size of each region was inferred from the proportion of women aged 20–49 years in stratified regions to the total female population in Taiwan, which was recorded in the Ministry of Health and Welfare, Taiwan. Oversampling was implemented in the eastern region because of the relatively low density of women.

The participants enrolled in this study signed the informed consent form, which was approved by the Ethics Committee of the National Cheng Kung University Hospital (Tainan, Taiwan, encoded: A-ER-108-433), the Chung Shan Medical University Hospital (Taichung, Taiwan, encoded: CS2-20022), the Taipei Veterans General Hospital (Taipei, Taiwan, encoded: 2020-03-008B), and Hualien Tzu Chi Hospital, Buddhist Tzu Chi Medical Foundation (Hualien, Taiwan, encoded: IRB109-077-B). The questionnaire interviews and blood samplings were conducted for each participant.

Interviewer-Administered Questionnaire

Demographic information was obtained using face-to-face or online questionnaires. The questionnaires consisted of five parts: personal characteristics (age, occupation, education, pregnant status, etc.), pollution sources, personal living habits (cigarette use, secondhand smoke exposure, burning incense, cosmetics, amalgam, etc.), dietary pattern within a month, and family and personal disease histories. A food frequency questionnaire was used to collect consumption information based on intake frequency, such as how many times daily, weekly, or monthly. The fish item was grouped as large-sized fish (tuna, salmon, swordfish, codfish, etc.), small-sized fish, freshwater fish (mackerel, pomfret, etc.), cephalopods (squid, cuttlefish, etc.), shrimp, shellfish, oysters, and echinoderms (sea cucumber, etc.). Quantitative data for intake of milk and other drinks were calculated based on a cup, spoon for oil and sauces, and bowls for other food items.

Whole Blood Sampling and Analysis of Blood Heavy Metal Concentrations

Blood samples (8 mL) of the participants were collected by licensed nurses in vacuum tubes with K2EDTA and then stored at − 20 °C until analysis. The metal levels in the blood samples, including As, Cd, Cr, Hg, and Pb, were measured through inductively coupled plasma mass spectrometry (ICP-MS, Nexion 2000, PerkinElmer). A final volume of 4 mL was obtained by mixing 200 μL blood samples with 0.1% Triton X-100 and 0.1% HNO3 and then centrifuging at 3,000 rpm for 10 min. The ICP-MS system was used to analyze the metal concentrations of the suspension. Matrix-matched calibration curves ranging from 0.01 to 20 μg/L were built for blood samples in the ICP-MS analysis to reduce the matrix effects in the blood samples. The standard reference material 995d (SRM 995d), produced by the US National Institute of Science and Technology, was applied to evaluate the accuracy of determining essential elements of human blood. In this study, the recovery rates for the five metals were approximately 70%–120% in the applied SRM.

Statistical Analysis

The demographic characteristics were summarized using mean (standard deviation, SD) and number (%) for continuous and categorical variables, respectively. The distribution of blood metals levels was presented as median and range. Independent sample t and Chi-square tests were applied to compare the demographic characteristics between pregnant and nonpregnant women. The Mann–Whitney U test and Kruskal–Wallis test were used to assess the differences in blood metal distributions between different groups. Spearman’s rank correlation coefficients were used to evaluate correlations among different metal levels and continuous variables, including ages and dietary intake. Data were analyzed using IBM SPSS for Windows version 20.0 (IBM, Armonk, NY) and R 4.1.1.

Results

The demographics of 837 women are shown in Table 1. Four hundred and fifty-one (53.9%) participants were pregnant women. Of these women, 78.4% were aged between 25 and 39 years (average age = 33.2 years), and 44%, 24%, and 26% were living in northern, central, and southern Taiwan, respectively. In addition, 55% responded that they lived in the vicinity of polluted sources (vehicle exhaust, industrial emissions, night markets, etc.) within 2.5-km radius. Meanwhile, 34% of the participants responded that they used incense burning, 64% used cosmetic products, and 35% received dental amalgam. Significant differences in age, living areas, occupations, educational attainment, and incense burning between pregnant and nonpregnant were found in this study.

Table 1 Demographic characteristics of all participants
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The concentrations of Cd and Pb were significantly higher in nonpregnant women than in pregnant women (Cd: 2.41 µg/L vs.2.12 µg/L; Pb: 0.83 µg/dL vs. 0.73 µg/dL), whereas the concentration of Cr was significantly lower in nonpregnant women than in pregnant women (Cr: 0.98 µg/L vs. 1.05 µg/L). Otherwise, no significant differences in As (9.02 µg/L vs. 9.51 µg/L) and Hg (3.71 µg/L vs. 3.79 µg/L) were found between the nonpregnant and pregnant women (Table 2).

