Prenatal mercury exposure and body mass index at 2 and 4 years: The Japan Environment and Children's Study

Whether prenatal mercury (Hg) exposure may increase obesity among children remains unclear. We here investigated the association between cord blood methylmercury (MeHg) and inorganic mercury (IHg) concentration and body mass index (BMI) z-scores, as well as overweight or obesity at the ages of 2 and 4 years. We used 3147 mother-child pairs data from the Japan Environment and Children's Study. The levels of MeHg and IHg in cord blood were quantified by liquid chromatography coupled with inductively coupled plasma mass spectrometry. Pediatric physicians examined the children's height and weight at ages 2 and 4, and BMI z-scores were calculated. Linear or logistic regression models were employed to examine the association between Hg concentrations and BMI outcomes. After adjusting for potential confounders, our findings indicated that a 2-fold increase in IHg concentrations was associated with increases in BMI z-scores of 0.05 (95% CI: 0.01, 0.09; p = 0.021) and 0.05 (95% CI: 0.02, 0.09; p = 0.0046) at ages 2 and 4, respectively. Conversely, no significant associations were observed for MeHg. When overweight or obesity was employed as an outcome, we did not observe consistent associations for either MeHg or IHg. In this birth cohort study, an increase in cord blood IHg, but not MeHg, was associated with a slight increase in BMI z-score in children at 2 and 4 years of age. However, the observed associations were weak and inconsistent, suggesting that the results might be due to residual confounding or mere chance rather than indicative of a causal relationship.