Introduction

Trace elements are bioelements that play a significant role in keeping the body healthy.¹ The human body cannot produce them, and therefore, we need to obtain them at appropriate doses from our diet. Both deficiency and excess may have a negative impact on body functions and the body's needs vary depending on the stage of the growth. Nutrition is vital during pregnancy for both the expectant mother and the developing fetus. Notably, the nutritional requirements of pregnant women differ from those of non-pregnant women of the same age. They also vary according to the week of pregnancy, with few differences compared to non-pregnant women during the first trimester; demands increasing during the second trimester; and a sharp rise during the third trimester, due to the rapid development of the fetus. Daily dietary reference intakes (DRIs) for pregnant women in Spain, published by the Spanish Federation of Nutrition, Food and Dietetics Societies² in 2010 were: 1000-1300 mg/d for calcium (Ca), 350-400 mg/d for magnesium (Mg), 1000 µg/d for copper (Cu), 27 mg/d for iron (Fe), 60 µg/d for selenium (Se) and 11-13 mg/d for zinc (Zn). Serum levels in the following ranges are considered normal: Ca 8.5-10.2 mg/dL, Fe 60-180 µg/dL, Mg 0.75-1.25 mmol/L, Cu 70-140 µg/dL, Se 70-150 ng/mL and Zn 0.66-1.10 µg/mL.³

Fe is necessary for proper development of the placenta, as well as bone and organ formation in the fetus. Iron-deficiency anemia is relatively common among pregnant women and may increase the risk of some complications, including preterm delivery and low infant birth weight. To prevent maternal anemia, it is recommended that pregnant women take a daily oral supplement of 30 to 60 mg of elemental Fe.⁴

Zn is important for protein synthesis, cell division, and nucleic acid metabolism. Deficiency in this element is manifested in different ways, depending on its severity, but in particular, may have a negative impact on the nervous and reproductive systems. Cu is essential for oxidative metabolism, cell growth, development of connective tissue and hemoglobin synthesis.⁵ Deficiency in this element can cause bone deformations, as well as cardiovascular problems. Se is part of the key enzyme glutathione peroxidase, seems to be involved in platelet function and helps to neutralize heavy metals such as mercury, lead, arsenic and cadmium. Further, deficiency in this element affects all components of the immune system. During pregnancy, there is active transport to the fetus through the placenta and Se deficiency is reported to be associated with a variety of adverse outcomes including miscarriage, preterm delivery, gestational diabetes and preeclampsia.⁶

Mg is a key element for fetal growth and is one of the most abundant nutrients in the human body. It is involved in cellular respiration, protein synthesis, maintenance of cardiovascular health, regulation of cellular function and the action of collagen, among many other functions. Deficiency in Mg during pregnancy has been associated with preeclampsia and preterm delivery, as well as low infant birth weight.⁷

Lastly, Ca is another element necessary for normal fetal growth.⁸ Among its functions, in addition to providing structure and rigidity to the bones, it plays a key role in muscle contractility, the transmission of signals from the brain nerves to the rest of the body, the circulation of the blood and the production of hormones and enzymes for various body functions. Lower maternal serum Ca levels have been associated with pregnancy-induced hypertension and preeclampsia.

The objective of this work was to describe serum levels of Ca, Fe, Mg, Se, Cu and Zn, and evaluate their relationship with maternal socio-demographic, anthropometric and dietary variables in women from cohorts from Valencia and Gipuzkoa (Spain) in their first trimester of pregnancy.

Method

Results

We analyzed data from serum samples of 1279 women (51.3% from the Valencia cohort). Sample size, percentages and missing values of all the variables are shown in Table 1. Overall, 92% had been born in Spain, 61.3% were 30 years old or older, with a mean age of 30.7 years (31.3 and 30.1 years for the Gipuzkoa and Valencia cohorts respectively), 50.1% lived in urban areas, 77.2% had secondary or university studies, 78.5% were active workers, 51% belonged to the lowest social class and 55% were primipara. In relation to other variables, 24.7% had excess body weight, 17.4% smoked during pregnancy, 14.9% had smoked but quit before pregnancy and 1.1% consumed more than 5 g of alcohol per day.