Table 2 Whole blood metal concentration of pregnant and nonpregnant women
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Table S2 shows the distribution of blood metal levels for childbearing-age women eating-specific food or not. The As and Hg levels were both significantly higher in the childbearing-age women eating brown rice, grain rice, offal, fish/seafood, leaf vegetables, fruits, and marine plants than in those who did not eat these types of food. Higher Pb levels were found in the childbearing-age women eating small fish, seafood, offal, leaf vegetables, and marine plants than those who did not eat these types of food. Additionally, the current study found significant differences in blood Pb levels between those consuming Chinese medicine and those not consuming these medicines. Meanwhile, no significant differences were found in the Cd and Cr blood metal levels of childbearing-age women with different dietary patterns. Furthermore, the blood metal levels of childbearing-age women from low to high consumption in specific food are shown in Fig. 1. For As accumulation, the significant differences of As levels among their consumption quantity from less than Q1 to over Q3 groups were only found for brown rice, large fish, and marginal for freshwater fish; for Hg, the significant differences were found for large fish, small fish, freshwater fish, and cephalopods; for Pb, the marginal significant difference was only found for leaf crops in reproductive-age women.

Fig. 1
figure 1figure 1

The blood metal levels of reproductive-age women from low to high consumption in specific food. The significant difference was shown as *p-value < 0.05 and **p-value < 0.001 by Kruskal–Wallis test

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The distribution of blood metal levels in alternative trimester pregnant stages were further evaluated (Fig. 2). A significantly decreasing trend was found for Cd and Hg, and significant differences in Cd, Hg, and Pb were found among the alternative pregnancy stages. No significant differences in As and Cr were observed. Furthermore, As and Hg highly correlated with seafood intake (Fig. 3).

Fig. 2
figure 2

Trend test of blood metal levels of trimester pregnant stages. The different superscript letters are significantly different (p < 0.05) and with the same superscript letters are not significantly different (p ≥ 0.05) by Kruskal–Wallis test. Ɨp < 0.05 by Jonckheere–Terpstra test (trend test)

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Fig. 3
figure 3

Spearman correlation coefficient of metal levels of whole blood and their corresponding dietary intake of alternative foodstuffs

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Discussion

Element Concentrations in Pregnant and Nonpregnant Women

As, Hg, Pb, Cd, and Cr are not only harmful to adults and children but also affect the fetus and infants. Exposure to these metals might be associated with birth outcomes, such as low birth weight, low birth length, spontaneous abortion, stillbirth, preterm birth, and head circumference (Cempaka et al. 2019; Gokoel et al. 2020; Milton et al. 2017; Murcia et al. 2016; Nyanza et al. 2020; Shih et al. 2021, 2020; Wang et al. 2021; Xia et al. 2016). For instance, in a previous study, the placenta of participants that underwent spontaneous abortion were found to contain significantly higher levels of As, Cd, Hg, Pb, Sb, Sn, Se, Mn, and cobalt (Co) compared with that of the control group (Baser et al. 2020). However, with exception of Cd, all levels were below the risk levels defined by the health authority (35 µg/L for As, 10 µg/L for Cr, 10 µg/L for Hg, 2 µg/L for Cd, and 10 µg/dL for Pb in blood of adults) (Rios et al. 2020). The effects of metals on pregnant women with high burden must be considered because those metals may be delivered to the embryo through umbilical cord blood and placenta during pregnancy (Li et al. 2019; Rudge et al. 2009) and cause neural tube and birth size defects in offspring (Araujo et al. 2020; Brender et al. 2006; Kippler et al. 2012). For Taiwanese infants, breastfeeding has been promoted by the Health Promotion Administration, Ministry of Health and Welfare since 2001 (Chen et al. 2020). About 40% and 29% of Taiwanese mothers reported exclusive breastfeeding at one month and two months postpartum, respectively (Chang et al. 2019). Although the advantages of breastfeeding are well-known, heavy metal toxicity has received increased research attention (Chao et al. 2014; Dix-Cooper and Kosatsky 2018; Jeong et al. 2017b; Vahidinia et al. 2019; Lin et al. 2022).

In this study, we found significant differences in blood Cd, Cr, and Pb levels and no significant differences in blood As and Hg levels between pregnant and nonpregnant women. The current results were partially consistent with those of Gong et al. (2021), who also reported significant differences in Cr and Pb (but not Cd) and no significant differences in As and Hg in the whole blood between 192 healthy pregnant women and 210 healthy nonpregnant women in China. Another study found that pregnant women in the US NHANES who have been exposed to several trace metals showed lower levels of blood Cd and Pb than nonpregnant women (Watson et al. 2020), which were similar to the current results. By contrast, the present results showed that the pregnant women had higher levels of Hg, along with As and Cr, than the nonpregnant women as opposed to the findings of Watson et al. (2020). However, the trend of increasing Hg levels by increasing their dental amalgams cannot be evidenced in the childbearing-age women (Table S1).

