Distinct demethylation pathways of rice- and fish-derived methylmercury in pregnant women revealed by compound-specific isotope analysis

Understanding mercury (Hg) metabolism and exposure pathways in pregnant women is essential for protecting fetal health. However, most previous studies have relied primarily on Hg concentrations in single biomarkers, limiting insight into <i>in vivo</i> metabolic differences and maternal-fetal transfer, especially regarding source-specific MeHg metabolism. This study applies compound-specific stable isotope analysis to investigate total Hg (THg) and methylmercury (MeHg) concentrations and isotopic signatures in rice and maternal biomarkers from the Wanshan Hg mining area, China. Even after remediation, THg in all cord blood samples still exceeded the U.S. EPA recommended limit. Widespread mass-dependent fractionation (MDF) was observed across different biomarkers. The MDF of THg was driven by internal redistribution and demethylation of MeHg, while the MDF of MeHg specifically reflected metabolic demethylation processes in the gut and brain. In contrast, no mass-independent fractionation (MIF) was observed during maternal or transplacental THg transfer, suggesting that Δ¹⁹⁹Hg values in maternal biomarkers reflect food-source signals. This study identifies the biomarker-specific contribution of dietary sources: MeHg isotope mixing models revealed that rice contributed a higher proportion of Hg in maternal blood than in hair, comparable to that in placenta and cord blood. Comparisons with previously reported fish-derived MeHg isotopes suggest that rice-derived MeHg undergoes preferential demethylation and elimination, highlighting the complexity of <i>in vivo</i> Hg dynamics. The use of both THg and MeHg isotopes in biomarkers enables more precise tracing of dietary Hg sources. These findings highlight the value of multi-biomarker isotope analysis for tracing Hg sources and improving exposure assessments.