Sample size, missing data, mean, SD, minimum, maximum and percentiles (25^th, 50^th and 75^th) for each metal are presented in Table 2 by cohort. Mean and standard deviation of the levels were 94030.7 ± 4278.0 µg/L for Ca, 1614.8 ± 279.5 µg/L for Cu, 1113.9 ± 325.7 µg/L for Fe, 17032.5 ± 1006.4 µg/L for Mg, 79.56 ± 9.64 µg/L for Se and 642.1 ± 110.8 µg/L for Zn.

Table 3 summarizes the results of multiple linear regression models for each metal. Ca levels were related to daily white meat consumption (p = 0.026). Higher levels of Cu were related to excess body weight (p < 0.001), low social class (p = 0.003) and being multipara (p < 0.001). On the other hand,lower levels of Cu were related to bread intake of less than 42 g /day (p = 0.013) and residence in Gipuzkoa, after adjustment for the other variables. With reference to Se, higher levels were related to residence in Valencia (p < 0.001), having a university degree (p = 0.001),taking B6 vitamin supplements (p = 0.006) and a fish intake of more than 71 g/day (p = 0.008), after adjustment for the other variables. Women born outside Spain showed lower Se levels (p < 0.001). In regard to Fe, lower levels were related to having excess body weight (p = 0.021) or obesity (p < 0.01) and taking vitamin B12 supplements (p = 0.006). Higher levels of Mg were associated with being over 35 years old (p = 0.001), high social class (p = 0.044), primiparous status (p = 0.002) and low daily bread (p = 0.009) and legume (p = 0.020) intake, after adjustment for the other variables.

Finally, higher levels of Zn were related to having a university degree (p = 0.039) and being from the Gipuzkoa cohort (p < 0.001), and lower levels of Zn were related to being over 35 years old (p = 0.027), after adjustment for the other variables.

Table 4 shows that the multivariate variance in serum levels of Ca, Cu, Fe, Se and Zn was associated with body mass, parity, educational level and country of origin (socio-demographic model) and also with white bread and vitamin B6 intake (dietary model). BMI and country of origin emerge as the most important among the explanatory variables considered. Although the influence of both socio-demographic conditions and diet on serum levels appears to be rather modest, the partitioning of variance in the joint model (Fig. 1) suggests that, overall, socio-demographic conditions are more influential than diet itself. Table 5 summarizes the results of previous studies.

Figure 1. Venn diagram showing how multivariate variation in serum levels of Ca, Cu, Fe, Se and Zn was partitioned among a diet component and a socio-demographic component. Numbers are R² values (%). The diet component includes white bread intake and vitamin B₆ intake. The socio-demographic component includes body mass index, parity, education level and country of origin. 

Discussion

We analyzed trace element levels in pregnant women in their first trimester of pregnancy. Our results reveal that the main variables related to these levels are BMI, social class and cohort. According to Abbassi-Ghanavati et al.¹¹, serum Ca between 88,000 µg/L and 106,000 µg/L can be considered normal and our results fall within this range (94,030.7 µg/L). Further, the Ca levels in our sample of pregnant women are in line with those observed in several previous studies^7,12,13 (with values of 95,900 µg/L, 97,000 µg/L and 92,000 µg/L respectively). On the other hand, they are a little higher than those obtained by Kanagal et al.¹⁴ (89,700 µg/L), and Punthumapol and Kittichotpanich¹⁵ (89,900 µg/L), and much higher than those found in a study carried out in China by Tong¹⁶ (21,600 µg/L). Some studies have shown Ca levels to be related to body weight, levels being significantly lower in those with excess body weight,^17,18 but we did not find any such relationship in our study. Regarding age, unlike other authors,¹⁸ we found an association close to statistical significance, levels decreasing with increasing age (p = 0.077).

Regarding Mg, our results (17,000 µg/L) are within the normal range for Mg of between 16,000 µg/L and 22,000 µg/L¹¹. The levels we observed were similar to those reported by Kanagal et al.¹⁴ (15,700 µg/L) and slightly lower than those observed by Punthumapol and Kittichotpanich¹⁵ (20,400 µg/L), Arun et al.⁷ (20,300 µg/L) and Sukonpan and Phupong¹² (20,700 µg/L). In contrast, a study carried out in China by Li¹⁹, showed levels of serum Mg well below those obtained in this study (9800 µg/L).