Blood Metal Levels Compared with Other Countries

We compared the blood essential element/metals in pregnant and nonpregnant women with measurements obtained from other countries (Table S3). The average level of As in Taiwan was slightly lower than that in the early report of Lei et al. (2015) in Taiwan, Brazil (Araujo et al. 2020), China (Wang et al. 2020c), and Iran (Vigeh et al. 2015), whereas the measured levels in this study were higher than those from other countries (Bank-Nielsen et al. 2019; Ettinger et al. 2017). The measurement data on Cd levels in the current study were lower than those in Egypt (Motawei and Gouda 2016) but higher than those in other countries (Lee et al. 2020; Ma et al. 2021; Pesce et al. 2021). The levels of Cr in the current study were lower than those obtained from China (Kim et al. 2020; Wang et al. 2020a) and Greenland (Bank-Nielsen et al. 2019). Our data on Hg were comparable with those obtained from Greenland (Bank-Nielsen et al. 2019), South Korea (Jeong et al. 2017a; Kim et al. 2013; Lee et al. 2021a,b, 2020; Song et al. 2016), and Japan (Kobayashi et al. 2019; Ma et al. 2021; Nakayama et al. 2019) but higher than those in other countries. For Pb, the current measured data were not higher than those found in other countries and slightly lower than the 2015 data in Taiwan (Lei et al. 2015). The blood metal concentrations varied in childbearing-age women worldwide. Overall, the levels of As, Hg, and Cd were higher in women from Taiwan than in those from other countries.

Association of Dietary Intake and Blood Metal Levels

Metals are emitted to the environment artificially because of industrialization and urbanization, and those metals might accumulate in humans through the soil, groundwater, atmosphere, and the food chain (Al-Saleh and Abduljabbar 2017). A cross-sectional study reported that age and body mass index are significantly negatively associated with plasma As and Cd (Ugwuja et al. 2020). Therefore, the different industrial developments in residential areas and the different demographic data of participants should be considered simultaneously.

As and Hg showed a similar accumulation trend in fish and seafood, and these metals accumulated in the pregnant women in this study. Given the high accumulation of inorganic As in crops and organic As in fish and seafood, crop intake was regarded as the main contributor and fish/seafood intake as the second contributor to As exposure (Ma et al. 2008; Nriagu and Lin 1995; Park and Lee 2013; Williams et al. 2007). On the other hand, previous studies have also shown that fish and crop are the main contributors to Hg exposure (Jiang et al. 2012; Li et al. 2010; Mahaffey et al. 2009), and the long sea line between Japan and Taiwan might be the major cause for the higher blood Hg levels of the residents eating large amounts of fish and seafood than those living in western countries (Bocca et al. 2020; de Figueiredo et al. 2020; Donohue et al. 2018; Park and Lee 2013). Furthermore, we found that Taiwanese breast milk donors who reported using incense had higher average breast milk levels of Hg and Pb compared with those that do not use incense in our recently published findings (Lin et al. 2022). This can be supported with the result of another study showing that incense burning affects the indoor Hg level (Shen et al. 2017). Thus, we suspect that incense usage might be another influencing factor for Hg exposure in Taiwanese reproductive-age women. Aside from the above different ways, human activity, such as coal-fired power plants, industrial processes, and coal combustions, are another major cause of Hg release and exposure. Hg released can enter aquatic and terrestrial systems, and the emitted inorganic Hg can be transformed to methyl Hg by bacteria, which can be absorbed by organisms, such as fish and crustaceans (Erickson and Lin 2015). Therefore, consumption of these marine organisms may result in the exposure and accumulation of methyl Hg in human bodies. Hg exposure causes neurological and immunological dysfunctions in adults and in development stage (Bjørklund et al. 2017; WHO 2017); hence, prenatal Hg exposure might cause adverse effects on pregnancy outcome (Vigeh et al. 2018). Overall, pregnant women and women of reproductive-age should avoid Hg exposure.

