This is the first time examine prenatal blood metal mixture using data from two countries with variation in race, socioeconomic status, and environmental exposure. The present research revealed differences in metal levels and anthropometric characteristics between the two populations. Pearson’s correlation coefficients and unadjusted linear regression analysis found some correlations between prenatal blood metals concentrations and birth weight. Adjusted linear regression analysis found a weak reverse relationship between metals and birth weight. Similarly, the BKMR overall exposure-response function showed decreasing birth weight in response to increase metal levels.
Lead, a well-known toxic metal, extensively studied for toxicities on reproductive system [18, 19, 39]. In the present study, the average blood Pb was three times lower in Japanese women than Iranian (Table 2). Lower Pb in Japanese women could be influenced by earlier phased-out leaded gasoline in Japan than Iran and better protection regulations for pregnant women. The BKMR bivariate analysis showed a U-shape relationship between Pb and birth weight. In addition, the single-variable predictor model found the largest adverse effect on birth weight by blood Pb. Although the underlying mechanisms of adverse health effects are not fully understood, Pb might involve disruption of reproductive hormones [40], induction reactive oxygen species production [41], and interaction with essential elements functions [4].
On average, Rb level was 70% (763µg/L) higher in Japanese women than Iranian. Pearson’s and linear regression analyses showed a significant negative correlation between Rb and birth weight. Although the reproductive system toxicities of Rb have not been sufficiently studied, a few reports have indicated pregnancy concentration increasing [42], fetal growth decreasing [43], and increasing the risk of pregnancy induced-hypertension [44] in Rb exposure groups.
Selenium, critical for human reproduction, thyroid hormone metabolism, DNA synthesis, and protection from oxidative damage [45], showed 76% higher level in Japanese women than Iranian, which may relate to higher tuna fish consumption, one of the popular food in Japan [45]. Our statistical analyses showed a weak negative correlation between Se and birth weight. Similarly, a few previous studies have reported inverse association between birth weight and Se level in blood, serum, and the placenta [46, 47].
Concentrations of blood Mn and As were higher in Iranian pregnant women than Japanese participants. There was not a strong correlation between these two elements and birth weight. On the other hand, some researchers, including our previous study, have reported adverse pregnancy outcomes, including low birth size, induced by prenatal blood Mn [7, 15, 17, 48, 49] and As [50, 51].
The present study also investigated Zn and Cu, necessary for fetal growth and development via many physiological pathways, such as enzyme activities, and protein synthesis [52–54]. Blood concentrations of these two metals may vary based on factors like gender, age, diets, and supplements intakes [55]. The comparison revealed higher Zn and lower Cu levels (roughly 10%) in Iranian pregnant women than Japanese.
While many previous studies have examined the relationship between prenatal metal exposure and birth weight, this study is unique in employing data from two different study populations. Variations in subject characteristics enhance the study’s power to generalize findings to the target population. In addition, the current work considered metal that has been not studied (Rb) enough for effects on reproductive system. However, our findings may be influenced by variations in characteristics between the two populations. Additionally, the study used first-trimester blood metals concentrations for exposure assessment instead of serial measurements. Many studies have reported a higher probability of adverse pregnancy outcomes with early-stage gestation metal exposure [16, 30]. However, blood metals’ half-life is from days to weeks, and concentrations can change during subsequent trimesters. Finally, none of study participants were occupationally exposed to metal, thus, they might be encountered from their diet and polluted environment, but, the study did not measure metal concentrations in their foods and/or environment.