The mean Cu level in our study population (1615 µg/L) was within the normal range of between 1120 µg/L and 1990 µg/L¹¹. Similar levels have been observed in other studies^20-22 (1614 µg/L, and 1650 µg/L and 1750 µg/L, respectively). On the other hand, Cu levels considerably different from ours have been observed in China²³ and in Poland²⁴ (1026 µg/L and 1980 µg/L, respectively). Wilson et al.²⁵ reported significantly higher levels in obese women than underweight, normal weight or overweight women, as in our study. We found a trend related to BMI (with the highest levels in obese women), while others have observed an opposite trend.¹⁷

Again, in the case of Zn, the levels we observed (642 µg/L) lie within the normal range of between 570 µg/L and 880 µg/L.¹¹ These results were in agreement with those found in previous studies in Spain²⁶ and Iran²⁷ (654 µg/L and 749 µg/L, respectively). Studies carried out in China^23,28 and Poland²⁴ showed higher levels (920 µg/L, 900 µg/L and 910 µg/L, respectively), while studies carried out in Korea²² and India²⁰ showed lower levels (570 µg/L and 514 µg/L respectively). We found an association between Zn and educational level, which could be an indicator of social class. Nevertheless, other authors did not find this pattern²⁹.

As for Se, our results were consistent with studies carried out in Spain³⁰ and Finland³¹ with reported values of 81 µg/L, while a study from India²⁰ found lower values of 70 µg/L. On the other hand, higher values were found in Korea,²² Spain²⁶ and Africa³² (94 µg/L, 99.59 µg/L and 112.3 µg/L, respectively). In our study, Se levels appear to be related to fish consumption, which is consistent with other research findings.³³

Fe levels between 720 µg/L and 1430 µg/L can be considered normal,¹¹ and our results are in that range (1114 µg/L). Similar results have been reported in China²³ and India²⁰ (900 µg/L and 1132 µg/L respectively), while some studies have shown slightly lower values^21,27,28 (690 µg/L, 800 µg/L and 744 µg/L respectively), and another study in China,³⁴ found values slightly higher than ours (1500 µg/L). We observed that being overweight or obese was associated with significantly lower serum Fe levels, a pattern that has been observed previously^15,25.

Social inequalities in dietary habits have been widely described and indicate that the lowest social classes tend to have less balanced and less healthy diets. Pregnant women with lower social status have fewer healthy habits, including a less healthy diet, more harmful behaviors, and poorer monitoring during pregnancy. Indeed, the role of education as a social determinant of diet has been confirmed by several authors as the most robust independent predictor of healthy dietary habits.³⁵

The strengths of the study are the sample size (n = 1279) and the fact that we gathered data on and adjusted models for a large number of factors in the first trimester, especially socio-demographic characteristics. Regarding its limitations, the descriptive and crosssectional character of the design means that we cannot ascertain causality in the associations observed, measurements were taken at only one point during pregnancy, and the spectroscopy measurements of the metals were element-specific not species-specific. Another limitation is that the information regarding some of the behavioral variables (smoking and alcohol consumption) was selfreported, an approach which may underestimate the prevalence of socially disapproved behaviors.

Conclusions

Our results indicate that the Ca, Cu, Se, Fe, Mg and Zn levels in both cohorts were adequate and within the ranges considered normal for pregnant women. More than half of the sample did not take supplements, except in the case of vitamin B₁₂ (72%). Among the variables considered, those with the best explanatory value of the levels of these trace elements were anthropometric and socio-demographic variables and within these, BMI, social class, educational level and cohort. This study will serve as a basis for future research, in particular, to identify trends in these elements through pregnancy and their possible relationship with the later development of the child.

Transparency declaration

The corresponding author on behalf of the other authors guarantee the accuracy, transparency and honesty of the data and information contained in the study, that no relevant information has been omitted and that all discrepancies between authors have been adequately resolved and described.