Pb is harmful in the central nervous system (Finkelstein et al. 1998; Zhou et al. 2020), renal function (Finkelstein et al. 1998), cardiovascular disease (Zheng et al. 2019), reproductive system (Sudjarwo and Giftania Wardani Sudjarwo 2017), and blood system (Alvarez-Ortega et al. 2019). The well-known sources of Pb exposure are mining, battery manufacturing, drinking water tubes, paint, makeup, and other consumer products (Cardoso et al. 2014; Nriagu et al. 1996; Piomelli 2002). Therefore, heavy metals can be found in the soil, water, air, and the food chain after disposal of those products. The exposure of Pb from daily life and occupation may enter the human body through inhalation of Pb particles discharged from industrial activities or ingestion of contaminated drinking water and food, which can be harmful to human health, especially for susceptible populations, such as child, infants, and pregnant women (Al Osman et al. 2019; Levallois et al. 2018). Furthermore, metal contaminants including As, Cd, Hg, and Pb are also regularly present in Chinese herbs according to previous findings (Cooper et al. 2007; Ernst and Coon 2001; Wang et al. 2019). It is noteworthy that a significantly higher Pb level in childbearing-age women consuming Chinese medicine was found in this study although the blood Pb levels of these women were not that high. Therefore, preventing the amount of Chinese medicine intake should still be given attention, especially for pregnant women.

Meanwhile, for Cd exposure, previous reports have mentioned that the main exposure route is tobacco exposure and food consumption in nonoccupational exposure (Järup 2003; UNEP 2010). However, Garner and Levallois (2016) reported that age and cigarette usage but not dietary intake are the major factors influencing Cd accumulation in Canadian population.

Association of Dietary Intake and Blood Metal Levels in Different Pregnancy Stages

A previous study collected urine samples during the first and third trimesters of pregnancy from 489 mothers living in a highly industrialized town in Spain. The concentrations of Mo, Se, Cd, Cs, and Sb were higher in the first than the third trimester, whereas the opposite trend was found for Co, Cu, and Zn (Fort et al. 2014). The decreasing Cd trend by the pregnancy stage was similar to the measurement of the current study. In contrast, the measured Cd level in 2738 pregnant Korean women was lower (1.41 µg/L) in the early pregnancy stage than in the later pregnancy stage (1.51 µg/L) (Jeong et al. 2017a). Furthermore, maternal blood Hg levels of Canadian and Japanese women also decreased significantly from first to third trimester of pregnancy (Morrissette et al. 2004; Vigeh et al. 2018), which is in accordance to the decreasing trend found in the current study. Meanwhile, for As, a previous study in Iran showed that the blood As levels increased from the first pregnancy stage to the third pregnancy stage (14.4 to 74.7 µg/L) (Vigeh et al. 2015), which is in opposition to our current findings that no significant differences in As levels were found among pregnancy stages. For Cr, our results agree with the findings of Gong et al. (2021), where no significant differences were found in the whole blood Cr concentrations among the different trimesters in Chinese pregnant women.

Furthermore, we assessed the correlations of metal levels of whole blood and their corresponding dietary intake of alternative foodstuffs. Results showed that the levels of As and Hg were highly correlated with seafood intake (Fig. 2), supporting the evidence that fish and seafood are the major contributors to As and Hg exposure (Jiang et al. 2012; Mahaffey et al. 2009; Park and Lee 2013; Williams et al. 2007). However, reasons responsible for the decreasing trends in Cd and Hg levels in the pregnancy stages of Taiwanese pregnant women are still unknown.

Strength and Limitations

The strength of this study was the recruitment of reproductive-age women, who may breastfeed their infants within a few hours after childbirth. Through this study, the participants can have knowledge of whether they have high metal accumulations and accordingly change their lifestyle or food consumption pattern to protect their infants from metal exposure through breastfeeding. However, one limitation of this study was that spot sample was obtained only once and the measured concentrations in the blood samples may not be enough to represent long-term exposure to all metals. Another limitation of this study was the potential issues of generalizability. In specific, we mainly focused on metal exposure through food consumption and excluded those with occupational or environmental exposures. However, the diet and lifestyle of those with occupational exposure versus those without may be different. To address these limitations, it is recommended for future studies that other human biological samples, such as urine or hair, can be additionally collected to assess metal exposure. In addition, metal exposure in reproductive-age women through other sources such as occupational and environmental exposures can be further clarified in future studies.

Conclusions

The concentrations of Cd and Pb were significantly higher in nonpregnant women than in pregnant women, whereas the concentration of Cr was significantly lower in nonpregnant women than in pregnant women. Although all levels in these childbearing-age women were lower than the risk levels defined by the health authority (except for Cd), the levels of As, Hg, and Cd were higher in women from Taiwan than in women from other countries. Moreover, the concentrations of As and Hg were highly correlated with seafood intake. Dietary diversification in addition to prevention of exposure is advocated (Ugwuja et al. 2020), and healthy diets and lifestyle, such as reducing burning incense usage and Chinese herb intake, should be advised for pregnant women. Prioritizing adequate nutrition and healthy dietary patterns in the risks and benefits of metals during pregnancy may play a key role for ensuring optimal health in a child. This topic warrants a follow-up in future studies.